diff --git a/src/Executor/ExecutionContext.php b/src/Executor/ExecutionContext.php
index 59b6454..8acf3db 100644
--- a/src/Executor/ExecutionContext.php
+++ b/src/Executor/ExecutionContext.php
@@ -1,6 +1,7 @@
 <?php
 namespace GraphQL\Executor;
 
+use GraphQL\Error;
 use GraphQL\Language\AST\OperationDefinition;
 use GraphQL\Schema;
 
@@ -25,7 +26,7 @@ class ExecutionContext
     /**
      * @var
      */
-    public $root;
+    public $rootValue;
 
     /**
      * @var OperationDefinition
@@ -35,7 +36,7 @@ class ExecutionContext
     /**
      * @var array
      */
-    public $variables;
+    public $variableValues;
 
     /**
      * @var array
@@ -46,13 +47,13 @@ class ExecutionContext
     {
         $this->schema = $schema;
         $this->fragments = $fragments;
-        $this->root = $root;
+        $this->rootValue = $root;
         $this->operation = $operation;
-        $this->variables = $variables;
+        $this->variableValues = $variables;
         $this->errors = $errors ?: [];
     }
 
-    public function addError($error)
+    public function addError(Error $error)
     {
         $this->errors[] = $error;
         return $this;
diff --git a/src/Executor/ExecutionResult.php b/src/Executor/ExecutionResult.php
new file mode 100644
index 0000000..f1b4487
--- /dev/null
+++ b/src/Executor/ExecutionResult.php
@@ -0,0 +1,38 @@
+<?php
+namespace GraphQL\Executor;
+
+use GraphQL\Error;
+
+class ExecutionResult
+{
+    /**
+     * @var array
+     */
+    public $data;
+
+    /**
+     * @var Error[]
+     */
+    public $errors;
+
+    /**
+     * @param array $data
+     * @param array $errors
+     */
+    public function __construct(array $data = null, array $errors = [])
+    {
+        $this->data = $data;
+        $this->errors = $errors;
+    }
+
+    public function toArray()
+    {
+        $result = ['data' => $this->data];
+
+        if (!empty($this->errors)) {
+            $result['errors'] = array_map(['GraphQL\Error', 'formatError'], $this->errors);
+        }
+
+        return $result;
+    }
+}
diff --git a/src/Executor/Executor.php b/src/Executor/Executor.php
index 284ffd4..983e720 100644
--- a/src/Executor/Executor.php
+++ b/src/Executor/Executor.php
@@ -9,6 +9,7 @@ use GraphQL\Language\AST\Node;
 use GraphQL\Language\AST\OperationDefinition;
 use GraphQL\Language\AST\SelectionSet;
 use GraphQL\Schema;
+use GraphQL\Type\Definition\AbstractType;
 use GraphQL\Type\Definition\Directive;
 use GraphQL\Type\Definition\EnumType;
 use GraphQL\Type\Definition\FieldDefinition;
@@ -16,6 +17,7 @@ use GraphQL\Type\Definition\InterfaceType;
 use GraphQL\Type\Definition\ListOfType;
 use GraphQL\Type\Definition\NonNull;
 use GraphQL\Type\Definition\ObjectType;
+use GraphQL\Type\Definition\ResolveInfo;
 use GraphQL\Type\Definition\ScalarType;
 use GraphQL\Type\Definition\Type;
 use GraphQL\Type\Definition\UnionType;
@@ -45,40 +47,70 @@ class Executor
 {
     private static $UNDEFINED;
 
-    public static function execute(Schema $schema, $root, Document $ast, $operationName = null, array $args = null)
+    private static $defaultResolveFn;
+
+    /**
+     * Custom default resolve function
+     *
+     * @param $fn
+     * @throws \Exception
+     */
+    public function setDefaultResolveFn($fn)
+    {
+        Utils::invariant(is_callable($fn), 'Expecting callable, but got ' . Utils::getVariableType($fn));
+        self::$defaultResolveFn = $fn;
+    }
+
+    /**
+     * @param Schema $schema
+     * @param Document $ast
+     * @param $rootValue
+     * @param array|\ArrayAccess $variableValues
+     * @param null $operationName
+     * @return ExecutionResult
+     */
+    public static function execute(Schema $schema, Document $ast, $rootValue = null, $variableValues = null, $operationName = null)
     {
         if (!self::$UNDEFINED) {
             self::$UNDEFINED = new \stdClass();
         }
 
+        if (null !== $variableValues) {
+            Utils::invariant(
+                is_array($variableValues) || $variableValues instanceof \ArrayAccess,
+                "Variable values are expected to be array or instance of ArrayAccess, got " . Utils::getVariableType($variableValues)
+            );
+        }
+        if (null !== $operationName) {
+            Utils::invariant(
+                is_string($operationName),
+                "Operation name is supposed to be string, got " . Utils::getVariableType($operationName)
+            );
+        }
+
+        $exeContext = self::buildExecutionContext($schema, $ast, $rootValue, $variableValues, $operationName);
+
         try {
-            $errors = new \ArrayObject();
-            $exeContext = self::buildExecutionContext($schema, $root, $ast, $operationName, $args, $errors);
-            $data = self::executeOperation($exeContext, $root, $exeContext->operation);
-        } catch (\Exception $e) {
-            $errors[] = $e;
+            $data = self::executeOperation($exeContext, $exeContext->operation, $rootValue);
+        } catch (Error $e) {
+            $exeContext->addError($e);
+            $data = null;
         }
 
-        $result = [
-            'data' => isset($data) ? $data : null
-        ];
-        if (count($errors) > 0) {
-            $result['errors'] = array_map(['GraphQL\Error', 'formatError'], $errors->getArrayCopy());
-        }
-
-        return $result;
+        return new ExecutionResult($data, $exeContext->errors);
     }
 
     /**
      * Constructs a ExecutionContext object from the arguments passed to
      * execute, which we will pass throughout the other execution methods.
      */
-    private static function buildExecutionContext(Schema $schema, $root, Document $ast, $operationName = null, array $args = null, &$errors)
+    private static function buildExecutionContext(Schema $schema, Document $documentAst, $rootValue, $rawVariableValues, $operationName = null)
     {
+        $errors = [];
         $operations = [];
         $fragments = [];
 
-        foreach ($ast->definitions as $statement) {
+        foreach ($documentAst->definitions as $statement) {
             switch ($statement->kind) {
                 case Node::OPERATION_DEFINITION:
                     $operations[$statement->name ? $statement->name->value : ''] = $statement;
@@ -91,32 +123,34 @@ class Executor
 
         if (!$operationName && count($operations) !== 1) {
             throw new Error(
-                'Must provide operation name if query contains multiple operations'
+                'Must provide operation name if query contains multiple operations.'
             );
         }
 
         $opName = $operationName ?: key($operations);
-        if (!isset($operations[$opName])) {
-            throw new Error('Unknown operation name: ' . $opName);
+        if (empty($operations[$opName])) {
+            throw new Error('Unknown operation named ' . $opName);
         }
         $operation = $operations[$opName];
 
-        $variables = Values::getVariableValues($schema, $operation->variableDefinitions ?: array(), $args ?: []);
-        $exeContext = new ExecutionContext($schema, $fragments, $root, $operation, $variables, $errors);
+        $variableValues = Values::getVariableValues($schema, $operation->variableDefinitions ?: [], $rawVariableValues ?: []);
+        $exeContext = new ExecutionContext($schema, $fragments, $rootValue, $operation, $variableValues, $errors);
         return $exeContext;
     }
 
     /**
      * Implements the "Evaluating operations" section of the spec.
      */
-    private static function executeOperation(ExecutionContext $exeContext, $root, OperationDefinition $operation)
+    private static function executeOperation(ExecutionContext $exeContext, OperationDefinition $operation, $rootValue)
     {
         $type = self::getOperationRootType($exeContext->schema, $operation);
         $fields = self::collectFields($exeContext, $type, $operation->selectionSet, new \ArrayObject(), new \ArrayObject());
+
         if ($operation->operation === 'mutation') {
-            return self::executeFieldsSerially($exeContext, $type, $root, $fields->getArrayCopy());
+            return self::executeFieldsSerially($exeContext, $type, $rootValue, $fields->getArrayCopy());
         }
-        return self::executeFields($exeContext, $type, $root, $fields);
+
+        return self::executeFields($exeContext, $type, $rootValue, $fields);
     }
 
 
@@ -154,11 +188,11 @@ class Executor
      * Implements the "Evaluating selection sets" section of the spec
      * for "write" mode.
      */
-    private static function executeFieldsSerially(ExecutionContext $exeContext, ObjectType $parentType, $source, $fields)
+    private static function executeFieldsSerially(ExecutionContext $exeContext, ObjectType $parentType, $sourceValue, $fields)
     {
         $results = [];
         foreach ($fields as $responseName => $fieldASTs) {
-            $result = self::resolveField($exeContext, $parentType, $source, $fieldASTs);
+            $result = self::resolveField($exeContext, $parentType, $sourceValue, $fieldASTs);
 
             if ($result !== self::$UNDEFINED) {
                 // Undefined means that field is not defined in schema
@@ -250,18 +284,36 @@ class Executor
     }
 
     /**
-     * Determines if a field should be included based on @if and @unless directives.
+     * Determines if a field should be included based on the @include and @skip
+     * directives, where @skip has higher precedence than @include.
      */
     private static function shouldIncludeNode(ExecutionContext $exeContext, $directives)
     {
-        $ifDirective = Values::getDirectiveValue(Directive::ifDirective(), $directives, $exeContext->variables);
-        if ($ifDirective !== null) {
-            return $ifDirective;
+        $skipDirective = Directive::skipDirective();
+        $includeDirective = Directive::includeDirective();
+
+        /** @var \GraphQL\Language\AST\Directive $skipAST */
+        $skipAST = $directives
+            ? Utils::find($directives, function(\GraphQL\Language\AST\Directive $directive) use ($skipDirective) {
+                return $directive->name->value === $skipDirective->name;
+            })
+            : null;
+
+        if ($skipAST) {
+            $argValues = Values::getArgumentValues($skipDirective->args, $skipAST->arguments, $exeContext->variableValues);
+            return empty($argValues['if']);
         }
 
-        $unlessDirective = Values::getDirectiveValue(Directive::unlessDirective(), $directives, $exeContext->variables);
-        if ($unlessDirective !== null) {
-            return !$unlessDirective;
+        /** @var \GraphQL\Language\AST\Directive $includeAST */
+        $includeAST = $directives
+            ? Utils::find($directives, function(\GraphQL\Language\AST\Directive $directive) use ($includeDirective) {
+                return $directive->name->value === $includeDirective->name;
+            })
+            : null;
+
+        if ($includeAST) {
+            $argValues = Values::getArgumentValues($includeDirective->args, $includeAST->arguments, $exeContext->variableValues);
+            return !empty($argValues['if']);
         }
 
         return true;
@@ -293,97 +345,106 @@ class Executor
     }
 
     /**
-     * A wrapper function for resolving the field, that catches the error
-     * and adds it to the context's global if the error is not rethrowable.
+     * Resolves the field on the given source object. In particular, this
+     * figures out the value that the field returns by calling its resolve function,
+     * then calls completeValue to complete promises, serialize scalars, or execute
+     * the sub-selection-set for objects.
      */
     private static function resolveField(ExecutionContext $exeContext, ObjectType $parentType, $source, $fieldASTs)
     {
-        $fieldDef = self::getFieldDef($exeContext->schema, $parentType, $fieldASTs[0]);
+        $fieldAST = $fieldASTs[0];
+        $fieldName = $fieldAST->name->value;
+
+        $fieldDef = self::getFieldDef($exeContext->schema, $parentType, $fieldName);
+
         if (!$fieldDef) {
             return self::$UNDEFINED;
         }
 
+        $returnType = $fieldDef->getType();
+
+        if (isset($fieldDef->resolve)) {
+            $resolveFn = $fieldDef->resolve;
+        } else if (isset(self::$defaultResolveFn)) {
+            $resolveFn = self::$defaultResolveFn;
+        } else {
+            $resolveFn = [__CLASS__, 'defaultResolveFn'];
+        }
+
+        // Build hash of arguments from the field.arguments AST, using the
+        // variables scope to fulfill any variable references.
+        // TODO: find a way to memoize, in case this field is within a List type.
+        $args = Values::getArgumentValues(
+            $fieldDef->args,
+            $fieldAST->arguments,
+            $exeContext->variableValues
+        );
+
+        // The resolve function's optional third argument is a collection of
+        // information about the current execution state.
+        $info = new ResolveInfo([
+            'fieldName' => $fieldName,
+            'fieldASTs' => $fieldASTs,
+            'returnType' => $returnType,
+            'parentType' => $parentType,
+            'schema' => $exeContext->schema,
+            'fragments' => $exeContext->fragments,
+            'rootValue' => $exeContext->rootValue,
+            'operation' => $exeContext->operation,
+            'variableValues' => $exeContext->variableValues,
+        ]);
+
+        // If an error occurs while calling the field `resolve` function, ensure that
+        // it is wrapped as a GraphQLError with locations. Log this error and return
+        // null if allowed, otherwise throw the error so the parent field can handle
+        // it.
+        try {
+            $result = call_user_func($resolveFn, $source, $args, $info);
+        } catch (\Exception $error) {
+            $reportedError = Error::createLocatedError($error, $fieldASTs);
+
+            if ($returnType instanceof NonNull) {
+                throw $reportedError;
+            }
+
+            $exeContext->addError($reportedError);
+            return null;
+        }
+
+        return self::completeValueCatchingError(
+            $exeContext,
+            $returnType,
+            $fieldASTs,
+            $info,
+            $result
+        );
+    }
+
+
+    public static function completeValueCatchingError(
+        ExecutionContext $exeContext,
+        Type $returnType,
+        $fieldASTs,
+        ResolveInfo $info,
+        $result
+    )
+    {
         // If the field type is non-nullable, then it is resolved without any
         // protection from errors.
-        if ($fieldDef->getType() instanceof NonNull) {
-            return self::resolveFieldOrError(
-                $exeContext,
-                $parentType,
-                $source,
-                $fieldASTs,
-                $fieldDef
-            );
+        if ($returnType instanceof NonNull) {
+            return self::completeValue($exeContext, $returnType, $fieldASTs, $info, $result);
         }
 
         // Otherwise, error protection is applied, logging the error and resolving
         // a null value for this field if one is encountered.
         try {
-            $result = self::resolveFieldOrError(
-                $exeContext,
-                $parentType,
-                $source,
-                $fieldASTs,
-                $fieldDef
-            );
-
-            return $result;
-        } catch (\Exception $error) {
-            $exeContext->addError($error);
+            return self::completeValue($exeContext, $returnType, $fieldASTs, $info, $result);
+        } catch (Error $err) {
+            $exeContext->addError($err);
             return null;
         }
     }
 
-    /**
-     * Resolves the field on the given source object. In particular, this
-     * figures out the object that the field returns using the resolve function,
-     * then calls completeField to coerce scalars or execute the sub
-     * selection set for objects.
-     */
-    private static function resolveFieldOrError(
-        ExecutionContext $exeContext,
-        ObjectType $parentType,
-        $source,
-        /*array<Field>*/ $fieldASTs,
-        FieldDefinition $fieldDef
-    )
-    {
-        $fieldAST = $fieldASTs[0];
-        $fieldType = $fieldDef->getType();
-        $resolveFn = $fieldDef->resolve ?: [__CLASS__, 'defaultResolveFn'];
-
-        // Build a JS object of arguments from the field.arguments AST, using the
-        // variables scope to fulfill any variable references.
-        // TODO: find a way to memoize, in case this field is within a Array type.
-        $args = Values::getArgumentValues(
-            $fieldDef->args,
-            $fieldAST->arguments,
-            $exeContext->variables
-        );
-
-        try {
-            $result = call_user_func($resolveFn,
-                $source,
-                $args,
-                $exeContext->root,
-                // TODO: provide all fieldASTs, not just the first field
-                $fieldAST,
-                $fieldType,
-                $parentType,
-                $exeContext->schema
-            );
-        } catch (\Exception $error) {
-            throw Error::createLocatedError($error, [$fieldAST]);
-        }
-
-        return self::completeField(
-            $exeContext,
-            $fieldType,
-            $fieldASTs,
-            $result
-        );
-    }
-
-
     /**
      * Implements the instructions for completeValue as defined in the
      * "Field entries" section of the spec.
@@ -396,20 +457,22 @@ class Executor
      * for the inner type on each item in the list.
      *
      * If the field type is a Scalar or Enum, ensures the completed value is a legal
-     * value of the type by calling the `coerce` method of GraphQL type definition.
+     * value of the type by calling the `serialize` method of GraphQL type
+     * definition.
      *
      * Otherwise, the field type expects a sub-selection set, and will complete the
      * value by evaluating all sub-selections.
      */
-    private static function completeField(ExecutionContext $exeContext, Type $fieldType,/* Array<Field> */ $fieldASTs, &$result)
+    private static function completeValue(ExecutionContext $exeContext, Type $returnType,/* Array<Field> */ $fieldASTs, ResolveInfo $info, &$result)
     {
         // If field type is NonNull, complete for inner type, and throw field error
         // if result is null.
-        if ($fieldType instanceof NonNull) {
-            $completed = self::completeField(
+        if ($returnType instanceof NonNull) {
+            $completed = self::completeValue(
                 $exeContext,
-                $fieldType->getWrappedType(),
+                $returnType->getWrappedType(),
                 $fieldASTs,
+                $info,
                 $result
             );
             if ($completed === null) {
@@ -427,8 +490,8 @@ class Executor
         }
 
         // If field type is List, complete each item in the list with the inner type
-        if ($fieldType instanceof ListOfType) {
-            $itemType = $fieldType->getWrappedType();
+        if ($returnType instanceof ListOfType) {
+            $itemType = $returnType->getWrappedType();
             Utils::invariant(
                 is_array($result) || $result instanceof \Traversable,
                 'User Error: expected iterable, but did not find one.'
@@ -436,32 +499,48 @@ class Executor
 
             $tmp = [];
             foreach ($result as $item) {
-                $tmp[] = self::completeField($exeContext, $itemType, $fieldASTs, $item);
+                $tmp[] = self::completeValueCatchingError($exeContext, $itemType, $fieldASTs, $info, $item);
             }
             return $tmp;
         }
 
-        // If field type is Scalar or Enum, coerce to a valid value, returning null
-        // if coercion is not possible.
-        if ($fieldType instanceof ScalarType ||
-            $fieldType instanceof EnumType
-        ) {
-            Utils::invariant(method_exists($fieldType, 'coerce'), 'Missing coerce method on type');
-            return $fieldType->coerce($result);
+        // If field type is Scalar or Enum, serialize to a valid value, returning
+        // null if serialization is not possible.
+        if ($returnType instanceof ScalarType ||
+            $returnType instanceof EnumType) {
+            Utils::invariant(method_exists($returnType, 'serialize'), 'Missing serialize method on type');
+            return $returnType->serialize($result);
         }
 
         // Field type must be Object, Interface or Union and expect sub-selections.
+        if ($returnType instanceof ObjectType) {
+            $objectType = $returnType;
+        } else if ($returnType instanceof AbstractType) {
+            $objectType = $returnType->getObjectType($result, $info);
 
-        $objectType =
-            $fieldType instanceof ObjectType ? $fieldType :
-                ($fieldType instanceof InterfaceType ||
-                $fieldType instanceof UnionType ? $fieldType->resolveType($result) :
-                    null);
+            if ($objectType && !$returnType->isPossibleType($objectType)) {
+                throw new Error(
+                    "Runtime Object type \"$objectType\" is not a possible type for \"$returnType\"."
+                );
+            }
+        } else {
+            $objectType = null;
+        }
 
         if (!$objectType) {
             return null;
         }
 
+        // If there is an isTypeOf predicate function, call it with the
+        // current result. If isTypeOf returns false, then raise an error rather
+        // than continuing execution.
+        if (false === $objectType->isTypeOf($result, $info)) {
+            throw new Error(
+                "Expected value of type $objectType but got: $result.",
+                $fieldASTs
+            );
+        }
+
         // Collect sub-fields to execute to complete this value.
         $subFieldASTs = new \ArrayObject();
         $visitedFragmentNames = new \ArrayObject();
@@ -488,17 +567,18 @@ class Executor
      * and returns it as the result, or if it's a function, returns the result
      * of calling that function.
      */
-    public static function defaultResolveFn($source, $args, $root, $fieldAST)
+    public static function defaultResolveFn($source, $args, ResolveInfo $info)
     {
+        $fieldName = $info->fieldName;
         $property = null;
+
         if (is_array($source) || $source instanceof \ArrayAccess) {
-            if (isset($source[$fieldAST->name->value])) {
-                $property = $source[$fieldAST->name->value];
+            if (isset($source[$fieldName])) {
+                $property = $source[$fieldName];
             }
         } else if (is_object($source)) {
-            if (property_exists($source, $fieldAST->name->value)) {
-                $e = func_get_args();
-                $property = $source->{$fieldAST->name->value};
+            if (property_exists($source, $fieldName)) {
+                $property = $source->{$fieldName};
             }
         }
 
@@ -516,25 +596,21 @@ class Executor
      *
      * @return FieldDefinition
      */
-    private static function getFieldDef(Schema $schema, ObjectType $parentType, Field $fieldAST)
+    private static function getFieldDef(Schema $schema, ObjectType $parentType, $fieldName)
     {
-        $name = $fieldAST->name->value;
         $schemaMetaFieldDef = Introspection::schemaMetaFieldDef();
         $typeMetaFieldDef = Introspection::typeMetaFieldDef();
         $typeNameMetaFieldDef = Introspection::typeNameMetaFieldDef();
 
-        if ($name === $schemaMetaFieldDef->name &&
-            $schema->getQueryType() === $parentType
-        ) {
+        if ($fieldName === $schemaMetaFieldDef->name && $schema->getQueryType() === $parentType) {
             return $schemaMetaFieldDef;
-        } else if ($name === $typeMetaFieldDef->name &&
-            $schema->getQueryType() === $parentType
-        ) {
+        } else if ($fieldName === $typeMetaFieldDef->name && $schema->getQueryType() === $parentType) {
             return $typeMetaFieldDef;
-        } else if ($name === $typeNameMetaFieldDef->name) {
+        } else if ($fieldName === $typeNameMetaFieldDef->name) {
             return $typeNameMetaFieldDef;
         }
+
         $tmp = $parentType->getFields();
-        return isset($tmp[$name]) ? $tmp[$name] : null;
+        return isset($tmp[$fieldName]) ? $tmp[$fieldName] : null;
     }
 }
diff --git a/src/Executor/Values.php b/src/Executor/Values.php
index 3636eb7..5d0caaf 100644
--- a/src/Executor/Values.php
+++ b/src/Executor/Values.php
@@ -3,16 +3,25 @@ namespace GraphQL\Executor;
 
 
 use GraphQL\Error;
+use GraphQL\Language\AST\Argument;
+use GraphQL\Language\AST\ListType;
+use GraphQL\Language\AST\ListValue;
 use GraphQL\Language\AST\Node;
+use GraphQL\Language\AST\ObjectValue;
+use GraphQL\Language\AST\Value;
+use GraphQL\Language\AST\Variable;
 use GraphQL\Language\AST\VariableDefinition;
 use GraphQL\Language\Printer;
 use GraphQL\Schema;
 use GraphQL\Type\Definition\Directive;
 use GraphQL\Type\Definition\EnumType;
+use GraphQL\Type\Definition\FieldArgument;
 use GraphQL\Type\Definition\FieldDefinition;
 use GraphQL\Type\Definition\InputObjectType;
+use GraphQL\Type\Definition\InputType;
 use GraphQL\Type\Definition\ListOfType;
 use GraphQL\Type\Definition\NonNull;
+use GraphQL\Type\Definition\ObjectType;
 use GraphQL\Type\Definition\ScalarType;
 use GraphQL\Type\Definition\Type;
 use GraphQL\Utils;
@@ -23,8 +32,14 @@ class Values
      * Prepares an object map of variables of the correct type based on the provided
      * variable definitions and arbitrary input. If the input cannot be coerced
      * to match the variable definitions, a Error will be thrown.
+     *
+     * @param Schema $schema
+     * @param VariableDefinition[] $definitionASTs
+     * @param array $inputs
+     * @return array
+     * @throws Error
      */
-    public static function getVariableValues(Schema $schema, /* Array<VariableDefinition> */ $definitionASTs, array $inputs)
+    public static function getVariableValues(Schema $schema, $definitionASTs, array $inputs)
     {
         $values = [];
         foreach ($definitionASTs as $defAST) {
@@ -37,11 +52,16 @@ class Values
     /**
      * Prepares an object map of argument values given a list of argument
      * definitions and list of argument AST nodes.
+     *
+     * @param FieldArgument[] $argDefs
+     * @param Argument[] $argASTs
+     * @param $variableValues
+     * @return array
      */
-    public static function getArgumentValues(/* Array<GraphQLFieldArgument>*/ $argDefs, /*Array<Argument>*/ $argASTs, $variables)
+    public static function getArgumentValues($argDefs, $argASTs, $variableValues)
     {
-        if (!$argDefs || count($argDefs) === 0) {
-            return null;
+        if (!$argDefs || !$argASTs) {
+            return [];
         }
         $argASTMap = $argASTs ? Utils::keyMap($argASTs, function ($arg) {
             return $arg->name->value;
@@ -50,28 +70,71 @@ class Values
         foreach ($argDefs as $argDef) {
             $name = $argDef->name;
             $valueAST = isset($argASTMap[$name]) ? $argASTMap[$name]->value : null;
-            $result[$name] = self::coerceValueAST($argDef->getType(), $valueAST, $variables);
+            $value = self::valueFromAST($valueAST, $argDef->getType(), $variableValues);
+
+            if (null === $value) {
+                $value = $argDef->defaultValue;
+            }
+            if (null !== $value) {
+                $result[$name] = $value;
+            }
         }
         return $result;
     }
 
-    public static function getDirectiveValue(Directive $directiveDef, /* Array<Directive> */ $directives, $variables)
+    public static function valueFromAST($valueAST, InputType $type, $variables = null)
     {
-        $directiveAST = null;
-        if ($directives) {
-            foreach ($directives as $directive) {
-                if ($directive->name->value === $directiveDef->name) {
-                    $directiveAST = $directive;
-                    break;
-                }
-            }
+        if ($type instanceof NonNull) {
+            return self::valueFromAST($valueAST, $type->getWrappedType(), $variables);
         }
-        if ($directiveAST) {
-            if (!$directiveDef->type) {
+
+        if (!$valueAST) {
+            return null;
+        }
+
+        if ($valueAST instanceof Variable) {
+            $variableName = $valueAST->name->value;
+
+            if (!$variables || !isset($variables[$variableName])) {
                 return null;
             }
-            return self::coerceValueAST($directiveDef->type, $directiveAST->value, $variables);
+            return $variables[$variableName];
         }
+
+        if ($type instanceof ListOfType) {
+            $itemType = $type->getWrappedType();
+            if ($valueAST instanceof ListValue) {
+                return array_map(function($itemAST) use ($itemType, $variables) {
+                    return Values::valueFromAST($itemAST, $itemType, $variables);
+                }, $valueAST->values);
+            } else {
+                return [self::valueFromAST($valueAST, $itemType, $variables)];
+            }
+        }
+
+        if ($type instanceof InputObjectType) {
+            $fields = $type->getFields();
+            if (!$valueAST instanceof ObjectValue) {
+                return null;
+            }
+            $fieldASTs = Utils::keyMap($valueAST->fields, function($field) {return $field->name->value;});
+            $values = [];
+            foreach ($fields as $field) {
+                $fieldAST = isset($fieldASTs[$field->name]) ? $fieldASTs[$field->name] : null;
+                $fieldValue = self::valueFromAST($fieldAST ? $fieldAST->value : null, $field->getType(), $variables);
+
+                if (null === $fieldValue) {
+                    $fieldValue = $field->defaultValue;
+                }
+                if (null !== $fieldValue) {
+                    $values[$field->name] = $fieldValue;
+                }
+            }
+            return $values;
+        }
+
+        Utils::invariant($type instanceof ScalarType || $type instanceof EnumType, 'Must be input type');
+        return $type->parseLiteral($valueAST);
     }
 
     /**
@@ -81,14 +144,21 @@ class Values
     private static function getVariableValue(Schema $schema, VariableDefinition $definitionAST, $input)
     {
         $type = Utils\TypeInfo::typeFromAST($schema, $definitionAST->type);
-        if (!$type) {
-            return null;
+        $variable = $definitionAST->variable;
+
+        if (!$type || !Type::isInputType($type)) {
+            $printed = Printer::doPrint($definitionAST->type);
+            throw new Error(
+                "Variable \"\${$variable->name->value}\" expected value of type " .
+                "\"$printed\" which cannot be used as an input type.",
+                [ $definitionAST ]
+            );
         }
-        if (self::isValidValue($type, $input)) {
+        if (self::isValidValue($input, $type)) {
             if (null === $input) {
                 $defaultValue = $definitionAST->defaultValue;
                 if ($defaultValue) {
-                    return self::coerceValueAST($type, $defaultValue);
+                    return self::valueFromAST($defaultValue, $type);
                 }
             }
             return self::coerceValue($type, $input);
@@ -103,15 +173,21 @@ class Values
 
 
     /**
-     * Given a type and any value, return true if that value is valid.
+     * Given a PHP value and a GraphQL type, determine if the value will be
+     * accepted for that type. This is primarily useful for validating the
+     * runtime values of query variables.
+     *
+     * @param $value
+     * @param Type $type
+     * @return bool
      */
-    private static function isValidValue(Type $type, $value)
+    private static function isValidValue($value, Type $type)
     {
         if ($type instanceof NonNull) {
             if (null === $value) {
                 return false;
             }
-            return self::isValidValue($type->getWrappedType(), $value);
+            return self::isValidValue($value, $type->getWrappedType());
         }
 
         if ($value === null) {
@@ -122,34 +198,44 @@ class Values
             $itemType = $type->getWrappedType();
             if (is_array($value)) {
                 foreach ($value as $item) {
-                    if (!self::isValidValue($itemType, $item)) {
+                    if (!self::isValidValue($item, $itemType)) {
                         return false;
                     }
                 }
                 return true;
             } else {
-                return self::isValidValue($itemType, $value);
+                return self::isValidValue($value, $itemType);
             }
         }
 
         if ($type instanceof InputObjectType) {
+            if (!is_array($value)) {
+                return false;
+            }
             $fields = $type->getFields();
+            $fieldMap = [];
+
+            // Ensure every defined field is valid.
             foreach ($fields as $fieldName => $field) {
                 /** @var FieldDefinition $field */
-                if (!self::isValidValue($field->getType(), isset($value[$fieldName]) ? $value[$fieldName] : null)) {
+                if (!self::isValidValue(isset($value[$fieldName]) ? $value[$fieldName] : null, $field->getType())) {
                     return false;
                 }
+                $fieldMap[$field->name] = $field;
             }
+
+            // Ensure every provided field is defined.
+            $diff = array_diff_key($value, $fieldMap);
+
+            if (!empty($diff)) {
+                return false;
+            }
+
             return true;
         }
 
-        if ($type instanceof ScalarType ||
-            $type instanceof EnumType
-        ) {
-            return null !== $type->coerce($value);
-        }
-
-        return false;
+        Utils::invariant($type instanceof ScalarType || $type instanceof EnumType, 'Must be input type');
+        return null !== $type->parseValue($value);
     }
 
     /**
@@ -183,93 +269,18 @@ class Values
             $fields = $type->getFields();
             $obj = [];
             foreach ($fields as $fieldName => $field) {
-                $fieldValue = self::coerceValue($field->getType(), $value[$fieldName]);
-                $obj[$fieldName] = $fieldValue === null ? $field->defaultValue : $fieldValue;
-            }
-            return $obj;
-
-        }
-
-        if ($type instanceof ScalarType ||
-            $type instanceof EnumType
-        ) {
-            $coerced = $type->coerce($value);
-            if (null !== $coerced) {
-                return $coerced;
-            }
-        }
-
-        return null;
-    }
-
-    /**
-     * Given a type and a value AST node known to match this type, build a
-     * runtime value.
-     */
-    private static function coerceValueAST(Type $type, $valueAST, $variables)
-    {
-        if ($type instanceof NonNull) {
-            // Note: we're not checking that the result of coerceValueAST is non-null.
-            // We're assuming that this query has been validated and the value used
-            // here is of the correct type.
-            return self::coerceValueAST($type->getWrappedType(), $valueAST, $variables);
-        }
-
-        if (!$valueAST) {
-            return null;
-        }
-
-        if ($valueAST->kind === Node::VARIABLE) {
-            $variableName = $valueAST->name->value;
-
-            if (!isset($variables, $variables[$variableName])) {
-                return null;
-            }
-
-            // Note: we're not doing any checking that this variable is correct. We're
-            // assuming that this query has been validated and the variable usage here
-            // is of the correct type.
-            return $variables[$variableName];
-        }
-
-        if ($type instanceof ListOfType) {
-            $itemType = $type->getWrappedType();
-            if ($valueAST->kind === Node::ARR) {
-                $tmp = [];
-                foreach ($valueAST->values as $itemAST) {
-                    $tmp[] = self::coerceValueAST($itemType, $itemAST, $variables);
+                $fieldValue = self::coerceValue($field->getType(), isset($value[$fieldName]) ? $value[$fieldName] : null);
+                if (null === $fieldValue) {
+                    $fieldValue = $field->defaultValue;
+                }
+                if (null !== $fieldValue) {
+                    $obj[$fieldName] = $fieldValue;
                 }
-                return $tmp;
-            } else {
-                return [self::coerceValueAST($itemType, $valueAST, $variables)];
-            }
-        }
-
-        if ($type instanceof InputObjectType) {
-            $fields = $type->getFields();
-            if ($valueAST->kind !== Node::OBJECT) {
-                return null;
-            }
-            $fieldASTs = Utils::keyMap($valueAST->fields, function ($field) {
-                return $field->name->value;
-            });
-
-            $obj = [];
-            foreach ($fields as $fieldName => $field) {
-                $fieldAST = $fieldASTs[$fieldName];
-                $fieldValue = self::coerceValueAST($field->getType(), $fieldAST ? $fieldAST->value : null, $variables);
-                $obj[$fieldName] = $fieldValue === null ? $field->defaultValue : $fieldValue;
             }
             return $obj;
-        }
 
-        if ($type instanceof ScalarType || $type instanceof EnumType) {
-            $coerced = $type->coerceLiteral($valueAST);
-            if (null !== $coerced) {
-                return $coerced;
-            }
         }
-
-        return null;
+        Utils::invariant($type instanceof ScalarType || $type instanceof EnumType, 'Must be input type');
+        return $type->parseValue($value);
     }
 }
diff --git a/src/GraphQL.php b/src/GraphQL.php
index f2c7cab..42811c3 100644
--- a/src/GraphQL.php
+++ b/src/GraphQL.php
@@ -11,25 +11,25 @@ class GraphQL
     /**
      * @param Schema $schema
      * @param $requestString
-     * @param mixed $rootObject
+     * @param mixed $rootValue
      * @param array <string, string>|null $variableValues
      * @param string|null $operationName
      * @return array
      */
-    public static function execute(Schema $schema, $requestString, $rootObject = null, $variableValues = null, $operationName = null)
+    public static function execute(Schema $schema, $requestString, $rootValue = null, $variableValues = null, $operationName = null)
     {
         try {
             $source = new Source($requestString ?: '', 'GraphQL request');
-            $ast = Parser::parse($source);
-            $validationResult = DocumentValidator::validate($schema, $ast);
+            $documentAST = Parser::parse($source);
+            $validationErrors = DocumentValidator::validate($schema, $documentAST);
 
-            if (empty($validationResult['isValid'])) {
-                return ['errors' => $validationResult['errors']];
+            if (!empty($validationErrors)) {
+                return ['errors' => array_map(['GraphQL\Error', 'formatError'], $validationErrors)];
             } else {
-                return Executor::execute($schema, $rootObject, $ast, $operationName, $variableValues);
+                return Executor::execute($schema, $documentAST, $rootValue, $variableValues, $operationName)->toArray();
             }
-        } catch (\Exception $e) {
-            return ['errors' => Error::formatError($e)];
+        } catch (Error $e) {
+            return ['errors' => [Error::formatError($e)]];
         }
     }
 }
diff --git a/src/Language/AST/Argument.php b/src/Language/AST/Argument.php
index 6d1bc9d..18579f0 100644
--- a/src/Language/AST/Argument.php
+++ b/src/Language/AST/Argument.php
@@ -1,7 +1,7 @@
 <?php
 namespace GraphQL\Language\AST;
 
-class Argument extends NamedType
+class Argument extends Node
 {
     public $kind = Node::ARGUMENT;
 
@@ -9,4 +9,9 @@ class Argument extends NamedType
      * @var Value
      */
     public $value;
+
+    /**
+     * @var Name
+     */
+    public $name;
 }
diff --git a/src/Language/AST/Field.php b/src/Language/AST/Field.php
index 4348246..6535b0b 100644
--- a/src/Language/AST/Field.php
+++ b/src/Language/AST/Field.php
@@ -1,10 +1,15 @@
 <?php
 namespace GraphQL\Language\AST;
 
-class Field extends NamedType
+class Field extends Node
 {
     public $kind = Node::FIELD;
 
+    /**
+     * @var Name
+     */
+    public $name;
+
     /**
      * @var Name|null
      */
diff --git a/src/Language/AST/FragmentDefinition.php b/src/Language/AST/FragmentDefinition.php
index a8053cc..8715584 100644
--- a/src/Language/AST/FragmentDefinition.php
+++ b/src/Language/AST/FragmentDefinition.php
@@ -2,10 +2,15 @@
 namespace GraphQL\Language\AST;
 
 
-class FragmentDefinition extends NamedType implements Definition
+class FragmentDefinition extends Node implements Definition
 {
     public $kind = Node::FRAGMENT_DEFINITION;
 
+    /**
+     * @var Name
+     */
+    public $name;
+
     /**
      * @var NamedType
      */
diff --git a/src/Language/AST/FragmentSpread.php b/src/Language/AST/FragmentSpread.php
index 0c32dfc..a989bc2 100644
--- a/src/Language/AST/FragmentSpread.php
+++ b/src/Language/AST/FragmentSpread.php
@@ -1,10 +1,15 @@
 <?php
 namespace GraphQL\Language\AST;
 
-class FragmentSpread extends NamedType
+class FragmentSpread extends Node
 {
     public $kind = Node::FRAGMENT_SPREAD;
 
+    /**
+     * @var Name
+     */
+    public $name;
+
     /**
      * @var array<Directive>
      */
diff --git a/src/Language/AST/NamedType.php b/src/Language/AST/NamedType.php
index ce6a6aa..b524364 100644
--- a/src/Language/AST/NamedType.php
+++ b/src/Language/AST/NamedType.php
@@ -1,7 +1,7 @@
 <?php
 namespace GraphQL\Language\AST;
 
-class NamedType extends Node
+class NamedType extends Node implements Type
 {
     public $kind = Node::NAMED_TYPE;
 
diff --git a/src/Language/AST/ObjectField.php b/src/Language/AST/ObjectField.php
index 6833a51..10d0ac1 100644
--- a/src/Language/AST/ObjectField.php
+++ b/src/Language/AST/ObjectField.php
@@ -2,10 +2,15 @@
 namespace GraphQL\Language\AST;
 
 
-class ObjectField extends NamedType
+class ObjectField extends Node
 {
     public $kind = Node::OBJECT_FIELD;
 
+    /**
+     * @var Name
+     */
+    public $name;
+
     /**
      * @var Value
      */
diff --git a/src/Language/AST/OperationDefinition.php b/src/Language/AST/OperationDefinition.php
index c649a54..681f07d 100644
--- a/src/Language/AST/OperationDefinition.php
+++ b/src/Language/AST/OperationDefinition.php
@@ -1,13 +1,18 @@
 <?php
 namespace GraphQL\Language\AST;
 
-class OperationDefinition extends NamedType implements Definition
+class OperationDefinition extends Node implements Definition
 {
     /**
      * @var string
      */
     public $kind = Node::OPERATION_DEFINITION;
 
+    /**
+     * @var Name
+     */
+    public $name;
+
     /**
      * @var string (oneOf 'query', 'mutation'))
      */
diff --git a/src/Language/AST/Variable.php b/src/Language/AST/Variable.php
index 1e2f89f..b7a7dfe 100644
--- a/src/Language/AST/Variable.php
+++ b/src/Language/AST/Variable.php
@@ -1,7 +1,12 @@
 <?php
 namespace GraphQL\Language\AST;
 
-class Variable extends NamedType
+class Variable extends Node
 {
     public $kind = Node::VARIABLE;
+
+    /**
+     * @var Name
+     */
+    public $name;
 }
diff --git a/src/Schema.php b/src/Schema.php
index f2fb7a1..f177689 100644
--- a/src/Schema.php
+++ b/src/Schema.php
@@ -2,7 +2,12 @@
 namespace GraphQL;
 
 use GraphQL\Type\Definition\Directive;
+use GraphQL\Type\Definition\FieldArgument;
+use GraphQL\Type\Definition\FieldDefinition;
+use GraphQL\Type\Definition\InputObjectType;
 use GraphQL\Type\Definition\InterfaceType;
+use GraphQL\Type\Definition\ListOfType;
+use GraphQL\Type\Definition\NonNull;
 use GraphQL\Type\Definition\ObjectType;
 use GraphQL\Type\Definition\Type;
 use GraphQL\Type\Definition\UnionType;
@@ -24,6 +29,99 @@ class Schema
         Utils::invariant($querySchema || $mutationSchema, "Either query or mutation type must be set");
         $this->querySchema = $querySchema;
         $this->mutationSchema = $mutationSchema;
+
+        // Build type map now to detect any errors within this schema.
+        $map = [];
+        foreach ([$this->getQueryType(), $this->getMutationType(), Introspection::_schema()] as $type) {
+            $this->_extractTypes($type, $map);
+        }
+        $this->_typeMap = $map + Type::getInternalTypes();
+
+        // Enforce correct interface implementations
+        foreach ($this->_typeMap as $typeName => $type) {
+            if ($type instanceof ObjectType) {
+                foreach ($type->getInterfaces() as $iface) {
+                    $this->assertObjectImplementsInterface($type, $iface);
+                }
+            }
+        }
+    }
+
+    /**
+     * @param ObjectType $object
+     * @param InterfaceType $iface
+     * @throws \Exception
+     */
+    private function assertObjectImplementsInterface(ObjectType $object, InterfaceType $iface)
+    {
+        $objectFieldMap = $object->getFields();
+        $ifaceFieldMap = $iface->getFields();
+
+        foreach ($ifaceFieldMap as $fieldName => $ifaceField) {
+            Utils::invariant(
+                isset($objectFieldMap[$fieldName]),
+                "\"$iface\" expects field \"$fieldName\" but \"$object\" does not provide it"
+            );
+
+            /** @var $ifaceField FieldDefinition */
+            /** @var $objectField FieldDefinition */
+            $objectField = $objectFieldMap[$fieldName];
+
+            Utils::invariant(
+                $this->isEqualType($ifaceField->getType(), $objectField->getType()),
+                "$iface.$fieldName expects type \"{$ifaceField->getType()}\" but " .
+                "$object.$fieldName provides type \"{$objectField->getType()}"
+            );
+
+            foreach ($ifaceField->args as $ifaceArg) {
+                /** @var $ifaceArg FieldArgument */
+                /** @var $objectArg FieldArgument */
+                $argName = $ifaceArg->name;
+                $objectArg = $objectField->getArg($argName);
+
+                // Assert interface field arg exists on object field.
+                Utils::invariant(
+                    $objectArg,
+                    "$iface.$fieldName expects argument \"$argName\" but $object.$fieldName does not provide it."
+                );
+
+                // Assert interface field arg type matches object field arg type.
+                // (invariant)
+                Utils::invariant(
+                    $this->isEqualType($ifaceArg->getType(), $objectArg->getType()),
+                    "$iface.$fieldName($argName:) expects type \"{$ifaceArg->getType()}\" " .
+                    "but $object.$fieldName($argName:) provides " .
+                    "type \"{$objectArg->getType()}\""
+                );
+
+                // Assert argument set invariance.
+                foreach ($objectField->args as $objectArg) {
+                    $argName = $objectArg->name;
+                    $ifaceArg = $ifaceField->getArg($argName);
+                    Utils::invariant(
+                        $ifaceArg,
+                        "$iface.$fieldName does not define argument \"$argName\" but " .
+                        "$object.$fieldName provides it."
+                    );
+                }
+            }
+        }
+    }
+
+    /**
+     * @param $typeA
+     * @param $typeB
+     * @return bool
+     */
+    private function isEqualType($typeA, $typeB)
+    {
+        if ($typeA instanceof NonNull && $typeB instanceof NonNull) {
+            return $this->isEqualType($typeA->getWrappedType(), $typeB->getWrappedType());
+        }
+        if ($typeA instanceof ListOfType && $typeB instanceof ListOfType) {
+            return $this->isEqualType($typeA->getWrappedType(), $typeB->getWrappedType());
+        }
+        return $typeA === $typeB;
     }
 
     public function getQueryType()
@@ -57,8 +155,8 @@ class Schema
     {
         if (!$this->_directives) {
             $this->_directives = [
-                Directive::ifDirective(),
-                Directive::unlessDirective()
+                Directive::includeDirective(),
+                Directive::skipDirective()
             ];
         }
         return $this->_directives;
@@ -66,27 +164,26 @@ class Schema
 
     public function getTypeMap()
     {
-        if (null === $this->_typeMap) {
-            $map = [];
-            foreach ([$this->getQueryType(), $this->getMutationType(), Introspection::_schema()] as $type) {
-                $this->_extractTypes($type, $map);
-            }
-            $this->_typeMap = $map + Type::getInternalTypes();
-        }
         return $this->_typeMap;
     }
 
     private function _extractTypes($type, &$map)
     {
+        if (!$type) {
+            return $map;
+        }
+
         if ($type instanceof WrappingType) {
             return $this->_extractTypes($type->getWrappedType(), $map);
         }
 
-        if (!$type instanceof Type || !empty($map[$type->name])) {
-            // TODO: warning?
+        if (!empty($map[$type->name])) {
+            Utils::invariant(
+                $map[$type->name] === $type,
+                "Schema must contain unique named types but contains multiple types named \"$type\"."
+            );
             return $map;
         }
-
         $map[$type->name] = $type;
 
         $nestedTypes = [];
@@ -97,13 +194,12 @@ class Schema
         if ($type instanceof ObjectType) {
             $nestedTypes = array_merge($nestedTypes, $type->getInterfaces());
         }
-        if ($type instanceof ObjectType || $type instanceof InterfaceType) {
+        if ($type instanceof ObjectType || $type instanceof InterfaceType || $type instanceof InputObjectType) {
             foreach ((array) $type->getFields() as $fieldName => $field) {
-                if (null === $field->args) {
-                    trigger_error('WTF  ' . $field->name . ' has no args?'); // gg
+                if (isset($field->args)) {
+                    $fieldArgTypes = array_map(function($arg) { return $arg->getType(); }, $field->args);
+                    $nestedTypes = array_merge($nestedTypes, $fieldArgTypes);
                 }
-                $fieldArgTypes = array_map(function($arg) { return $arg->getType(); }, $field->args);
-                $nestedTypes = array_merge($nestedTypes, $fieldArgTypes);
                 $nestedTypes[] = $field->getType();
             }
         }
diff --git a/src/Type/Definition/AbstractType.php b/src/Type/Definition/AbstractType.php
index af5a55b..453af24 100644
--- a/src/Type/Definition/AbstractType.php
+++ b/src/Type/Definition/AbstractType.php
@@ -13,4 +13,15 @@ GraphQLUnionType;
      * @return array<ObjectType>
      */
     public function getPossibleTypes();
+
+    /**
+     * @return ObjectType
+     */
+    public function getObjectType($value, ResolveInfo $info);
+
+    /**
+     * @param Type $type
+     * @return bool
+     */
+    public function isPossibleType(Type $type);
 }
diff --git a/src/Type/Definition/BooleanType.php b/src/Type/Definition/BooleanType.php
index d3b2676..3446094 100644
--- a/src/Type/Definition/BooleanType.php
+++ b/src/Type/Definition/BooleanType.php
@@ -7,12 +7,17 @@ class BooleanType extends ScalarType
 {
     public $name = Type::BOOLEAN;
 
-    public function coerce($value)
+    public function serialize($value)
     {
         return !!$value;
     }
 
-    public function coerceLiteral($ast)
+    public function parseValue($value)
+    {
+        return !!$value;
+    }
+
+    public function parseLiteral($ast)
     {
         if ($ast instanceof BooleanValue) {
             return (bool) $ast->value;
diff --git a/src/Type/Definition/Directive.php b/src/Type/Definition/Directive.php
index 9545b9b..abb9b41 100644
--- a/src/Type/Definition/Directive.php
+++ b/src/Type/Definition/Directive.php
@@ -8,39 +8,51 @@ class Directive
     /**
      * @return Directive
      */
-    public static function ifDirective()
+    public static function includeDirective()
     {
         $internal = self::getInternalDirectives();
-        return $internal['if'];
+        return $internal['include'];
     }
 
     /**
      * @return Directive
      */
-    public static function unlessDirective()
+    public static function skipDirective()
     {
         $internal = self::getInternalDirectives();
-        return $internal['unless'];
+        return $internal['skip'];
     }
 
     public static function getInternalDirectives()
     {
         if (!self::$internalDirectives) {
             self::$internalDirectives = [
-                'if' => new self([
-                    'name' => 'if',
-                    'description' => 'Directs the executor to omit this field if the argument provided is false.',
-                    'type' => Type::nonNull(Type::boolean()),
+                'include' => new self([
+                    'name' => 'include',
+                    'description' => 'Directs the executor to include this field or fragment only when the `if` argument is true.',
+                    'args' => [
+                        new FieldArgument([
+                            'name' => 'if',
+                            'type' => Type::nonNull(Type::boolean()),
+                            'description' => 'Included when true.'
+                        ])
+                    ],
                     'onOperation' => false,
-                    'onFragment' => false,
+                    'onFragment' => true,
                     'onField' => true
                 ]),
-                'unless' => new self([
-                    'name' => 'unless',
-                    'description' => 'Directs the executor to omit this field if the argument provided is true.',
-                    'type' => Type::nonNull(Type::boolean()),
+                'skip' => new self([
+                    'name' => 'skip',
+                    'description' => 'Directs the executor to skip this field or fragment when the `if` argument is true.',
+                    'args' => [
+                        new FieldArgument([
+                            'name' => 'if',
+                            'type' => Type::nonNull(Type::boolean()),
+                            'description' => 'Skipped when true'
+                        ])
+                    ],
                     'onOperation' => false,
-                    'onFragment' => false,
+                    'onFragment' => true,
                     'onField' => true
                 ])
             ];
@@ -59,9 +71,9 @@ class Directive
     public $description;
 
     /**
-     * @var Type
+     * @var FieldArgument[]
      */
-    public $type;
+    public $args;
 
     /**
      * @var boolean
diff --git a/src/Type/Definition/EnumType.php b/src/Type/Definition/EnumType.php
index 0d7a8c8..0f58c28 100644
--- a/src/Type/Definition/EnumType.php
+++ b/src/Type/Definition/EnumType.php
@@ -53,13 +53,13 @@ class EnumType extends Type implements InputType, OutputType
         return $this->_values;
     }
 
-    public function coerce($value)
+    public function serialize($value)
     {
         $enumValue = $this->_getValueLookup()->offsetGet($value);
         return $enumValue ? $enumValue->name : null;
     }
 
-    public function coerceLiteral($value)
+    public function parseLiteral($value)
     {
         if ($value instanceof EnumValue) {
             $lookup = $this->_getNameLookup();
diff --git a/src/Type/Definition/FieldArgument.php b/src/Type/Definition/FieldArgument.php
index 1acb3ac..c3e0e6a 100644
--- a/src/Type/Definition/FieldArgument.php
+++ b/src/Type/Definition/FieldArgument.php
@@ -4,6 +4,12 @@ namespace GraphQL\Type\Definition;
 
 use GraphQL\Utils;
 
+/**
+ * Class FieldArgument
+ *
+ * @package GraphQL\Type\Definition
+ * @todo Rename to Argument as it is also applicable to directives, not only fields
+ */
 class FieldArgument
 {
     /**
diff --git a/src/Type/Definition/FieldDefinition.php b/src/Type/Definition/FieldDefinition.php
index dd76eb6..b8d8cf3 100644
--- a/src/Type/Definition/FieldDefinition.php
+++ b/src/Type/Definition/FieldDefinition.php
@@ -1,6 +1,8 @@
 <?php
 namespace GraphQL\Type\Definition;
 
+use GraphQL\Utils;
+
 class FieldDefinition
 {
     /**
@@ -98,12 +100,28 @@ class FieldDefinition
         $this->deprecationReason = isset($config['deprecationReason']) ? $config['deprecationReason'] : null;
     }
 
+    /**
+     * @param $name
+     * @return FieldArgument|null
+     */
+    public function getArg($name)
+    {
+        foreach ($this->args ?: [] as $arg) {
+            /** @var FieldArgument $arg */
+            if ($arg->name === $name) {
+                return $arg;
+            }
+        }
+        return null;
+    }
+
     /**
      * @return Type
      */
     public function getType()
     {
         if (null === $this->resolvedType) {
+            // TODO: deprecate types as callbacks - instead just allow field definitions to be callbacks
             $this->resolvedType = Type::resolve($this->type);
         }
         return $this->resolvedType;
diff --git a/src/Type/Definition/FloatType.php b/src/Type/Definition/FloatType.php
index 30b204a..da58b47 100644
--- a/src/Type/Definition/FloatType.php
+++ b/src/Type/Definition/FloatType.php
@@ -8,12 +8,22 @@ class FloatType extends ScalarType
 {
     public $name = Type::FLOAT;
 
-    public function coerce($value)
+    public function serialize($value)
+    {
+        return $this->coerceFloat($value);
+    }
+
+    public function parseValue($value)
+    {
+        return $this->coerceFloat($value);
+    }
+
+    private function coerceFloat($value)
     {
         return is_numeric($value) || $value === true || $value === false ? (float) $value : null;
     }
 
-    public function coerceLiteral($ast)
+    public function parseLiteral($ast)
     {
         if ($ast instanceof FloatValue || $ast instanceof IntValue) {
             return (float) $ast->value;
diff --git a/src/Type/Definition/IDType.php b/src/Type/Definition/IDType.php
index d8947fb..e6f1deb 100644
--- a/src/Type/Definition/IDType.php
+++ b/src/Type/Definition/IDType.php
@@ -8,12 +8,17 @@ class IDType extends ScalarType
 {
     public $name = 'ID';
 
-    public function coerce($value)
+    public function serialize($value)
     {
         return (string) $value;
     }
 
-    public function coerceLiteral($ast)
+    public function parseValue($value)
+    {
+        return (string) $value;
+    }
+
+    public function parseLiteral($ast)
     {
         if ($ast instanceof StringValue || $ast instanceof IntValue) {
             return $ast->value;
diff --git a/src/Type/Definition/InputObjectType.php b/src/Type/Definition/InputObjectType.php
index e85d766..8230c72 100644
--- a/src/Type/Definition/InputObjectType.php
+++ b/src/Type/Definition/InputObjectType.php
@@ -32,7 +32,7 @@ class InputObjectType extends Type implements InputType
     }
 
     /**
-     * @return array<InputObjectField>
+     * @return InputObjectField[]
      */
     public function getFields()
     {
diff --git a/src/Type/Definition/IntType.php b/src/Type/Definition/IntType.php
index 0c7a160..8370e53 100644
--- a/src/Type/Definition/IntType.php
+++ b/src/Type/Definition/IntType.php
@@ -2,12 +2,23 @@
 namespace GraphQL\Type\Definition;
 
 use GraphQL\Language\AST\IntValue;
+use GraphQL\Language\AST\Value;
 
 class IntType extends ScalarType
 {
     public $name = Type::INT;
 
-    public function coerce($value)
+    public function serialize($value)
+    {
+        return $this->coerceInt($value);
+    }
+
+    public function parseValue($value)
+    {
+        return $this->coerceInt($value);
+    }
+
+    private function coerceInt($value)
     {
         if (false === $value || true === $value) {
             return (int) $value;
@@ -18,7 +29,7 @@ class IntType extends ScalarType
         return null;
     }
 
-    public function coerceLiteral($ast)
+    public function parseLiteral($ast)
     {
         if ($ast instanceof IntValue) {
             $val = (int) $ast->value;
diff --git a/src/Type/Definition/InterfaceType.php b/src/Type/Definition/InterfaceType.php
index 5f15396..f1d1650 100644
--- a/src/Type/Definition/InterfaceType.php
+++ b/src/Type/Definition/InterfaceType.php
@@ -35,11 +35,11 @@ class InterfaceType extends Type implements AbstractType, OutputType, CompositeT
      * implementation for Interface types.
      *
      * @param ObjectType $impl
-     * @param array<InterfaceType> $interfaces
+     * @param InterfaceType[] $interfaces
      */
-    public static function addImplementationToInterfaces(ObjectType $impl, array $interfaces)
+    public static function addImplementationToInterfaces(ObjectType $impl)
     {
-        foreach ($interfaces as $interface) {
+        foreach ($impl->getInterfaces() as $interface) {
             $interface->_implementations[] = $impl;
         }
     }
@@ -84,10 +84,10 @@ class InterfaceType extends Type implements AbstractType, OutputType, CompositeT
         return $this->_implementations;
     }
 
-    public function isPossibleType(ObjectType $type)
+    public function isPossibleType(Type $type)
     {
         $possibleTypeNames = $this->_possibleTypeNames;
-        if (!$possibleTypeNames) {
+        if (null === $possibleTypeNames) {
             $this->_possibleTypeNames = $possibleTypeNames = array_reduce($this->getPossibleTypes(), function(&$map, Type $possibleType) {
                 $map[$possibleType->name] = true;
                 return $map;
@@ -98,11 +98,13 @@ class InterfaceType extends Type implements AbstractType, OutputType, CompositeT
 
     /**
      * @param $value
-     * @return ObjectType|null
+     * @param ResolveInfo $info
+     * @return Type|null
+     * @throws \Exception
      */
-    public function resolveType($value)
+    public function getObjectType($value, ResolveInfo $info)
     {
         $resolver = $this->_resolveType;
-        return $resolver ? call_user_func($resolver, $value) : Type::getTypeOf($value, $this);
+        return $resolver ? call_user_func($resolver, $value, $info) : Type::getTypeOf($value, $info, $this);
     }
 }
diff --git a/src/Type/Definition/ObjectType.php b/src/Type/Definition/ObjectType.php
index acf8607..0378e59 100644
--- a/src/Type/Definition/ObjectType.php
+++ b/src/Type/Definition/ObjectType.php
@@ -45,7 +45,7 @@ class ObjectType extends Type implements OutputType, CompositeType
     /**
      * @var array<Field>
      */
-    private $_fields = [];
+    private $_fields;
 
     /**
      * @var array<InterfaceType>
@@ -57,9 +57,46 @@ class ObjectType extends Type implements OutputType, CompositeType
      */
     private $_isTypeOf;
 
+    /**
+     * Keeping reference of config for late bindings
+     *
+     * @var array
+     */
+    private $_config;
+
+    private $_initialized = false;
+
     public function __construct(array $config)
     {
-        Config::validate($config, [
+        $this->name = $config['name'];
+        $this->description = isset($config['description']) ? $config['description'] : null;
+        $this->_config = $config;
+
+        if (isset($config['interfaces'])) {
+            InterfaceType::addImplementationToInterfaces($this);
+        }
+    }
+
+    /**
+     * Late instance initialization
+     */
+    private function initialize()
+    {
+        if ($this->_initialized) {
+            return ;
+        }
+        $config = $this->_config;
+
+        if (isset($config['fields']) && is_callable($config['fields'])) {
+            $config['fields'] = call_user_func($config['fields']);
+        }
+        if (isset($config['interfaces']) && is_callable($config['interfaces'])) {
+            $config['interfaces'] = call_user_func($config['interfaces']);
+        }
+
+        // Note: this validation is disabled by default, because it is resource-consuming
+        // TODO: add bin/validate script to check if schema is valid during development
+        Config::validate($this->_config, [
             'name' => Config::STRING | Config::REQUIRED,
             'fields' => Config::arrayOf(
                 FieldDefinition::getDefinition(),
@@ -69,22 +106,13 @@ class ObjectType extends Type implements OutputType, CompositeType
             'interfaces' => Config::arrayOf(
                 Config::INTERFACE_TYPE
             ),
-            'isTypeOf' => Config::CALLBACK,
+            'isTypeOf' => Config::CALLBACK, // ($value, ResolveInfo $info) => boolean
         ]);
 
-        $this->name = $config['name'];
-        $this->description = isset($config['description']) ? $config['description'] : null;
-
-        if (isset($config['fields'])) {
-            $this->_fields = FieldDefinition::createMap($config['fields']);
-        }
-
+        $this->_fields = FieldDefinition::createMap($config['fields']);
         $this->_interfaces = isset($config['interfaces']) ? $config['interfaces'] : [];
         $this->_isTypeOf = isset($config['isTypeOf']) ? $config['isTypeOf'] : null;
-
-        if (!empty($this->_interfaces)) {
-            InterfaceType::addImplementationToInterfaces($this, $this->_interfaces);
-        }
+        $this->_initialized = true;
     }
 
     /**
@@ -92,6 +120,9 @@ class ObjectType extends Type implements OutputType, CompositeType
      */
     public function getFields()
     {
+        if (false === $this->_initialized) {
+            $this->initialize();
+        }
         return $this->_fields;
     }
 
@@ -102,6 +133,9 @@ class ObjectType extends Type implements OutputType, CompositeType
      */
     public function getField($name)
     {
+        if (false === $this->_initialized) {
+            $this->initialize();
+        }
         Utils::invariant(isset($this->_fields[$name]), "Field '%s' is not defined for type '%s'", $name, $this->name);
         return $this->_fields[$name];
     }
@@ -111,6 +145,9 @@ class ObjectType extends Type implements OutputType, CompositeType
      */
     public function getInterfaces()
     {
+        if (false === $this->_initialized) {
+            $this->initialize();
+        }
         return $this->_interfaces;
     }
 
@@ -118,8 +155,8 @@ class ObjectType extends Type implements OutputType, CompositeType
      * @param $value
      * @return bool|null
      */
-    public function isTypeOf($value)
+    public function isTypeOf($value, ResolveInfo $info)
     {
-        return isset($this->_isTypeOf) ? call_user_func($this->_isTypeOf, $value) : null;
+        return isset($this->_isTypeOf) ? call_user_func($this->_isTypeOf, $value, $info) : null;
     }
 }
diff --git a/src/Type/Definition/ResolveInfo.php b/src/Type/Definition/ResolveInfo.php
new file mode 100644
index 0000000..8313d7b
--- /dev/null
+++ b/src/Type/Definition/ResolveInfo.php
@@ -0,0 +1,61 @@
+<?php
+namespace GraphQL\Type\Definition;
+
+use GraphQL\Language\AST\Field;
+use GraphQL\Language\AST\FragmentDefinition;
+use GraphQL\Language\AST\OperationDefinition;
+use GraphQL\Schema;
+use GraphQL\Utils;
+
+class ResolveInfo
+{
+    /**
+     * @var string
+     */
+    public $fieldName;
+
+    /**
+     * @var Field[]
+     */
+    public $fieldASTs;
+
+    /**
+     * @var OutputType
+     */
+    public $returnType;
+
+    /**
+     * @var Type|CompositeType
+     */
+    public $parentType;
+
+    /**
+     * @var Schema
+     */
+    public $schema;
+
+    /**
+     * @var array<fragmentName, FragmentDefinition>
+     */
+    public $fragments;
+
+    /**
+     * @var mixed
+     */
+    public $rootValue;
+
+    /**
+     * @var OperationDefinition
+     */
+    public $operation;
+
+    /**
+     * @var array<variableName, mixed>
+     */
+    public $variableValues;
+
+    public function __construct(array $values)
+    {
+        Utils::assign($this, $values);
+    }
+}
diff --git a/src/Type/Definition/ScalarType.php b/src/Type/Definition/ScalarType.php
index 04ad3da..cc840ca 100644
--- a/src/Type/Definition/ScalarType.php
+++ b/src/Type/Definition/ScalarType.php
@@ -14,7 +14,7 @@ use GraphQL\Utils;
  *
  *     var OddType = new GraphQLScalarType({
  *       name: 'Odd',
- *       coerce(value) {
+ *       serialize(value) {
  *         return value % 2 === 1 ? value : null;
  *       }
  *     });
@@ -27,7 +27,9 @@ abstract class ScalarType extends Type implements OutputType, InputType
         Utils::invariant($this->name, 'Type must be named.');
     }
 
-    abstract public function coerce($value);
+    abstract public function serialize($value);
 
-    abstract public function coerceLiteral($ast);
+    abstract public function parseValue($value);
+
+    abstract public function parseLiteral(/* GraphQL\Language\AST\Value */$valueAST);
 }
diff --git a/src/Type/Definition/ScalarTypeConfig.php b/src/Type/Definition/ScalarTypeConfig.php
deleted file mode 100644
index eb5255d..0000000
--- a/src/Type/Definition/ScalarTypeConfig.php
+++ /dev/null
@@ -1,26 +0,0 @@
-<?php
-namespace GraphQL\Type\Definition;
-
-
-class ScalarTypeConfig
-{
-    /**
-     * @var string
-     */
-    public $name;
-
-    /**
-     * @var string|null
-     */
-    public $description;
-
-    /**
-     * @var \Closure
-     */
-    public $coerce;
-
-    /**
-     * @var \Closure
-     */
-    public $coerceLiteral;
-}
diff --git a/src/Type/Definition/StringType.php b/src/Type/Definition/StringType.php
index 5d2596e..cccea49 100644
--- a/src/Type/Definition/StringType.php
+++ b/src/Type/Definition/StringType.php
@@ -7,7 +7,12 @@ class StringType extends ScalarType
 {
     public $name = Type::STRING;
 
-    public function coerce($value)
+    public function serialize($value)
+    {
+        return $this->parseValue($value);
+    }
+
+    public function parseValue($value)
     {
         if ($value === true) {
             return 'true';
@@ -18,7 +23,7 @@ class StringType extends ScalarType
         return (string) $value;
     }
 
-    public function coerceLiteral($ast)
+    public function parseLiteral($ast)
     {
         if ($ast instanceof StringValue) {
             return $ast->value;
diff --git a/src/Type/Definition/Type.php b/src/Type/Definition/Type.php
index 2bf188d..22e90d9 100644
--- a/src/Type/Definition/Type.php
+++ b/src/Type/Definition/Type.php
@@ -111,7 +111,7 @@ GraphQLNonNull;
      */
     public static function isInputType($type)
     {
-        $nakedType = self::getUnmodifiedType($type);
+        $nakedType = self::getNamedType($type);
         return $nakedType instanceof InputType;
     }
 
@@ -121,13 +121,14 @@ GraphQLNonNull;
      */
     public static function isOutputType($type)
     {
-        $nakedType = self::getUnmodifiedType($type);
+        $nakedType = self::getNamedType($type);
         return $nakedType instanceof OutputType;
     }
 
     public static function isLeafType($type)
     {
-        $nakedType = self::getUnmodifiedType($type);
+        // TODO: add LeafType interface
+        $nakedType = self::getNamedType($type);
         return (
             $nakedType instanceof ScalarType ||
             $nakedType instanceof EnumType
@@ -136,19 +137,12 @@ GraphQLNonNull;
 
     public static function isCompositeType($type)
     {
-        return (
-            $type instanceof ObjectType ||
-            $type instanceof InterfaceType ||
-            $type instanceof UnionType
-        );
+        return $type instanceof CompositeType;
     }
 
     public static function isAbstractType($type)
     {
-        return (
-            $type instanceof InterfaceType ||
-            $type instanceof UnionType
-        );
+        return $type instanceof AbstractType;
     }
 
     /**
@@ -164,7 +158,7 @@ GraphQLNonNull;
      * @param $type
      * @return UnmodifiedType
      */
-    public static function getUnmodifiedType($type)
+    public static function getNamedType($type)
     {
         if (null === $type) {
             return null;
@@ -195,21 +189,21 @@ GraphQLNonNull;
      * @return Type
      * @throws \Exception
      */
-    public static function getTypeOf($value, AbstractType $abstractType)
+    public static function getTypeOf($value, ResolveInfo $info, AbstractType $abstractType)
     {
         $possibleTypes = $abstractType->getPossibleTypes();
 
         for ($i = 0; $i < count($possibleTypes); $i++) {
             /** @var ObjectType $type */
             $type = $possibleTypes[$i];
-            $isTypeOf = $type->isTypeOf($value);
+            $isTypeOf = $type->isTypeOf($value, $info);
 
             if ($isTypeOf === null) {
                 // TODO: move this to a JS impl specific type system validation step
                 // so the error can be found before execution.
                 throw new \Exception(
                     'Non-Object Type ' . $abstractType->name . ' does not implement ' .
-                    'resolveType and Object Type ' . $type->name . ' does not implement ' .
+                    'getObjectType and Object Type ' . $type->name . ' does not implement ' .
                     'isTypeOf. There is no way to determine if a value is of this type.'
                 );
             }
diff --git a/src/Type/Definition/UnionType.php b/src/Type/Definition/UnionType.php
index 529b991..c72de31 100644
--- a/src/Type/Definition/UnionType.php
+++ b/src/Type/Definition/UnionType.php
@@ -73,9 +73,9 @@ class UnionType extends Type implements AbstractType, OutputType, CompositeType
      * @param ObjectType $value
      * @return Type
      */
-    public function resolveType($value)
+    public function getObjectType($value, ResolveInfo $info)
     {
         $resolver = $this->_resolveType;
-        return $resolver ? call_user_func($resolver, $value) : Type::getTypeOf($value, $this);
+        return $resolver ? call_user_func($resolver, $value) : Type::getTypeOf($value, $info, $this);
     }
 }
diff --git a/src/Type/Introspection.php b/src/Type/Introspection.php
index 002c5f6..dbd85fb 100644
--- a/src/Type/Introspection.php
+++ b/src/Type/Introspection.php
@@ -3,6 +3,7 @@ namespace GraphQL\Type;
 
 
 use GraphQL\Schema;
+use GraphQL\Type\Definition\Directive;
 use GraphQL\Type\Definition\EnumType;
 use GraphQL\Type\Definition\FieldDefinition;
 use GraphQL\Type\Definition\InputObjectType;
@@ -10,6 +11,7 @@ use GraphQL\Type\Definition\InterfaceType;
 use GraphQL\Type\Definition\ListOfType;
 use GraphQL\Type\Definition\NonNull;
 use GraphQL\Type\Definition\ObjectType;
+use GraphQL\Type\Definition\ResolveInfo;
 use GraphQL\Type\Definition\ScalarType;
 use GraphQL\Type\Definition\Type;
 use GraphQL\Type\Definition\UnionType;
@@ -30,6 +32,165 @@ class Introspection
 {
     private static $_map = [];
 
+    /**
+     * @return string
+     */
+    public static function getIntrospectionQuery($includeDescription = true)
+    {
+        $withDescription = <<<'EOD'
+  query IntrospectionQuery {
+    __schema {
+      queryType { name }
+      mutationType { name }
+      types {
+        ...FullType
+      }
+      directives {
+        name
+        description
+        args {
+          ...InputValue
+        }
+        onOperation
+        onFragment
+        onField
+      }
+    }
+  }
+
+  fragment FullType on __Type {
+    kind
+    name
+    description
+    fields {
+      name
+      description
+      args {
+        ...InputValue
+      }
+      type {
+        ...TypeRef
+      }
+      isDeprecated
+      deprecationReason
+    }
+    inputFields {
+      ...InputValue
+    }
+    interfaces {
+      ...TypeRef
+    }
+    enumValues {
+      name
+      description
+      isDeprecated
+      deprecationReason
+    }
+    possibleTypes {
+      ...TypeRef
+    }
+  }
+
+  fragment InputValue on __InputValue {
+    name
+    description
+    type { ...TypeRef }
+    defaultValue
+  }
+
+  fragment TypeRef on __Type {
+    kind
+    name
+    ofType {
+      kind
+      name
+      ofType {
+        kind
+        name
+        ofType {
+          kind
+          name
+        }
+      }
+    }
+  }
+EOD;
+        $withoutDescription = <<<'EOD'
+  query IntrospectionQuery {
+    __schema {
+      queryType { name }
+      mutationType { name }
+      types {
+        ...FullType
+      }
+      directives {
+        name
+        args {
+          ...InputValue
+        }
+        onOperation
+        onFragment
+        onField
+      }
+    }
+  }
+
+  fragment FullType on __Type {
+    kind
+    name
+    fields {
+      name
+      args {
+        ...InputValue
+      }
+      type {
+        ...TypeRef
+      }
+      isDeprecated
+      deprecationReason
+    }
+    inputFields {
+      ...InputValue
+    }
+    interfaces {
+      ...TypeRef
+    }
+    enumValues {
+      name
+      isDeprecated
+      deprecationReason
+    }
+    possibleTypes {
+      ...TypeRef
+    }
+  }
+
+  fragment InputValue on __InputValue {
+    name
+    type { ...TypeRef }
+    defaultValue
+  }
+
+  fragment TypeRef on __Type {
+    kind
+    name
+    ofType {
+      kind
+      name
+      ofType {
+        kind
+        name
+        ofType {
+          kind
+          name
+        }
+      }
+    }
+  }
+EOD;
+        return $includeDescription ? $withDescription : $withoutDescription;
+    }
+
     public static function _schema()
     {
         if (!isset(self::$_map['__Schema'])) {
@@ -83,12 +244,15 @@ class Introspection
             self::$_map['__Directive'] = new ObjectType([
                 'name' => '__Directive',
                 'fields' => [
-                    'name' => ['type' => Type::string()],
+                    'name' => ['type' => Type::nonNull(Type::string())],
                     'description' => ['type' => Type::string()],
-                    'type' => ['type' => [__CLASS__, '_type']],
-                    'onOperation' => ['type' => Type::boolean()],
-                    'onFragment' => ['type' => Type::boolean()],
-                    'onField' => ['type' => Type::boolean()]
+                    'args' => [
+                        'type' => Type::nonNull(Type::listOf(Type::nonNull(self::_inputValue()))),
+                        'resolve' => function(Directive $directive) {return $directive->args ?: [];}
+                    ],
+                    'onOperation' => ['type' => Type::nonNull(Type::boolean())],
+                    'onFragment' => ['type' => Type::nonNull(Type::boolean())],
+                    'onField' => ['type' => Type::nonNull(Type::boolean())]
                 ]
             ]);
         }
@@ -349,14 +513,9 @@ class Introspection
                 'resolve' => function (
                     $source,
                     $args,
-                    $root,
-                    $fieldAST,
-                    $fieldType,
-                    $parentType,
-                    $schema
+                    ResolveInfo $info
                 ) {
-                    // TODO: move 3+ args to separate object
-                    return $schema;
+                    return $info->schema;
                 }
             ]);
         }
@@ -373,8 +532,8 @@ class Introspection
                 'args' => [
                     ['name' => 'name', 'type' => Type::nonNull(Type::string())]
                 ],
-                'resolve' => function ($source, $args, $root, $fieldAST, $fieldType, $parentType, $schema) {
-                    return $schema->getType($args['name']);
+                'resolve' => function ($source, $args, ResolveInfo $info) {
+                    return $info->schema->getType($args['name']);
                 }
             ]);
         }
@@ -392,12 +551,9 @@ class Introspection
                 'resolve' => function (
                     $source,
                     $args,
-                    $root,
-                    $fieldAST,
-                    $fieldType,
-                    $parentType
+                    ResolveInfo $info
                 ) {
-                    return $parentType->name;
+                    return $info->parentType->name;
                 }
             ]);
         }
diff --git a/src/Type/SchemaValidator.php b/src/Type/SchemaValidator.php
index 04131d0..adc8ffb 100644
--- a/src/Type/SchemaValidator.php
+++ b/src/Type/SchemaValidator.php
@@ -169,11 +169,6 @@ class SchemaValidator
                 $errors = array_merge($errors, $newErrors);
             }
         }
-        $isValid = empty($errors);
-        $result = [
-            'isValid' => $isValid,
-            'errors' => $isValid ? null : array_map(['GraphQL\Error', 'formatError'], $errors)
-        ];
-        return (object) $result;
+        return $errors;
     }
 }
\ No newline at end of file
diff --git a/src/Utils/TypeInfo.php b/src/Utils/TypeInfo.php
index fc1a09f..46ed40c 100644
--- a/src/Utils/TypeInfo.php
+++ b/src/Utils/TypeInfo.php
@@ -4,9 +4,12 @@ namespace GraphQL\Utils;
 use GraphQL\Language\AST\Field;
 use GraphQL\Language\AST\ListType;
 use GraphQL\Language\AST\Name;
+use GraphQL\Language\AST\NamedType;
 use GraphQL\Language\AST\Node;
 use GraphQL\Language\AST\NonNullType;
 use GraphQL\Schema;
+use GraphQL\Type\Definition\Directive;
+use GraphQL\Type\Definition\FieldArgument;
 use GraphQL\Type\Definition\FieldDefinition;
 use GraphQL\Type\Definition\InputObjectType;
 use GraphQL\Type\Definition\InputType;
@@ -38,8 +41,8 @@ class TypeInfo
             return $innerType ? new NonNull($innerType) : null;
         }
 
-        Utils::invariant($inputTypeAst instanceof Name, 'Must be a type name');
-        return $schema->getType($inputTypeAst->value);
+        Utils::invariant($inputTypeAst->kind === Node::NAMED_TYPE, 'Must be a named type');
+        return $schema->getType($inputTypeAst->name->value);
     }
 
     /**
@@ -103,6 +106,15 @@ class TypeInfo
      */
     private $_fieldDefStack;
 
+    /**
+     * @var Directive
+     */
+    private $_directive;
+
+    /**
+     * @var FieldArgument
+     */
+    private $_argument;
 
     public function __construct(Schema $schema)
     {
@@ -157,6 +169,21 @@ class TypeInfo
         return null;
     }
 
+    /**
+     * @return Directive|null
+     */
+    function getDirective()
+    {
+        return $this->_directive;
+    }
+
+    /**
+     * @return FieldArgument|null
+     */
+    function getArgument()
+    {
+        return $this->_argument;
+    }
 
     function enter(Node $node)
     {
@@ -164,16 +191,19 @@ class TypeInfo
 
         switch ($node->kind) {
             case Node::SELECTION_SET:
-                // var $compositeType: ?GraphQLCompositeType;
-                $rawType = Type::getUnmodifiedType($this->getType());
+                $namedType = Type::getNamedType($this->getType());
                 $compositeType = null;
-                if (Type::isCompositeType($rawType)) {
+                if (Type::isCompositeType($namedType)) {
                     // isCompositeType is a type refining predicate, so this is safe.
-                    $compositeType = $rawType;
+                    $compositeType = $namedType;
                 }
                 array_push($this->_parentTypeStack, $compositeType);
                 break;
 
+            case Node::DIRECTIVE:
+                $this->_directive = $schema->getDirective($node->name->value);
+                break;
+
             case Node::FIELD:
                 $parentType = $this->getParentType();
                 $fieldDef = null;
@@ -196,7 +226,7 @@ class TypeInfo
 
             case Node::INLINE_FRAGMENT:
             case Node::FRAGMENT_DEFINITION:
-                $type = $schema->getType($node->typeCondition->value);
+                $type = self::typeFromAST($schema, $node->typeCondition);
                 array_push($this->_typeStack, $type);
                 break;
 
@@ -205,32 +235,28 @@ class TypeInfo
                 break;
 
             case Node::ARGUMENT:
-                $field = $this->getFieldDef();
-                $argType = null;
-                if ($field) {
-                    $argDef = Utils::find($field->args, function($arg) use ($node) {return $arg->name === $node->name->value;});
+                $fieldOrDirective = $this->getDirective() ?: $this->getFieldDef();
+                $argDef = $argType = null;
+                if ($fieldOrDirective) {
+                    $argDef = Utils::find($fieldOrDirective->args, function($arg) use ($node) {return $arg->name === $node->name->value;});
                     if ($argDef) {
                         $argType = $argDef->getType();
                     }
                 }
+                $this->_argument = $argDef;
                 array_push($this->_inputTypeStack, $argType);
                 break;
 
-            case Node::DIRECTIVE:
-                $directive = $schema->getDirective($node->name->value);
-                array_push($this->_inputTypeStack, $directive ? $directive->type : null);
-                break;
-
             case Node::LST:
-                $arrayType = Type::getNullableType($this->getInputType());
+                $listType = Type::getNullableType($this->getInputType());
                 array_push(
                     $this->_inputTypeStack,
-                    $arrayType instanceof ListOfType ? $arrayType->getWrappedType() : null
+                    $listType instanceof ListOfType ? $listType->getWrappedType() : null
                 );
                 break;
 
             case Node::OBJECT_FIELD:
-                $objectType = Type::getUnmodifiedType($this->getInputType());
+                $objectType = Type::getNamedType($this->getInputType());
                 $fieldType = null;
                 if ($objectType instanceof InputObjectType) {
                     $tmp = $objectType->getFields();
@@ -248,10 +274,16 @@ class TypeInfo
             case Node::SELECTION_SET:
                 array_pop($this->_parentTypeStack);
                 break;
+
             case Node::FIELD:
                 array_pop($this->_fieldDefStack);
                 array_pop($this->_typeStack);
                 break;
+
+            case Node::DIRECTIVE:
+                $this->_directive = null;
+                break;
+
             case Node::OPERATION_DEFINITION:
             case Node::INLINE_FRAGMENT:
             case Node::FRAGMENT_DEFINITION:
@@ -261,9 +293,9 @@ class TypeInfo
                 array_pop($this->_inputTypeStack);
                 break;
             case Node::ARGUMENT:
+                $this->_argument = null;
                 array_pop($this->_inputTypeStack);
                 break;
-            case Node::DIRECTIVE:
             case Node::LST:
             case Node::OBJECT_FIELD:
                 array_pop($this->_inputTypeStack);
diff --git a/src/Validator/DocumentValidator.php b/src/Validator/DocumentValidator.php
index 2aa9d43..05157a1 100644
--- a/src/Validator/DocumentValidator.php
+++ b/src/Validator/DocumentValidator.php
@@ -2,7 +2,7 @@
 namespace GraphQL\Validator;
 
 use GraphQL\Error;
-use GraphQL\Language\AST\ArrayValue;
+use GraphQL\Language\AST\ListValue;
 use GraphQL\Language\AST\Document;
 use GraphQL\Language\AST\FragmentSpread;
 use GraphQL\Language\AST\Node;
@@ -33,6 +33,7 @@ use GraphQL\Validator\Rules\NoUnusedFragments;
 use GraphQL\Validator\Rules\NoUnusedVariables;
 use GraphQL\Validator\Rules\OverlappingFieldsCanBeMerged;
 use GraphQL\Validator\Rules\PossibleFragmentSpreads;
+use GraphQL\Validator\Rules\ProvidedNonNullArguments;
 use GraphQL\Validator\Rules\ScalarLeafs;
 use GraphQL\Validator\Rules\VariablesAreInputTypes;
 use GraphQL\Validator\Rules\VariablesInAllowedPosition;
@@ -45,23 +46,28 @@ class DocumentValidator
     {
         if (null === self::$allRules) {
             self::$allRules = [
-                new ArgumentsOfCorrectType(),
-                new DefaultValuesOfCorrectType(),
-                new FieldsOnCorrectType(),
-                new FragmentsOnCompositeTypes(),
-                new KnownArgumentNames(),
-                new KnownDirectives(),
-                new KnownFragmentNames(),
-                new KnownTypeNames(),
-                new NoFragmentCycles(),
-                new NoUndefinedVariables(),
-                new NoUnusedFragments(),
-                new NoUnusedVariables(),
-                new OverlappingFieldsCanBeMerged(),
-                new PossibleFragmentSpreads(),
-                new ScalarLeafs(),
-                new VariablesAreInputTypes(),
-                new VariablesInAllowedPosition()
+                // new UniqueOperationNames,
+                // new LoneAnonymousOperation,
+                new KnownTypeNames,
+                new FragmentsOnCompositeTypes,
+                new VariablesAreInputTypes,
+                new ScalarLeafs,
+                new FieldsOnCorrectType,
+                // new UniqueFragmentNames,
+                new KnownFragmentNames,
+                new NoUnusedFragments,
+                new PossibleFragmentSpreads,
+                new NoFragmentCycles,
+                new NoUndefinedVariables,
+                new NoUnusedVariables,
+                new KnownDirectives,
+                new KnownArgumentNames,
+                // new UniqueArgumentNames,
+                new ArgumentsOfCorrectType,
+                new ProvidedNonNullArguments,
+                new DefaultValuesOfCorrectType,
+                new VariablesInAllowedPosition,
+                new OverlappingFieldsCanBeMerged,
             ];
         }
         return self::$allRules;
@@ -70,13 +76,7 @@ class DocumentValidator
     public static function validate(Schema $schema, Document $ast, array $rules = null)
     {
         $errors = self::visitUsingRules($schema, $ast, $rules ?: self::allRules());
-        $isValid = empty($errors);
-
-        $result = [
-            'isValid' => $isValid,
-            'errors' => $isValid ? null : array_map(['GraphQL\Error', 'formatError'], $errors)
-        ];
-        return $result;
+        return $errors;
     }
 
     static function isError($value)
@@ -121,7 +121,7 @@ class DocumentValidator
         // Lists accept a non-list value as a list of one.
         if ($type instanceof ListOfType) {
             $itemType = $type->getWrappedType();
-            if ($valueAST instanceof ArrayValue) {
+            if ($valueAST instanceof ListValue) {
                 foreach($valueAST->values as $itemAST) {
                     if (!self::isValidLiteralValue($itemAST, $itemType)) {
                         return false;
@@ -133,10 +133,10 @@ class DocumentValidator
             }
         }
 
-        // Scalar/Enum input checks to ensure the type can coerce the value to
+        // Scalar/Enum input checks to ensure the type can serialize the value to
         // a non-null value.
         if ($type instanceof ScalarType || $type instanceof EnumType) {
-            return $type->coerceLiteral($valueAST) !== null;
+            return $type->parseLiteral($valueAST) !== null;
         }
 
         // Input objects check each defined field, ensuring it is of the correct
@@ -294,7 +294,7 @@ class DocumentValidator
                                 if ($result->doBreak) {
                                     $instances[$i] = null;
                                 }
-                            } if (self::isError($result)) {
+                            } else if (self::isError($result)) {
                                 self::append($errors, $result);
                             } else if ($result !== null) {
                                 throw new \Exception("Config cannot edit document.");
diff --git a/src/Validator/Rules/ArgumentsOfCorrectType.php b/src/Validator/Rules/ArgumentsOfCorrectType.php
index d634828..8191c03 100644
--- a/src/Validator/Rules/ArgumentsOfCorrectType.php
+++ b/src/Validator/Rules/ArgumentsOfCorrectType.php
@@ -16,52 +16,22 @@ use GraphQL\Validator\ValidationContext;
 
 class ArgumentsOfCorrectType
 {
+    static function badValueMessage($argName, $type, $value)
+    {
+        return "Argument \"$argName\" expected type \"$type\" but got: $value.";
+    }
+
     public function __invoke(ValidationContext $context)
     {
         return [
-            Node::FIELD => function(Field $fieldAST) use ($context) {
-                $fieldDef = $context->getFieldDef();
-                if (!$fieldDef) {
-                    return Visitor::skipNode();
+            Node::ARGUMENT => function(Argument $argAST) use ($context) {
+                $argDef = $context->getArgument();
+                if ($argDef && !DocumentValidator::isValidLiteralValue($argAST->value, $argDef->getType())) {
+                    return new Error(
+                        self::badValueMessage($argAST->name->value, $argDef->getType(), Printer::doPrint($argAST->value)),
+                        [$argAST->value]
+                    );
                 }
-                $errors = [];
-                $argASTs = $fieldAST->arguments ?: [];
-                $argASTMap = Utils::keyMap($argASTs, function (Argument $arg) {
-                    return $arg->name->value;
-                });
-
-                foreach ($fieldDef->args as $argDef) {
-                    $argAST = isset($argASTMap[$argDef->name]) ? $argASTMap[$argDef->name] : null;
-                    if (!$argAST && $argDef->getType() instanceof NonNull) {
-                        $errors[] = new Error(
-                            Messages::missingArgMessage(
-                                $fieldAST->name->value,
-                                $argDef->name,
-                                $argDef->getType()
-                            ),
-                            [$fieldAST]
-                        );
-                    }
-                }
-
-                $argDefMap = Utils::keyMap($fieldDef->args, function ($def) {
-                    return $def->name;
-                });
-                foreach ($argASTs as $argAST) {
-                    $argDef = $argDefMap[$argAST->name->value];
-                    if ($argDef && !DocumentValidator::isValidLiteralValue($argAST->value, $argDef->getType())) {
-                        $errors[] = new Error(
-                            Messages::badValueMessage(
-                                $argAST->name->value,
-                                $argDef->getType(),
-                                Printer::doPrint($argAST->value)
-                            ),
-                            [$argAST->value]
-                        );
-                    }
-                }
-
-                return !empty($errors) ? $errors : null;
             }
         ];
     }
diff --git a/src/Validator/Rules/FragmentsOnCompositeTypes.php b/src/Validator/Rules/FragmentsOnCompositeTypes.php
index b6fe572..f3cee89 100644
--- a/src/Validator/Rules/FragmentsOnCompositeTypes.php
+++ b/src/Validator/Rules/FragmentsOnCompositeTypes.php
@@ -6,35 +6,43 @@ use GraphQL\Error;
 use GraphQL\Language\AST\FragmentDefinition;
 use GraphQL\Language\AST\InlineFragment;
 use GraphQL\Language\AST\Node;
+use GraphQL\Language\Printer;
 use GraphQL\Type\Definition\CompositeType;
+use GraphQL\Type\Definition\Type;
 use GraphQL\Validator\Messages;
 use GraphQL\Validator\ValidationContext;
 
 class FragmentsOnCompositeTypes
 {
+    static function inlineFragmentOnNonCompositeErrorMessage($type)
+    {
+        return "Fragment cannot condition on non composite type \"$type\".";
+    }
+
+    static function fragmentOnNonCompositeErrorMessage($fragName, $type)
+    {
+        return "Fragment \"$fragName\" cannot condition on non composite type \"$type\".";
+    }
+
     public function __invoke(ValidationContext $context)
     {
         return [
             Node::INLINE_FRAGMENT => function(InlineFragment $node) use ($context) {
-                $typeName = $node->typeCondition->value;
-                $type = $context->getSchema()->getType($typeName);
-                $isCompositeType = $type instanceof CompositeType;
+                $type = $context->getType();
 
-                if (!$isCompositeType) {
+                if ($type && !Type::isCompositeType($type)) {
                     return new Error(
-                        "Fragment cannot condition on non composite type \"$typeName\".",
+                        self::inlineFragmentOnNonCompositeErrorMessage($type),
                         [$node->typeCondition]
                     );
                 }
             },
             Node::FRAGMENT_DEFINITION => function(FragmentDefinition $node) use ($context) {
-                $typeName = $node->typeCondition->value;
-                $type = $context->getSchema()->getType($typeName);
-                $isCompositeType = $type instanceof CompositeType;
+                $type = $context->getType();
 
-                if (!$isCompositeType) {
+                if ($type && !Type::isCompositeType($type)) {
                     return new Error(
-                        Messages::fragmentOnNonCompositeErrorMessage($node->name->value, $typeName),
+                        self::fragmentOnNonCompositeErrorMessage($node->name->value, Printer::doPrint($node->typeCondition)),
                         [$node->typeCondition]
                     );
                 }
diff --git a/src/Validator/Rules/KnownArgumentNames.php b/src/Validator/Rules/KnownArgumentNames.php
index 84e7e5c..c610573 100644
--- a/src/Validator/Rules/KnownArgumentNames.php
+++ b/src/Validator/Rules/KnownArgumentNames.php
@@ -11,27 +11,57 @@ use GraphQL\Validator\ValidationContext;
 
 class KnownArgumentNames
 {
+    public static function unknownArgMessage($argName, $fieldName, $type)
+    {
+        return "Unknown argument \"$argName\" on field \"$fieldName\" of type \"$type\".";
+    }
+
+    public static function unknownDirectiveArgMessage($argName, $directiveName)
+    {
+        return "Unknown argument \"$argName\" on directive \"@$directiveName\".";
+    }
+
     public function __invoke(ValidationContext $context)
     {
         return [
-            Node::ARGUMENT => function(Argument $node) use ($context) {
-                $fieldDef = $context->getFieldDef();
-                if ($fieldDef) {
-                    $argDef = null;
-                    foreach ($fieldDef->args as $arg) {
-                        if ($arg->name === $node->name->value) {
-                            $argDef = $arg;
-                            break;
+            Node::ARGUMENT => function(Argument $node, $key, $parent, $path, $ancestors) use ($context) {
+                $argumentOf = $ancestors[count($ancestors) - 1];
+                if ($argumentOf->kind === Node::FIELD) {
+                    $fieldDef = $context->getFieldDef();
+
+                    if ($fieldDef) {
+                        $fieldArgDef = null;
+                        foreach ($fieldDef->args as $arg) {
+                            if ($arg->name === $node->name->value) {
+                                $fieldArgDef = $arg;
+                                break;
+                            }
+                        }
+                        if (!$fieldArgDef) {
+                            $parentType = $context->getParentType();
+                            Utils::invariant($parentType);
+                            return new Error(
+                                self::unknownArgMessage($node->name->value, $fieldDef->name, $parentType->name),
+                                [$node]
+                            );
                         }
                     }
-
-                    if (!$argDef) {
-                        $parentType = $context->getParentType();
-                        Utils::invariant($parentType);
-                        return new Error(
-                            Messages::unknownArgMessage($node->name->value, $fieldDef->name, $parentType->name),
-                            [$node]
-                        );
+                } else if ($argumentOf->kind === Node::DIRECTIVE) {
+                    $directive = $context->getDirective();
+                    if ($directive) {
+                        $directiveArgDef = null;
+                        foreach ($directive->args as $arg) {
+                            if ($arg->name === $node->name->value) {
+                                $directiveArgDef = $arg;
+                                break;
+                            }
+                        }
+                        if (!$directiveArgDef) {
+                            return new Error(
+                                self::unknownDirectiveArgMessage($node->name->value, $directive->name),
+                                [$node]
+                            );
+                        }
                     }
                 }
             }
diff --git a/src/Validator/Rules/KnownDirectives.php b/src/Validator/Rules/KnownDirectives.php
index eacc820..da8d3af 100644
--- a/src/Validator/Rules/KnownDirectives.php
+++ b/src/Validator/Rules/KnownDirectives.php
@@ -15,6 +15,16 @@ use GraphQL\Validator\ValidationContext;
 
 class KnownDirectives
 {
+    static function unknownDirectiveMessage($directiveName)
+    {
+        return "Unknown directive \"$directiveName\".";
+    }
+
+    static function misplacedDirectiveMessage($directiveName, $placement)
+    {
+        return "Directive \"$directiveName\" may not be used on \"$placement\".";
+    }
+
     public function __invoke(ValidationContext $context)
     {
         return [
@@ -29,7 +39,7 @@ class KnownDirectives
 
                 if (!$directiveDef) {
                     return new Error(
-                        Messages::unknownDirectiveMessage($node->name->value),
+                        self::unknownDirectiveMessage($node->name->value),
                         [$node]
                     );
                 }
@@ -37,13 +47,13 @@ class KnownDirectives
 
                 if ($appliedTo instanceof OperationDefinition && !$directiveDef->onOperation) {
                     return new Error(
-                        Messages::misplacedDirectiveMessage($node->name->value, 'operation'),
+                        self::misplacedDirectiveMessage($node->name->value, 'operation'),
                         [$node]
                     );
                 }
                 if ($appliedTo instanceof Field && !$directiveDef->onField) {
                     return new Error(
-                        Messages::misplacedDirectiveMessage($node->name->value, 'field'),
+                        self::misplacedDirectiveMessage($node->name->value, 'field'),
                         [$node]
                     );
                 }
@@ -56,7 +66,7 @@ class KnownDirectives
 
                 if ($fragmentKind && !$directiveDef->onFragment) {
                     return new Error(
-                        Messages::misplacedDirectiveMessage($node->name->value, 'fragment'),
+                        self::misplacedDirectiveMessage($node->name->value, 'fragment'),
                         [$node]
                     );
                 }
diff --git a/src/Validator/Rules/KnownFragmentNames.php b/src/Validator/Rules/KnownFragmentNames.php
index 5cc0495..3d0dbc8 100644
--- a/src/Validator/Rules/KnownFragmentNames.php
+++ b/src/Validator/Rules/KnownFragmentNames.php
@@ -9,6 +9,11 @@ use GraphQL\Validator\ValidationContext;
 
 class KnownFragmentNames
 {
+    static function unknownFragmentMessage($fragName)
+    {
+        return "Unknown fragment \"$fragName\".";
+    }
+
     public function __invoke(ValidationContext $context)
     {
         return [
@@ -17,7 +22,7 @@ class KnownFragmentNames
                 $fragment = $context->getFragment($fragmentName);
                 if (!$fragment) {
                     return new Error(
-                        "Undefined fragment $fragmentName.",
+                        self::unknownFragmentMessage($fragmentName),
                         [$node->name]
                     );
                 }
diff --git a/src/Validator/Rules/KnownTypeNames.php b/src/Validator/Rules/KnownTypeNames.php
index 91b9c46..93d5f4c 100644
--- a/src/Validator/Rules/KnownTypeNames.php
+++ b/src/Validator/Rules/KnownTypeNames.php
@@ -4,22 +4,27 @@ namespace GraphQL\Validator\Rules;
 
 use GraphQL\Error;
 use GraphQL\Language\AST\Name;
+use GraphQL\Language\AST\NamedType;
 use GraphQL\Language\AST\Node;
 use GraphQL\Validator\Messages;
 use GraphQL\Validator\ValidationContext;
 
 class KnownTypeNames
 {
+    static function unknownTypeMessage($type)
+    {
+        return "Unknown type \"$type\".";
+    }
+
     public function __invoke(ValidationContext $context)
     {
         return [
-            Node::NAME => function(Name $node, $key) use ($context) {
-
+            Node::NAMED_TYPE => function(NamedType $node, $key) use ($context) {
                 if ($key === 'type' || $key === 'typeCondition') {
-                    $typeName = $node->value;
+                    $typeName = $node->name->value;
                     $type = $context->getSchema()->getType($typeName);
                     if (!$type) {
-                        return new Error(Messages::unknownTypeMessage($typeName), [$node]);
+                        return new Error(self::unknownTypeMessage($typeName), [$node]);
                     }
                 }
             }
diff --git a/src/Validator/Rules/NoFragmentCycles.php b/src/Validator/Rules/NoFragmentCycles.php
index bf9bb15..8f41fb8 100644
--- a/src/Validator/Rules/NoFragmentCycles.php
+++ b/src/Validator/Rules/NoFragmentCycles.php
@@ -14,11 +14,16 @@ use GraphQL\Language\AST\FragmentDefinition;
 use GraphQL\Language\AST\FragmentSpread;
 use GraphQL\Language\AST\Node;
 use GraphQL\Language\Visitor;
-use GraphQL\Validator\Messages;
 use GraphQL\Validator\ValidationContext;
 
 class NoFragmentCycles
 {
+    static function cycleErrorMessage($fragName, array $spreadNames = [])
+    {
+        $via = !empty($spreadNames) ? ' via ' . implode(', ', $spreadNames) : '';
+        return "Cannot spread fragment \"$fragName\" within itself$via.";
+    }
+
     public function __invoke(ValidationContext $context)
     {
         // Gather all the fragment spreads ASTs for each fragment definition.
@@ -67,7 +72,7 @@ class NoFragmentCycles
                     $knownToLeadToCycle[$spread] = true;
                 }
                 $errors[] = new Error(
-                    Messages::cycleErrorMessage($initialName, array_map(function ($s) {
+                    self::cycleErrorMessage($initialName, array_map(function ($s) {
                         return $s->name->value;
                     }, $spreadPath)),
                     $cyclePath
diff --git a/src/Validator/Rules/NoUndefinedVariables.php b/src/Validator/Rules/NoUndefinedVariables.php
index 56a3c70..dca3ad9 100644
--- a/src/Validator/Rules/NoUndefinedVariables.php
+++ b/src/Validator/Rules/NoUndefinedVariables.php
@@ -23,6 +23,16 @@ use GraphQL\Validator\ValidationContext;
  */
 class NoUndefinedVariables
 {
+    static function undefinedVarMessage($varName)
+    {
+        return "Variable \"$$varName\" is not defined.";
+    }
+
+    static function undefinedVarByOpMessage($varName, $opName)
+    {
+        return "Variable \"$$varName\" is not defined by operation \"$opName\".";
+    }
+
     public function __invoke(ValidationContext $context)
     {
         $operation = null;
@@ -53,12 +63,12 @@ class NoUndefinedVariables
                     }
                     if ($withinFragment && $operation && $operation->name) {
                         return new Error(
-                            Messages::undefinedVarByOpMessage($varName, $operation->name->value),
+                            self::undefinedVarByOpMessage($varName, $operation->name->value),
                             [$variable, $operation]
                         );
                     }
                     return new Error(
-                        Messages::undefinedVarMessage($varName),
+                        self::undefinedVarMessage($varName),
                         [$variable]
                     );
                 }
diff --git a/src/Validator/Rules/NoUnusedFragments.php b/src/Validator/Rules/NoUnusedFragments.php
index 99d10ea..bea8422 100644
--- a/src/Validator/Rules/NoUnusedFragments.php
+++ b/src/Validator/Rules/NoUnusedFragments.php
@@ -11,6 +11,11 @@ use GraphQL\Validator\ValidationContext;
 
 class NoUnusedFragments
 {
+    static function unusedFragMessage($fragName)
+    {
+        return "Fragment \"$fragName\" is never used.";
+    }
+
     public function __invoke(ValidationContext $context)
     {
         $fragmentDefs = [];
@@ -43,7 +48,7 @@ class NoUnusedFragments
                     foreach ($fragmentDefs as $def) {
                         if (empty($fragmentNameUsed[$def->name->value])) {
                             $errors[] = new Error(
-                                Messages::unusedFragMessage($def->name->value),
+                                self::unusedFragMessage($def->name->value),
                                 [$def]
                             );
                         }
@@ -59,11 +64,14 @@ class NoUnusedFragments
         foreach ($spreads as $fragName => $fragment) {
             if (empty($fragmentNameUsed[$fragName])) {
                 $fragmentNameUsed[$fragName] = true;
-                $this->reduceSpreadFragments(
-                    $fragAdjacencies->{$fragName},
-                    $fragmentNameUsed,
-                    $fragAdjacencies
-                );
+
+                if (isset($fragAdjacencies->{$fragName})) {
+                    $this->reduceSpreadFragments(
+                        $fragAdjacencies->{$fragName},
+                        $fragmentNameUsed,
+                        $fragAdjacencies
+                    );
+                }
             }
         }
     }
diff --git a/src/Validator/Rules/NoUnusedVariables.php b/src/Validator/Rules/NoUnusedVariables.php
index 7299153..9a63a80 100644
--- a/src/Validator/Rules/NoUnusedVariables.php
+++ b/src/Validator/Rules/NoUnusedVariables.php
@@ -10,6 +10,11 @@ use GraphQL\Validator\ValidationContext;
 
 class NoUnusedVariables
 {
+    static function unusedVariableMessage($varName)
+    {
+        return "Variable \"$$varName\" is never used.";
+    }
+
     public function __invoke(ValidationContext $context)
     {
         $visitedFragmentNames = new \stdClass();
@@ -30,7 +35,7 @@ class NoUnusedVariables
                     foreach ($variableDefs as $def) {
                         if (empty($variableNameUsed->{$def->variable->name->value})) {
                             $errors[] = new Error(
-                                Messages::unusedVariableMessage($def->variable->name->value),
+                                self::unusedVariableMessage($def->variable->name->value),
                                 [$def]
                             );
                         }
diff --git a/src/Validator/Rules/OverlappingFieldsCanBeMerged.php b/src/Validator/Rules/OverlappingFieldsCanBeMerged.php
index c185a3c..fb27759 100644
--- a/src/Validator/Rules/OverlappingFieldsCanBeMerged.php
+++ b/src/Validator/Rules/OverlappingFieldsCanBeMerged.php
@@ -3,19 +3,40 @@ namespace GraphQL\Validator\Rules;
 
 
 use GraphQL\Error;
+use GraphQL\Language\AST\Directive;
 use GraphQL\Language\AST\FragmentSpread;
 use GraphQL\Language\AST\InlineFragment;
+use GraphQL\Language\AST\NamedType;
 use GraphQL\Language\AST\Node;
 use GraphQL\Language\AST\SelectionSet;
 use GraphQL\Language\Printer;
 use GraphQL\Type\Definition\Type;
+use GraphQL\Utils;
 use GraphQL\Utils\PairSet;
 use GraphQL\Utils\TypeInfo;
-use GraphQL\Validator\Messages;
 use GraphQL\Validator\ValidationContext;
 
 class OverlappingFieldsCanBeMerged
 {
+    static function fieldsConflictMessage($responseName, $reason)
+    {
+        $reasonMessage = self::reasonMessage($reason);
+        return "Fields \"$responseName\" conflict because $reasonMessage.";
+    }
+
+    static function reasonMessage($reason)
+    {
+        if (is_array($reason)) {
+            $tmp = array_map(function ($tmp) {
+                list($responseName, $subReason) = $tmp;
+                $reasonMessage = self::reasonMessage($subReason);
+                return "subfields \"$responseName\" conflict because $reasonMessage";
+            }, $reason);
+            return implode(' and ', $tmp);
+        }
+        return $reason;
+    }
+
     public function __invoke(ValidationContext $context)
     {
         $comparedSet = new PairSet();
@@ -27,7 +48,7 @@ class OverlappingFieldsCanBeMerged
                 'leave' => function(SelectionSet $selectionSet) use ($context, $comparedSet) {
                     $fieldMap = $this->collectFieldASTsAndDefs(
                         $context,
-                        $context->getType(),
+                        $context->getParentType(),
                         $selectionSet
                     );
 
@@ -37,11 +58,11 @@ class OverlappingFieldsCanBeMerged
                         return array_map(function ($conflict) {
                             $responseName = $conflict[0][0];
                             $reason = $conflict[0][1];
-                            $blameNodes = $conflict[1];
+                            $fields = $conflict[1];
 
                             return new Error(
-                                Messages::fieldsConflictMessage($responseName, $reason),
-                                $blameNodes
+                                self::fieldsConflictMessage($responseName, $reason),
+                                $fields
                             );
                         }, $conflicts);
 
@@ -101,7 +122,7 @@ class OverlappingFieldsCanBeMerged
         $type1 = isset($def1) ? $def1->getType() : null;
         $type2 = isset($def2) ? $def2->getType() : null;
 
-        if (!$this->sameType($type1, $type2)) {
+        if ($type1 && $type2 && !$this->sameType($type1, $type2)) {
             return [
                 [$responseName, "they return differing types $type1 and $type2"],
                 [$ast1, $ast2]
@@ -111,7 +132,7 @@ class OverlappingFieldsCanBeMerged
         $args1 = isset($ast1->arguments) ? $ast1->arguments : [];
         $args2 = isset($ast2->arguments) ? $ast2->arguments : [];
 
-        if (!$this->sameNameValuePairs($args1, $args2)) {
+        if (!$this->sameArguments($args1, $args2)) {
             return [
                 [$responseName, 'they have differing arguments'],
                 [$ast1, $ast2]
@@ -121,7 +142,7 @@ class OverlappingFieldsCanBeMerged
         $directives1 = isset($ast1->directives) ? $ast1->directives : [];
         $directives2 = isset($ast2->directives) ? $ast2->directives : [];
 
-        if (!$this->sameNameValuePairs($directives1, $directives2)) {
+        if (!$this->sameDirectives($directives1, $directives2)) {
             return [
                 [$responseName, 'they have differing directives'],
                 [$ast1, $ast2]
@@ -136,13 +157,13 @@ class OverlappingFieldsCanBeMerged
 
             $subfieldMap = $this->collectFieldASTsAndDefs(
                 $context,
-                $type1,
+                Type::getNamedType($type1),
                 $selectionSet1,
                 $visitedFragmentNames
             );
             $subfieldMap = $this->collectFieldASTsAndDefs(
                 $context,
-                $type2,
+                Type::getNamedType($type2),
                 $selectionSet2,
                 $visitedFragmentNames,
                 $subfieldMap
@@ -152,7 +173,7 @@ class OverlappingFieldsCanBeMerged
             if (!empty($conflicts)) {
                 return [
                     [$responseName, array_map(function ($conflict) { return $conflict[0]; }, $conflicts)],
-                    array_reduce($conflicts, function ($list, $conflict) { return array_merge($list, $conflict[1]); }, [$ast1, $ast2])
+                    array_reduce($conflicts, function ($allFields, $conflict) { return array_merge($allFields, $conflict[1]); }, [$ast1, $ast2])
                 ];
             }
         }
@@ -183,8 +204,7 @@ class OverlappingFieldsCanBeMerged
 
             switch ($selection->kind) {
                 case Node::FIELD:
-                    $fieldAST = $selection;
-                    $fieldName = $fieldAST->name->value;
+                    $fieldName = $selection->name->value;
                     $fieldDef = null;
                     if ($parentType && method_exists($parentType, 'getFields')) {
                         $tmp = $parentType->getFields();
@@ -192,28 +212,26 @@ class OverlappingFieldsCanBeMerged
                             $fieldDef = $tmp[$fieldName];
                         }
                     }
-                    $responseName = $fieldAST->alias ? $fieldAST->alias->value : $fieldName;
+                    $responseName = $selection->alias ? $selection->alias->value : $fieldName;
 
                     if (!isset($_astAndDefs[$responseName])) {
                         $_astAndDefs[$responseName] = new \ArrayObject();
                     }
-                    $_astAndDefs[$responseName][] = [$fieldAST, $fieldDef];
+                    $_astAndDefs[$responseName][] = [$selection, $fieldDef];
                     break;
                 case Node::INLINE_FRAGMENT:
                     /** @var InlineFragment $inlineFragment */
-                    $inlineFragment = $selection;
                     $_astAndDefs = $this->collectFieldASTsAndDefs(
                         $context,
-                        TypeInfo::typeFromAST($context->getSchema(), $inlineFragment->typeCondition),
-                        $inlineFragment->selectionSet,
+                        TypeInfo::typeFromAST($context->getSchema(), $selection->typeCondition),
+                        $selection->selectionSet,
                         $_visitedFragmentNames,
                         $_astAndDefs
                     );
                     break;
                 case Node::FRAGMENT_SPREAD:
-                    /** @var FragmentSpread $fragmentSpread */
-                    $fragmentSpread = $selection;
-                    $fragName = $fragmentSpread->name->value;
+                    /** @var FragmentSpread $selection */
+                    $fragName = $selection->name->value;
                     if (!empty($_visitedFragmentNames[$fragName])) {
                         continue;
                     }
@@ -235,29 +253,53 @@ class OverlappingFieldsCanBeMerged
         return $_astAndDefs;
     }
 
+    private function sameDirectives(array $directives1, array $directives2)
+    {
+        if (count($directives1) !== count($directives2)) {
+            return false;
+        }
+
+        foreach ($directives1 as $directive1) {
+            $directive2 = null;
+            foreach ($directives2 as $tmp) {
+                if ($tmp->name->value === $directive1->name->value) {
+                    $directive2 = $tmp;
+                    break;
+                }
+            }
+            if (!$directive2) {
+                return false;
+            }
+            if (!$this->sameArguments($directive1->arguments, $directive2->arguments)) {
+                return false;
+            }
+        }
+        return true;
+    }
+
 
     /**
      * @param Array<Argument | Directive> $pairs1
      * @param Array<Argument | Directive> $pairs2
      * @return bool|string
      */
-    private function sameNameValuePairs(array $pairs1, array $pairs2)
+    private function sameArguments(array $arguments1, array $arguments2)
     {
-        if (count($pairs1) !== count($pairs2)) {
+        if (count($arguments1) !== count($arguments2)) {
             return false;
         }
-        foreach ($pairs1 as $pair1) {
-            $matchedPair2 = null;
-            foreach ($pairs2 as $pair2) {
-                if ($pair2->name->value === $pair1->name->value) {
-                    $matchedPair2 = $pair2;
+        foreach ($arguments1 as $arg1) {
+            $arg2 = null;
+            foreach ($arguments2 as $arg) {
+                if ($arg->name->value === $arg1->name->value) {
+                    $arg2 = $arg;
                     break;
                 }
             }
-            if (!$matchedPair2) {
+            if (!$arg2) {
                 return false;
             }
-            if (!$this->sameValue($pair1->value, $matchedPair2->value)) {
+            if (!$this->sameValue($arg1->value, $arg2->value)) {
                 return false;
             }
         }
@@ -271,6 +313,6 @@ class OverlappingFieldsCanBeMerged
 
     function sameType($type1, $type2)
     {
-        return (!$type1 && !$type2) || (string) $type1 === (string) $type2;
+        return (string) $type1 === (string) $type2;
     }
 }
diff --git a/src/Validator/Rules/PossibleFragmentSpreads.php b/src/Validator/Rules/PossibleFragmentSpreads.php
index cb6fa9f..e4fcce0 100644
--- a/src/Validator/Rules/PossibleFragmentSpreads.php
+++ b/src/Validator/Rules/PossibleFragmentSpreads.php
@@ -11,32 +11,41 @@ use GraphQL\Type\Definition\ObjectType;
 use GraphQL\Type\Definition\Type;
 use GraphQL\Type\Definition\UnionType;
 use GraphQL\Utils;
-use GraphQL\Validator\Messages;
 use GraphQL\Validator\ValidationContext;
 
 class PossibleFragmentSpreads
 {
+    static function typeIncompatibleSpreadMessage($fragName, $parentType, $fragType)
+    {
+        return "Fragment \"$fragName\" cannot be spread here as objects of type \"$parentType\" can never be of type \"$fragType\".";
+    }
+
+    static function typeIncompatibleAnonSpreadMessage($parentType, $fragType)
+    {
+        return "Fragment cannot be spread here as objects of type \"$parentType\" can never be of type \"$fragType\".";
+    }
+
     public function __invoke(ValidationContext $context)
     {
         return [
             Node::INLINE_FRAGMENT => function(InlineFragment $node) use ($context) {
-                $fragType = Type::getUnmodifiedType($context->getType());
+                $fragType = Type::getNamedType($context->getType());
                 $parentType = $context->getParentType();
                 if ($fragType && $parentType && !$this->doTypesOverlap($fragType, $parentType)) {
                     return new Error(
-                        Messages::typeIncompatibleAnonSpreadMessage($parentType, $fragType),
+                        self::typeIncompatibleAnonSpreadMessage($parentType, $fragType),
                         [$node]
                     );
                 }
             },
             Node::FRAGMENT_SPREAD => function(FragmentSpread $node) use ($context) {
                 $fragName = $node->name->value;
-                $fragType = Type::getUnmodifiedType($this->getFragmentType($context, $fragName));
+                $fragType = Type::getNamedType($this->getFragmentType($context, $fragName));
                 $parentType = $context->getParentType();
 
                 if ($fragType && $parentType && !$this->doTypesOverlap($fragType, $parentType)) {
                     return new Error(
-                        Messages::typeIncompatibleSpreadMessage($fragName, $parentType, $fragType),
+                        self::typeIncompatibleSpreadMessage($fragName, $parentType, $fragType),
                         [$node]
                     );
                 }
@@ -47,7 +56,7 @@ class PossibleFragmentSpreads
     private function getFragmentType(ValidationContext $context, $name)
     {
         $frag = $context->getFragment($name);
-        return $frag ? $context->getSchema()->getType($frag->typeCondition->value) : null;
+        return $frag ? Utils\TypeInfo::typeFromAST($context->getSchema(), $frag->typeCondition) : null;
     }
 
     private function doTypesOverlap($t1, $t2)
diff --git a/src/Validator/Rules/ProvidedNonNullArguments.php b/src/Validator/Rules/ProvidedNonNullArguments.php
new file mode 100644
index 0000000..f8595a0
--- /dev/null
+++ b/src/Validator/Rules/ProvidedNonNullArguments.php
@@ -0,0 +1,87 @@
+<?php
+namespace GraphQL\Validator\Rules;
+
+
+use GraphQL\Error;
+use GraphQL\Language\AST\Directive;
+use GraphQL\Language\AST\Field;
+use GraphQL\Language\AST\Node;
+use GraphQL\Language\Visitor;
+use GraphQL\Type\Definition\NonNull;
+use GraphQL\Utils;
+use GraphQL\Validator\ValidationContext;
+
+class ProvidedNonNullArguments
+{
+    static function missingFieldArgMessage($fieldName, $argName, $type)
+    {
+        return "Field \"$fieldName\" argument \"$argName\" of type \"$type\" is required but not provided.";
+    }
+
+    static function missingDirectiveArgMessage($directiveName, $argName, $type)
+    {
+        return "Directive \"@$directiveName\" argument \"$argName\" of type \"$type\" is required but not provided.";
+    }
+
+    public function __invoke(ValidationContext $context)
+    {
+        return [
+            Node::FIELD => [
+                'leave' => function(Field $fieldAST) use ($context) {
+                    $fieldDef = $context->getFieldDef();
+
+                    if (!$fieldDef) {
+                        return Visitor::skipNode();
+                    }
+                    $errors = [];
+                    $argASTs = $fieldAST->arguments ?: [];
+
+                    $argASTMap = [];
+                    foreach ($argASTs as $argAST) {
+                        $argASTMap[$argAST->name->value] = $argASTs;
+                    }
+                    foreach ($fieldDef->args as $argDef) {
+                        $argAST = isset($argASTMap[$argDef->name]) ? $argASTMap[$argDef->name] : null;
+                        if (!$argAST && $argDef->getType() instanceof NonNull) {
+                            $errors[] = new Error(
+                                self::missingFieldArgMessage($fieldAST->name->value, $argDef->name, $argDef->getType()),
+                                [$fieldAST]
+                            );
+                        }
+                    }
+
+                    if (!empty($errors)) {
+                        return $errors;
+                    }
+                }
+            ],
+            Node::DIRECTIVE => [
+                'leave' => function(Directive $directiveAST) use ($context) {
+                    $directiveDef = $context->getDirective();
+                    if (!$directiveDef) {
+                        return Visitor::skipNode();
+                    }
+                    $errors = [];
+                    $argASTs = $directiveAST->arguments ?: [];
+                    $argASTMap = [];
+                    foreach ($argASTs as $argAST) {
+                        $argASTMap[$argAST->name->value] = $argASTs;
+                    }
+
+                    foreach ($directiveDef->args as $argDef) {
+                        $argAST = isset($argASTMap[$argDef->name]) ? $argASTMap[$argDef->name] : null;
+                        if (!$argAST && $argDef->getType() instanceof NonNull) {
+                            $errors[] = new Error(
+                                self::missingDirectiveArgMessage($directiveAST->name->value, $argDef->name, $argDef->getType()),
+                                [$directiveAST]
+                            );
+                        }
+                    }
+                    if (!empty($errors)) {
+                        return $errors;
+                    }
+                }
+            ]
+        ];
+    }
+}
diff --git a/src/Validator/Rules/ScalarLeafs.php b/src/Validator/Rules/ScalarLeafs.php
index 898aed3..58238bd 100644
--- a/src/Validator/Rules/ScalarLeafs.php
+++ b/src/Validator/Rules/ScalarLeafs.php
@@ -11,6 +11,16 @@ use GraphQL\Validator\ValidationContext;
 
 class ScalarLeafs
 {
+    static function noSubselectionAllowedMessage($field, $type)
+    {
+        return "Field \"$field\" of type \"$type\" must not have a sub selection.";
+    }
+
+    static function requiredSubselectionMessage($field, $type)
+    {
+        return "Field \"$field\" of type \"$type\" must have a sub selection.";
+    }
+
     public function __invoke(ValidationContext $context)
     {
         return [
@@ -20,13 +30,13 @@ class ScalarLeafs
                     if (Type::isLeafType($type)) {
                         if ($node->selectionSet) {
                             return new Error(
-                                Messages::noSubselectionAllowedMessage($node->name->value, $type),
+                                self::noSubselectionAllowedMessage($node->name->value, $type),
                                 [$node->selectionSet]
                             );
                         }
                     } else if (!$node->selectionSet) {
                         return new Error(
-                            Messages::requiredSubselectionMessage($node->name->value, $type),
+                            self::requiredSubselectionMessage($node->name->value, $type),
                             [$node]
                         );
                     }
diff --git a/src/Validator/Rules/VariablesAreInputTypes.php b/src/Validator/Rules/VariablesAreInputTypes.php
index 7eca699..faca0ce 100644
--- a/src/Validator/Rules/VariablesAreInputTypes.php
+++ b/src/Validator/Rules/VariablesAreInputTypes.php
@@ -4,40 +4,35 @@ namespace GraphQL\Validator\Rules;
 
 use GraphQL\Error;
 use GraphQL\Language\AST\Node;
-use GraphQL\Language\AST\Type;
 use GraphQL\Language\AST\VariableDefinition;
 use GraphQL\Language\Printer;
 use GraphQL\Type\Definition\InputType;
+use GraphQL\Type\Definition\Type;
 use GraphQL\Utils;
-use GraphQL\Validator\Messages;
 use GraphQL\Validator\ValidationContext;
 
 class VariablesAreInputTypes
 {
+    static function nonInputTypeOnVarMessage($variableName, $typeName)
+    {
+        return "Variable \"\$$variableName\" cannot be non-input type \"$typeName\".";
+    }
+
     public function __invoke(ValidationContext $context)
     {
         return [
             Node::VARIABLE_DEFINITION => function(VariableDefinition $node) use ($context) {
-                $typeName = $this->getTypeASTName($node->type);
-                $type = $context->getSchema()->getType($typeName);
+                $type = Utils\TypeInfo::typeFromAST($context->getSchema(), $node->type);
 
-                if (!($type instanceof InputType)) {
+                // If the variable type is not an input type, return an error.
+                if ($type && !Type::isInputType($type)) {
                     $variableName = $node->variable->name->value;
                     return new Error(
-                        Messages::nonInputTypeOnVarMessage($variableName, Printer::doPrint($node->type)),
-                        [$node->type]
+                        self::nonInputTypeOnVarMessage($variableName, Printer::doPrint($node->type)),
+                        [ $node->type ]
                     );
                 }
             }
         ];
     }
-
-    private function getTypeASTName(Type $typeAST)
-    {
-        if ($typeAST->kind === Node::NAME) {
-            return $typeAST->value;
-        }
-        Utils::invariant($typeAST->type, 'Must be wrapping type');
-        return $this->getTypeASTName($typeAST->type);
-    }
 }
diff --git a/src/Validator/ValidationContext.php b/src/Validator/ValidationContext.php
index eaac972..27d56f1 100644
--- a/src/Validator/ValidationContext.php
+++ b/src/Validator/ValidationContext.php
@@ -113,4 +113,14 @@ class ValidationContext
     {
         return $this->_typeInfo->getFieldDef();
     }
+
+    function getDirective()
+    {
+        return $this->_typeInfo->getDirective();
+    }
+
+    function getArgument()
+    {
+        return $this->_typeInfo->getArgument();
+    }
 }
diff --git a/tests/Executor/AbstractTest.php b/tests/Executor/AbstractTest.php
new file mode 100644
index 0000000..378810f
--- /dev/null
+++ b/tests/Executor/AbstractTest.php
@@ -0,0 +1,244 @@
+<?php
+namespace GraphQL\Executor;
+
+require_once __DIR__ . '/TestClasses.php';
+
+use GraphQL\Language\Parser;
+use GraphQL\Schema;
+use GraphQL\Type\Definition\InterfaceType;
+use GraphQL\Type\Definition\ObjectType;
+use GraphQL\Type\Definition\Type;
+use GraphQL\Type\Definition\UnionType;
+
+class AbstractTest extends \PHPUnit_Framework_TestCase
+{
+
+    // Execute: Handles execution of abstract types
+    public function testIsTypeOfUsedToResolveRuntimeTypeForInterface()
+    {
+        // isTypeOf used to resolve runtime type for Interface
+        $petType = new InterfaceType([
+            'name' => 'Pet',
+            'fields' => [
+                'name' => ['type' => Type::string()]
+            ]
+        ]);
+
+        // Added to interface type when defined
+        $dogType = new ObjectType([
+            'name' => 'Dog',
+            'interfaces' => [$petType],
+            'isTypeOf' => function($obj) { return $obj instanceof Dog; },
+            'fields' => [
+                'name' => ['type' => Type::string()],
+                'woofs' => ['type' => Type::boolean()]
+            ]
+        ]);
+
+        $catType = new ObjectType([
+            'name' => 'Cat',
+            'interfaces' => [$petType],
+            'isTypeOf' => function ($obj) {
+                return $obj instanceof Cat;
+            },
+            'fields' => [
+                'name' => ['type' => Type::string()],
+                'meows' => ['type' => Type::boolean()],
+            ]
+        ]);
+
+        $schema = new Schema(
+            new ObjectType([
+                'name' => 'Query',
+                'fields' => [
+                    'pets' => [
+                        'type' => Type::listOf($petType),
+                        'resolve' => function () {
+                            return [new Dog('Odie', true), new Cat('Garfield', false)];
+                        }
+                    ]
+                ]
+            ])
+        );
+
+        $query = '{
+          pets {
+            name
+            ... on Dog {
+              woofs
+            }
+            ... on Cat {
+              meows
+            }
+          }
+        }';
+
+        $expected = new ExecutionResult([
+            'pets' => [
+                ['name' => 'Odie', 'woofs' => true],
+                ['name' => 'Garfield', 'meows' => false]
+            ]
+        ]);
+
+        $this->assertEquals($expected, Executor::execute($schema, Parser::parse($query)));
+    }
+
+    public function testIsTypeOfUsedToResolveRuntimeTypeForUnion()
+    {
+        // isTypeOf used to resolve runtime type for Union
+
+        $dogType = new ObjectType([
+            'name' => 'Dog',
+            'isTypeOf' => function($obj) { return $obj instanceof Dog; },
+            'fields' => [
+                'name' => ['type' => Type::string()],
+                'woofs' => ['type' => Type::boolean()]
+            ]
+        ]);
+
+        $catType = new ObjectType([
+            'name' => 'Cat',
+            'isTypeOf' => function ($obj) {
+                return $obj instanceof Cat;
+            },
+            'fields' => [
+                'name' => ['type' => Type::string()],
+                'meows' => ['type' => Type::boolean()],
+            ]
+        ]);
+
+        $petType = new UnionType([
+            'name' => 'Pet',
+            'types' => [$dogType, $catType]
+        ]);
+
+        $schema = new Schema(new ObjectType([
+            'name' => 'Query',
+            'fields' => [
+                'pets' => [
+                    'type' => Type::listOf($petType),
+                    'resolve' => function() {
+                        return [ new Dog('Odie', true), new Cat('Garfield', false) ];
+                    }
+                ]
+            ]
+        ]));
+
+        $query = '{
+          pets {
+            name
+            ... on Dog {
+              woofs
+            }
+            ... on Cat {
+              meows
+            }
+          }
+        }';
+
+        $expected = new ExecutionResult([
+            'pets' => [
+                ['name' => 'Odie', 'woofs' => true],
+                ['name' => 'Garfield', 'meows' => false]
+            ]
+        ]);
+
+        $this->assertEquals($expected, Executor::execute($schema, Parser::parse($query)));
+    }
+
+    function testResolveTypeOnInterfaceYieldsUsefulError()
+    {
+        $DogType = null;
+        $CatType = null;
+        $HumanType = null;
+
+        $PetType = new InterfaceType([
+            'name' => 'Pet',
+            'resolveType' => function ($obj) use (&$DogType, &$CatType, &$HumanType) {
+                if ($obj instanceof Dog) {
+                    return $DogType;
+                }
+                if ($obj instanceof Cat) {
+                    return $CatType;
+                }
+                if ($obj instanceof Human) {
+                    return $HumanType;
+                }
+                return null;
+            },
+            'fields' => [
+                'name' => ['type' => Type::string()]
+            ]
+        ]);
+
+        $HumanType = new ObjectType([
+            'name' => 'Human',
+            'fields' => [
+                'name' => ['type' => Type::string()],
+            ]
+        ]);
+
+        $DogType = new ObjectType([
+            'name' => 'Dog',
+            'interfaces' => [$PetType],
+            'fields' => [
+                'name' => ['type' => Type::string()],
+                'woofs' => ['type' => Type::boolean()],
+            ]
+        ]);
+
+        $CatType = new ObjectType([
+            'name' => 'Cat',
+            'interfaces' => [$PetType],
+            'fields' => [
+                'name' => ['type' => Type::string()],
+                'meows' => ['type' => Type::boolean()],
+            ]
+        ]);
+
+        $schema = new Schema(new ObjectType([
+            'name' => 'Query',
+            'fields' => [
+                'pets' => [
+                    'type' => Type::listOf($PetType),
+                    'resolve' => function () {
+                        return [
+                            new Dog('Odie', true),
+                            new Cat('Garfield', false),
+                            new Human('Jon')
+                        ];
+                    }
+                ]
+            ]
+        ]));
+
+
+        $query = '{
+          pets {
+            name
+            ... on Dog {
+              woofs
+            }
+            ... on Cat {
+              meows
+            }
+          }
+        }';
+
+        $expected = [
+            'data' => [
+                'pets' => [
+                    ['name' => 'Odie', 'woofs' => true],
+                    ['name' => 'Garfield', 'meows' => false],
+                    null
+                ]
+            ],
+            'errors' => [
+                [ 'message' => 'Runtime Object type "Human" is not a possible type for "Pet".' ]
+            ]
+        ];
+
+        $this->assertEquals($expected, Executor::execute($schema, Parser::parse($query))->toArray());
+    }
+
+}
diff --git a/tests/Executor/DirectivesTest.php b/tests/Executor/DirectivesTest.php
index 7c1df7b..78bc564 100644
--- a/tests/Executor/DirectivesTest.php
+++ b/tests/Executor/DirectivesTest.php
@@ -19,16 +19,16 @@ class DirectivesTest extends \PHPUnit_Framework_TestCase
     public function testWorksOnScalars()
     {
         // if true includes scalar
-        $this->assertEquals(['data' => ['a' => 'a', 'b' => 'b']], $this->executeTestQuery('{ a, b @if:true }'));
+        $this->assertEquals(['data' => ['a' => 'a', 'b' => 'b']], $this->executeTestQuery('{ a, b @include(if: true) }'));
 
         // if false omits on scalar
-        $this->assertEquals(['data' => ['a' => 'a']], $this->executeTestQuery('{ a, b @if:false }'));
+        $this->assertEquals(['data' => ['a' => 'a']], $this->executeTestQuery('{ a, b @include(if: false) }'));
 
         // unless false includes scalar
-        $this->assertEquals(['data' => ['a' => 'a', 'b' => 'b']], $this->executeTestQuery('{ a, b @unless:false }'));
+        $this->assertEquals(['data' => ['a' => 'a', 'b' => 'b']], $this->executeTestQuery('{ a, b @skip(if: false) }'));
 
         // unless true omits scalar
-        $this->assertEquals(['data' => ['a' => 'a']], $this->executeTestQuery('{ a, b @unless:true }'));
+        $this->assertEquals(['data' => ['a' => 'a']], $this->executeTestQuery('{ a, b @skip(if: true) }'));
     }
 
     public function testWorksOnFragmentSpreads()
@@ -37,7 +37,7 @@ class DirectivesTest extends \PHPUnit_Framework_TestCase
         $q = '
         query Q {
           a
-          ...Frag @if:false
+          ...Frag @include(if: false)
         }
         fragment Frag on TestType {
           b
@@ -49,7 +49,7 @@ class DirectivesTest extends \PHPUnit_Framework_TestCase
         $q = '
         query Q {
           a
-          ...Frag @if:true
+          ...Frag @include(if: true)
         }
         fragment Frag on TestType {
           b
@@ -61,7 +61,7 @@ class DirectivesTest extends \PHPUnit_Framework_TestCase
         $q = '
         query Q {
           a
-          ...Frag @unless:false
+          ...Frag @skip(if: false)
         }
         fragment Frag on TestType {
           b
@@ -73,7 +73,7 @@ class DirectivesTest extends \PHPUnit_Framework_TestCase
         $q = '
         query Q {
           a
-          ...Frag @unless:true
+          ...Frag @skip(if: true)
         }
         fragment Frag on TestType {
           b
@@ -88,7 +88,7 @@ class DirectivesTest extends \PHPUnit_Framework_TestCase
         $q = '
         query Q {
           a
-          ... on TestType @if:false {
+          ... on TestType @include(if: false) {
             b
           }
         }
@@ -102,7 +102,7 @@ class DirectivesTest extends \PHPUnit_Framework_TestCase
         $q = '
         query Q {
           a
-          ... on TestType @if:true {
+          ... on TestType @include(if: true) {
             b
           }
         }
@@ -116,7 +116,7 @@ class DirectivesTest extends \PHPUnit_Framework_TestCase
         $q = '
         query Q {
           a
-          ... on TestType @unless:false {
+          ... on TestType @skip(if: false) {
             b
           }
         }
@@ -130,7 +130,7 @@ class DirectivesTest extends \PHPUnit_Framework_TestCase
         $q = '
         query Q {
           a
-          ... on TestType @unless:true {
+          ... on TestType @skip(if: true) {
             b
           }
         }
@@ -149,7 +149,7 @@ class DirectivesTest extends \PHPUnit_Framework_TestCase
           a
           ...Frag
         }
-        fragment Frag on TestType @if:false {
+        fragment Frag on TestType @include(if: false) {
           b
         }
         ';
@@ -161,7 +161,7 @@ class DirectivesTest extends \PHPUnit_Framework_TestCase
           a
           ...Frag
         }
-        fragment Frag on TestType @if:true {
+        fragment Frag on TestType @include(if: true) {
           b
         }
         ';
@@ -173,7 +173,7 @@ class DirectivesTest extends \PHPUnit_Framework_TestCase
           a
           ...Frag
         }
-        fragment Frag on TestType @unless:false {
+        fragment Frag on TestType @skip(if: false) {
           b
         }
         ';
@@ -185,7 +185,7 @@ class DirectivesTest extends \PHPUnit_Framework_TestCase
           a
           ...Frag
         }
-        fragment Frag on TestType @unless:true {
+        fragment Frag on TestType @skip(if: true) {
           b
         }
         ';
@@ -213,13 +213,13 @@ class DirectivesTest extends \PHPUnit_Framework_TestCase
     private static function getData()
     {
         return self::$data ?: (self::$data = [
-            'a' => function() { return 'a'; },
-            'b' => function() { return 'b'; }
+            'a' => 'a',
+            'b' => 'b'
         ]);
     }
 
     private function executeTestQuery($doc)
     {
-        return Executor::execute(self::getSchema(), self::getData(), Parser::parse($doc));
+        return Executor::execute(self::getSchema(), Parser::parse($doc), self::getData())->toArray();
     }
 }
diff --git a/tests/Executor/ExecutorSchemaTest.php b/tests/Executor/ExecutorSchemaTest.php
index 6240dfe..8b23ae5 100644
--- a/tests/Executor/ExecutorSchemaTest.php
+++ b/tests/Executor/ExecutorSchemaTest.php
@@ -168,7 +168,7 @@ class ExecutorSchemaTest extends \PHPUnit_Framework_TestCase
             ]
         ];
 
-        $this->assertEquals($expected, Executor::execute($BlogSchema, null, Parser::parse($request), '', []));
+        $this->assertEquals($expected, Executor::execute($BlogSchema, Parser::parse($request))->toArray());
     }
 
     private function article($id)
diff --git a/tests/Executor/ExecutorTest.php b/tests/Executor/ExecutorTest.php
index 29ddf59..76d9784 100644
--- a/tests/Executor/ExecutorTest.php
+++ b/tests/Executor/ExecutorTest.php
@@ -1,6 +1,7 @@
 <?php
 namespace GraphQL\Executor;
 
+use GraphQL\Error;
 use GraphQL\FormattedError;
 use GraphQL\Language\Parser;
 use GraphQL\Language\SourceLocation;
@@ -132,7 +133,7 @@ class ExecutorTest extends \PHPUnit_Framework_TestCase
         ]);
         $schema = new Schema($dataType);
 
-        $this->assertEquals($expected, Executor::execute($schema, $data, $ast, 'Example', ['size' => 100]));
+        $this->assertEquals($expected, Executor::execute($schema, $ast, $data, ['size' => 100], 'Example')->toArray());
     }
 
     public function testMergesParallelFragments()
@@ -187,11 +188,12 @@ class ExecutorTest extends \PHPUnit_Framework_TestCase
             ]
         ];
 
-        $this->assertEquals($expected, Executor::execute($schema, null, $ast));
+        $this->assertEquals($expected, Executor::execute($schema, $ast)->toArray());
     }
 
     public function testThreadsContextCorrectly()
     {
+        // threads context correctly
         $doc = 'query Example { a }';
 
         $gotHere = false;
@@ -214,7 +216,7 @@ class ExecutorTest extends \PHPUnit_Framework_TestCase
             ]
         ]));
 
-        Executor::execute($schema, $data, $ast, 'Example', []);
+        Executor::execute($schema, $ast, $data, [], 'Example');
         $this->assertEquals(true, $gotHere);
     }
 
@@ -246,7 +248,7 @@ class ExecutorTest extends \PHPUnit_Framework_TestCase
                 ]
             ]
         ]));
-        Executor::execute($schema, null, $docAst, 'Example', []);
+        Executor::execute($schema, $docAst, null, [], 'Example');
         $this->assertSame($gotHere, true);
     }
 
@@ -255,6 +257,7 @@ class ExecutorTest extends \PHPUnit_Framework_TestCase
         $doc = '{
       sync,
       syncError,
+      syncRawError,
       async,
       asyncReject,
       asyncError
@@ -265,9 +268,11 @@ class ExecutorTest extends \PHPUnit_Framework_TestCase
                 return 'sync';
             },
             'syncError' => function () {
-                throw new \Exception('Error getting syncError');
+                throw new Error('Error getting syncError');
+            },
+            'syncRawError' => function() {
+                throw new \Exception('Error getting syncRawError');
             },
-
             // Following are inherited from JS reference implementation, but make no sense in this PHP impl
             // leaving them just to simplify migrations from newer js versions
             'async' => function() {
@@ -287,6 +292,7 @@ class ExecutorTest extends \PHPUnit_Framework_TestCase
             'fields' => [
                 'sync' => ['type' => Type::string()],
                 'syncError' => ['type' => Type::string()],
+                'syncRawError' => [ 'type' => Type::string() ],
                 'async' => ['type' => Type::string()],
                 'asyncReject' => ['type' => Type::string() ],
                 'asyncError' => ['type' => Type::string()],
@@ -297,20 +303,22 @@ class ExecutorTest extends \PHPUnit_Framework_TestCase
             'data' => [
                 'sync' => 'sync',
                 'syncError' => null,
+                'syncRawError' => null,
                 'async' => 'async',
                 'asyncReject' => null,
                 'asyncError' => null,
             ],
             'errors' => [
-                new FormattedError('Error getting syncError', [new SourceLocation(3, 7)]),
-                new FormattedError('Error getting asyncReject', [new SourceLocation(5, 7)]),
-                new FormattedError('Error getting asyncError', [new SourceLocation(6, 7)])
+                FormattedError::create('Error getting syncError', [new SourceLocation(3, 7)]),
+                FormattedError::create('Error getting syncRawError', [new SourceLocation(4, 7)]),
+                FormattedError::create('Error getting asyncReject', [new SourceLocation(6, 7)]),
+                FormattedError::create('Error getting asyncError', [new SourceLocation(7, 7)])
             ]
         ];
 
-        $result = Executor::execute($schema, $data, $docAst);
+        $result = Executor::execute($schema, $docAst, $data);
 
-        $this->assertEquals($expected, $result);
+        $this->assertEquals($expected, $result->toArray());
     }
 
     public function testUsesTheInlineOperationIfNoOperationIsProvided()
@@ -326,9 +334,9 @@ class ExecutorTest extends \PHPUnit_Framework_TestCase
             ]
         ]));
 
-        $ex = Executor::execute($schema, $data, $ast);
+        $ex = Executor::execute($schema, $ast, $data);
 
-        $this->assertEquals(['data' => ['a' => 'b']], $ex);
+        $this->assertEquals(['data' => ['a' => 'b']], $ex->toArray());
     }
 
     public function testUsesTheOnlyOperationIfNoOperationIsProvided()
@@ -343,8 +351,8 @@ class ExecutorTest extends \PHPUnit_Framework_TestCase
             ]
         ]));
 
-        $ex = Executor::execute($schema, $data, $ast);
-        $this->assertEquals(['data' => ['a' => 'b']], $ex);
+        $ex = Executor::execute($schema, $ast, $data);
+        $this->assertEquals(['data' => ['a' => 'b']], $ex->toArray());
     }
 
     public function testThrowsIfNoOperationIsProvidedWithMultipleOperations()
@@ -359,15 +367,12 @@ class ExecutorTest extends \PHPUnit_Framework_TestCase
             ]
         ]));
 
-        $ex = Executor::execute($schema, $data, $ast);
-
-        $this->assertEquals(
-            [
-                'data' => null,
-                'errors' => [new FormattedError('Must provide operation name if query contains multiple operations')]
-            ],
-            $ex
-        );
+        try {
+            Executor::execute($schema, $ast, $data);
+            $this->fail('Expected exception is not thrown');
+        } catch (Error $err) {
+            $this->assertEquals('Must provide operation name if query contains multiple operations.', $err->getMessage());
+        }
     }
 
     public function testUsesTheQuerySchemaForQueries()
@@ -390,8 +395,8 @@ class ExecutorTest extends \PHPUnit_Framework_TestCase
             ])
         );
 
-        $queryResult = Executor::execute($schema, $data, $ast, 'Q');
-        $this->assertEquals(['data' => ['a' => 'b']], $queryResult);
+        $queryResult = Executor::execute($schema, $ast, $data, [], 'Q');
+        $this->assertEquals(['data' => ['a' => 'b']], $queryResult->toArray());
     }
 
     public function testUsesTheMutationSchemaForMutations()
@@ -413,8 +418,8 @@ class ExecutorTest extends \PHPUnit_Framework_TestCase
                 ]
             ])
         );
-        $mutationResult = Executor::execute($schema, $data, $ast, 'M');
-        $this->assertEquals(['data' => ['c' => 'd']], $mutationResult);
+        $mutationResult = Executor::execute($schema, $ast, $data, [], 'M');
+        $this->assertEquals(['data' => ['c' => 'd']], $mutationResult->toArray());
     }
 
     public function testAvoidsRecursion()
@@ -440,8 +445,8 @@ class ExecutorTest extends \PHPUnit_Framework_TestCase
             ]
         ]));
 
-        $queryResult = Executor::execute($schema, $data, $ast, 'Q');
-        $this->assertEquals(['data' => ['a' => 'b']], $queryResult);
+        $queryResult = Executor::execute($schema, $ast, $data, [], 'Q');
+        $this->assertEquals(['data' => ['a' => 'b']], $queryResult->toArray());
     }
 
     public function testDoesNotIncludeIllegalFieldsInOutput()
@@ -464,7 +469,106 @@ class ExecutorTest extends \PHPUnit_Framework_TestCase
                 ]
             ])
         );
-        $mutationResult = Executor::execute($schema, null, $ast);
-        $this->assertEquals(['data' => []], $mutationResult);
+        $mutationResult = Executor::execute($schema, $ast);
+        $this->assertEquals(['data' => []], $mutationResult->toArray());
+    }
+
+    public function testDoesNotIncludeArgumentsThatWereNotSet()
+    {
+        $schema = new Schema(
+            new ObjectType([
+                'name' => 'Type',
+                'fields' => [
+                    'field' => [
+                        'type' => Type::string(),
+                        'resolve' => function($data, $args) {return $args ? json_encode($args) : '';},
+                        'args' => [
+                            'a' => ['type' => Type::boolean()],
+                            'b' => ['type' => Type::boolean()],
+                            'c' => ['type' => Type::boolean()],
+                            'd' => ['type' => Type::int()],
+                            'e' => ['type' => Type::int()]
+                        ]
+                    ]
+                ]
+            ])
+        );
+
+        $query = Parser::parse('{ field(a: true, c: false, e: 0) }');
+        $result = Executor::execute($schema, $query);
+        $expected = [
+            'data' => [
+                'field' => '{"a":true,"c":false,"e":0}'
+            ]
+        ];
+
+        $this->assertEquals($expected, $result->toArray());
+
+/*
+    var query = parse('{ field(a: true, c: false, e: 0) }');
+    var result = await execute(schema, query);
+
+    expect(result).to.deep.equal({
+      data: {
+        field: '{"a":true,"c":false,"e":0}'
+      }
+    });
+  });
+
+  it('fails when an isTypeOf check is not met', async () => {
+    class Special {
+      constructor(value) {
+        this.value = value;
+      }
+    }
+
+    class NotSpecial {
+      constructor(value) {
+        this.value = value;
+      }
+    }
+
+    var SpecialType = new GraphQLObjectType({
+      name: 'SpecialType',
+      isTypeOf(obj) {
+        return obj instanceof Special;
+      },
+      fields: {
+        value: { type: GraphQLString }
+      }
+    });
+
+    var schema = new GraphQLSchema({
+      query: new GraphQLObjectType({
+        name: 'Query',
+        fields: {
+          specials: {
+            type: new GraphQLList(SpecialType),
+            resolve: rootValue => rootValue.specials
+          }
+        }
+      })
+    });
+
+    var query = parse('{ specials { value } }');
+    var value = {
+      specials: [ new Special('foo'), new NotSpecial('bar') ]
+    };
+    var result = await execute(schema, query, value);
+
+    expect(result.data).to.deep.equal({
+      specials: [
+        { value: 'foo' },
+        null
+      ]
+    });
+    expect(result.errors).to.have.lengthOf(1);
+    expect(result.errors).to.containSubset([
+      { message:
+          'Expected value of type "SpecialType" but got: [object Object].',
+        locations: [ { line: 1, column: 3 } ] }
+    ]);
+  });
+ */
     }
 }
diff --git a/tests/Executor/ListsTest.php b/tests/Executor/ListsTest.php
index 36b0294..25a9bd1 100644
--- a/tests/Executor/ListsTest.php
+++ b/tests/Executor/ListsTest.php
@@ -1,6 +1,7 @@
 <?php
 namespace GraphQL\Executor;
 
+use GraphQL\Error;
 use GraphQL\FormattedError;
 use GraphQL\Language\Parser;
 use GraphQL\Language\SourceLocation;
@@ -24,7 +25,7 @@ class ListsTest extends \PHPUnit_Framework_TestCase
 
         $ast = Parser::parse($doc);
         $expected = ['data' => ['nest' => ['list' => [1,2]]]];
-        $this->assertEquals($expected, Executor::execute($this->schema(), $this->data(), $ast, 'Q', []));
+        $this->assertEquals($expected, Executor::execute($this->schema(), $ast, $this->data(), [], 'Q')->toArray());
     }
 
     public function testHandlesListsOfNonNullsWhenTheyReturnNonNullValues()
@@ -47,7 +48,7 @@ class ListsTest extends \PHPUnit_Framework_TestCase
             ]
         ];
 
-        $this->assertEquals($expected, Executor::execute($this->schema(), $this->data(), $ast, 'Q', []));
+        $this->assertEquals($expected, Executor::execute($this->schema(), $ast, $this->data(), [], 'Q')->toArray());
     }
 
     public function testHandlesNonNullListsOfWhenTheyReturnNonNullValues()
@@ -69,7 +70,7 @@ class ListsTest extends \PHPUnit_Framework_TestCase
                 ]
             ]
         ];
-        $this->assertEquals($expected, Executor::execute($this->schema(), $this->data(), $ast, 'Q', []));
+        $this->assertEquals($expected, Executor::execute($this->schema(), $ast, $this->data(), [], 'Q')->toArray());
     }
 
     public function testHandlesNonNullListsOfNonNullsWhenTheyReturnNonNullValues()
@@ -91,7 +92,7 @@ class ListsTest extends \PHPUnit_Framework_TestCase
                 ]
             ]
         ];
-        $this->assertEquals($expected, Executor::execute($this->schema(), $this->data(), $ast, 'Q', []));
+        $this->assertEquals($expected, Executor::execute($this->schema(), $ast, $this->data(), [], 'Q')->toArray());
     }
 
     public function testHandlesListsWhenTheyReturnNullAsAValue()
@@ -113,7 +114,7 @@ class ListsTest extends \PHPUnit_Framework_TestCase
                 ]
             ]
         ];
-        $this->assertEquals($expected, Executor::execute($this->schema(), $this->data(), $ast, 'Q', []));
+        $this->assertEquals($expected, Executor::execute($this->schema(), $ast, $this->data(), [], 'Q')->toArray());
     }
 
     public function testHandlesListsOfNonNullsWhenTheyReturnNullAsAValue()
@@ -135,13 +136,13 @@ class ListsTest extends \PHPUnit_Framework_TestCase
                 ]
             ],
             'errors' => [
-                new FormattedError(
+                FormattedError::create(
                     'Cannot return null for non-nullable type.',
                     [new SourceLocation(4, 11)]
                 )
             ]
         ];
-        $this->assertEquals($expected, Executor::execute($this->schema(), $this->data(), $ast, 'Q', []));
+        $this->assertEquals($expected, Executor::execute($this->schema(), $ast, $this->data(), [], 'Q')->toArray());
     }
 
     public function testHandlesNonNullListsOfWhenTheyReturnNullAsAValue()
@@ -160,7 +161,7 @@ class ListsTest extends \PHPUnit_Framework_TestCase
                 'nest' => ['nonNullListContainsNull' => [1, null, 2]]
             ]
         ];
-        $this->assertEquals($expected, Executor::execute($this->schema(), $this->data(), $ast, 'Q', []));
+        $this->assertEquals($expected, Executor::execute($this->schema(), $ast, $this->data(), [], 'Q')->toArray());
     }
 
     public function testHandlesNonNullListsOfNonNullsWhenTheyReturnNullAsAValue()
@@ -180,13 +181,13 @@ class ListsTest extends \PHPUnit_Framework_TestCase
                 'nest' => null
             ],
             'errors' => [
-                new FormattedError(
+                FormattedError::create(
                     'Cannot return null for non-nullable type.',
                     [new SourceLocation(4, 11)]
                 )
             ]
         ];
-        $this->assertEquals($expected, Executor::execute($this->schema(), $this->data(), $ast, 'Q', []));
+        $this->assertEquals($expected, Executor::execute($this->schema(), $ast, $this->data(), [], 'Q')->toArray());
     }
 
     public function testHandlesListsWhenTheyReturnNull()
@@ -208,7 +209,7 @@ class ListsTest extends \PHPUnit_Framework_TestCase
                 ]
             ]
         ];
-        $this->assertEquals($expected, Executor::execute($this->schema(), $this->data(), $ast, 'Q', []));
+        $this->assertEquals($expected, Executor::execute($this->schema(), $ast, $this->data(), [], 'Q')->toArray());
     }
 
     public function testHandlesListsOfNonNullsWhenTheyReturnNull()
@@ -230,7 +231,7 @@ class ListsTest extends \PHPUnit_Framework_TestCase
                 ]
             ]
         ];
-        $this->assertEquals($expected, Executor::execute($this->schema(), $this->data(), $ast, 'Q', []));
+        $this->assertEquals($expected, Executor::execute($this->schema(), $ast, $this->data(), [], 'Q')->toArray());
     }
 
     public function testHandlesNonNullListsOfWhenTheyReturnNull()
@@ -250,13 +251,13 @@ class ListsTest extends \PHPUnit_Framework_TestCase
                 'nest' => null,
             ],
             'errors' => [
-                new FormattedError(
+                FormattedError::create(
                     'Cannot return null for non-nullable type.',
                     [new SourceLocation(4, 11)]
                 )
             ]
         ];
-        $this->assertEquals($expected, Executor::execute($this->schema(), $this->data(), $ast, 'Q', []));
+        $this->assertEquals($expected, Executor::execute($this->schema(), $ast, $this->data(), [], 'Q')->toArray());
     }
 
     public function testHandlesNonNullListsOfNonNullsWhenTheyReturnNull()
@@ -276,13 +277,13 @@ class ListsTest extends \PHPUnit_Framework_TestCase
                 'nest' => null
             ],
             'errors' => [
-                new FormattedError(
+                FormattedError::create(
                     'Cannot return null for non-nullable type.',
                     [new SourceLocation(4, 11)]
                 )
             ]
         ];
-        $this->assertEquals($expected, Executor::execute($this->schema(), $this->data(), $ast, 'Q', []));
+        $this->assertEquals($expected, Executor::execute($this->schema(), $ast, $this->data(), [], 'Q')->toArray());
     }
 
 
diff --git a/tests/Executor/MutationsTest.php b/tests/Executor/MutationsTest.php
index 35e5653..768d4a3 100644
--- a/tests/Executor/MutationsTest.php
+++ b/tests/Executor/MutationsTest.php
@@ -31,7 +31,7 @@ class MutationsTest extends \PHPUnit_Framework_TestCase
       }
     }';
         $ast = Parser::parse($doc);
-        $mutationResult = Executor::execute($this->schema(), new Root(6), $ast, 'M');
+        $mutationResult = Executor::execute($this->schema(), $ast, new Root(6), null, 'M');
         $expected = [
             'data' => [
                 'first' => [
@@ -51,7 +51,7 @@ class MutationsTest extends \PHPUnit_Framework_TestCase
                 ]
             ]
         ];
-        $this->assertEquals($mutationResult, $expected);
+        $this->assertEquals($expected, $mutationResult->toArray());
     }
 
     public function testEvaluatesMutationsCorrectlyInThePresenseOfAFailedMutation()
@@ -77,7 +77,7 @@ class MutationsTest extends \PHPUnit_Framework_TestCase
       }
     }';
         $ast = Parser::parse($doc);
-        $mutationResult = Executor::execute($this->schema(), new Root(6), $ast, 'M');
+        $mutationResult = Executor::execute($this->schema(), $ast, new Root(6), null, 'M');
         $expected = [
             'data' => [
                 'first' => [
@@ -96,17 +96,17 @@ class MutationsTest extends \PHPUnit_Framework_TestCase
                 'sixth' => null,
             ],
             'errors' => [
-                new FormattedError(
+                FormattedError::create(
                     'Cannot change the number',
                     [new SourceLocation(8, 7)]
                 ),
-                new FormattedError(
+                FormattedError::create(
                     'Cannot change the number',
                     [new SourceLocation(17, 7)]
                 )
             ]
         ];
-        $this->assertEquals($expected, $mutationResult);
+        $this->assertEquals($expected, $mutationResult->toArray());
     }
 
     private function schema()
diff --git a/tests/Executor/NonNullTest.php b/tests/Executor/NonNullTest.php
index 76e00cc..4cd9fe6 100644
--- a/tests/Executor/NonNullTest.php
+++ b/tests/Executor/NonNullTest.php
@@ -86,13 +86,13 @@ class NonNullTest extends \PHPUnit_Framework_TestCase
                 'sync' => null,
             ],
             'errors' => [
-                new FormattedError(
+                FormattedError::create(
                     $this->syncError->message,
                     [new SourceLocation(3, 9)]
                 )
             ]
         ];
-        $this->assertEquals($expected, Executor::execute($this->schema, $this->throwingData, $ast, 'Q', []));
+        $this->assertEquals($expected, Executor::execute($this->schema, $ast, $this->throwingData, [], 'Q')->toArray());
     }
 
     public function testNullsASynchronouslyReturnedObjectThatContainsANonNullableFieldThatThrowsSynchronously()
@@ -113,10 +113,10 @@ class NonNullTest extends \PHPUnit_Framework_TestCase
                 'nest' => null
             ],
             'errors' => [
-                new FormattedError($this->nonNullSyncError->message, [new SourceLocation(4, 11)])
+                FormattedError::create($this->nonNullSyncError->message, [new SourceLocation(4, 11)])
             ]
         ];
-        $this->assertEquals($expected, Executor::execute($this->schema, $this->throwingData, $ast, 'Q', []));
+        $this->assertEquals($expected, Executor::execute($this->schema, $ast, $this->throwingData, [], 'Q')->toArray());
     }
 
     public function testNullsAComplexTreeOfNullableFieldsThatThrow()
@@ -144,11 +144,11 @@ class NonNullTest extends \PHPUnit_Framework_TestCase
                 ]
             ],
             'errors' => [
-                new FormattedError($this->syncError->message, [new SourceLocation(4, 11)]),
-                new FormattedError($this->syncError->message, [new SourceLocation(6, 13)]),
+                FormattedError::create($this->syncError->message, [new SourceLocation(4, 11)]),
+                FormattedError::create($this->syncError->message, [new SourceLocation(6, 13)]),
             ]
         ];
-        $this->assertEquals($expected, Executor::execute($this->schema, $this->throwingData, $ast, 'Q', []));
+        $this->assertEquals($expected, Executor::execute($this->schema, $ast, $this->throwingData, [], 'Q')->toArray());
     }
 
     public function testNullsANullableFieldThatSynchronouslyReturnsNull()
@@ -166,7 +166,7 @@ class NonNullTest extends \PHPUnit_Framework_TestCase
                 'sync' => null,
             ]
         ];
-        $this->assertEquals($expected, Executor::execute($this->schema, $this->nullingData, $ast, 'Q', []));
+        $this->assertEquals($expected, Executor::execute($this->schema, $ast, $this->nullingData, [], 'Q')->toArray());
     }
 
     public function test4()
@@ -187,10 +187,10 @@ class NonNullTest extends \PHPUnit_Framework_TestCase
                 'nest' => null
             ],
             'errors' => [
-                new FormattedError('Cannot return null for non-nullable type.', [new SourceLocation(4, 11)])
+                FormattedError::create('Cannot return null for non-nullable type.', [new SourceLocation(4, 11)])
             ]
         ];
-        $this->assertEquals($expected, Executor::execute($this->schema, $this->nullingData, $ast, 'Q', []));
+        $this->assertEquals($expected, Executor::execute($this->schema, $ast, $this->nullingData, [], 'Q')->toArray());
     }
 
     public function test5()
@@ -226,7 +226,7 @@ class NonNullTest extends \PHPUnit_Framework_TestCase
                 ],
             ]
         ];
-        $this->assertEquals($expected, Executor::execute($this->schema, $this->nullingData, $ast, 'Q', []));
+        $this->assertEquals($expected, Executor::execute($this->schema, $ast, $this->nullingData, [], 'Q')->toArray());
     }
 
     public function testNullsTheTopLevelIfSyncNonNullableFieldThrows()
@@ -238,10 +238,10 @@ class NonNullTest extends \PHPUnit_Framework_TestCase
         $expected = [
             'data' => null,
             'errors' => [
-                new FormattedError($this->nonNullSyncError->message, [new SourceLocation(2, 17)])
+                FormattedError::create($this->nonNullSyncError->message, [new SourceLocation(2, 17)])
             ]
         ];
-        $this->assertEquals($expected, Executor::execute($this->schema, $this->throwingData, Parser::parse($doc)));
+        $this->assertEquals($expected, Executor::execute($this->schema, Parser::parse($doc), $this->throwingData)->toArray());
     }
 
     public function testNullsTheTopLevelIfSyncNonNullableFieldReturnsNull()
@@ -254,9 +254,9 @@ class NonNullTest extends \PHPUnit_Framework_TestCase
         $expected = [
             'data' => null,
             'errors' => [
-                new FormattedError('Cannot return null for non-nullable type.', [new SourceLocation(2, 17)]),
+                FormattedError::create('Cannot return null for non-nullable type.', [new SourceLocation(2, 17)]),
             ]
         ];
-        $this->assertEquals($expected, Executor::execute($this->schema, $this->nullingData, Parser::parse($doc)));
+        $this->assertEquals($expected, Executor::execute($this->schema, Parser::parse($doc), $this->nullingData)->toArray());
     }
 }
diff --git a/tests/Executor/TestClasses.php b/tests/Executor/TestClasses.php
new file mode 100644
index 0000000..c1fe6ba
--- /dev/null
+++ b/tests/Executor/TestClasses.php
@@ -0,0 +1,78 @@
+<?php
+namespace GraphQL\Executor;
+
+use GraphQL\Type\Definition\ScalarType;
+
+class Dog
+{
+    function __construct($name, $woofs)
+    {
+        $this->name = $name;
+        $this->woofs = $woofs;
+    }
+}
+
+class Cat
+{
+    function __construct($name, $meows)
+    {
+        $this->name = $name;
+        $this->meows = $meows;
+    }
+}
+
+class Human
+{
+    function __construct($name)
+    {
+        $this->name = $name;
+    }
+}
+
+class Person
+{
+    public $name;
+    public $pets;
+    public $friends;
+
+    function __construct($name, $pets = null, $friends = null)
+    {
+        $this->name = $name;
+        $this->pets = $pets;
+        $this->friends = $friends;
+    }
+}
+
+class ComplexScalar extends ScalarType
+{
+    public static function create()
+    {
+        return new self();
+    }
+
+    public $name = 'ComplexScalar';
+
+    public function serialize($value)
+    {
+        if ($value === 'DeserializedValue') {
+            return 'SerializedValue';
+        }
+        return null;
+    }
+
+    public function parseValue($value)
+    {
+        if ($value === 'SerializedValue') {
+            return 'DeserializedValue';
+        }
+        return null;
+    }
+
+    public function parseLiteral($valueAST)
+    {
+        if ($valueAST->value === 'SerializedValue') {
+            return 'DeserializedValue';
+        }
+        return null;
+    }
+}
diff --git a/tests/Executor/UnionInterfaceTest.php b/tests/Executor/UnionInterfaceTest.php
index e04a9ae..6a7f03b 100644
--- a/tests/Executor/UnionInterfaceTest.php
+++ b/tests/Executor/UnionInterfaceTest.php
@@ -1,6 +1,8 @@
 <?php
 namespace GraphQL\Executor;
 
+require_once __DIR__ . '/TestClasses.php';
+
 use GraphQL\Language\Parser;
 use GraphQL\Schema;
 use GraphQL\Type\Definition\Config;
@@ -31,7 +33,7 @@ class UnionInterfaceTest extends \PHPUnit_Framework_TestCase
             'interfaces' => [$NamedType],
             'fields' => [
                 'name' => ['type' => Type::string()],
-                'barks' => ['type' => Type::boolean()]
+                'woofs' => ['type' => Type::boolean()]
             ],
             'isTypeOf' => function ($value) {
                 return $value instanceof Dog;
@@ -143,7 +145,7 @@ class UnionInterfaceTest extends \PHPUnit_Framework_TestCase
                 ]
             ]
         ];
-        $this->assertEquals($expected, Executor::execute($this->schema, null, $ast));
+        $this->assertEquals($expected, Executor::execute($this->schema, $ast)->toArray());
     }
 
     public function testExecutesUsingUnionTypes()
@@ -156,7 +158,7 @@ class UnionInterfaceTest extends \PHPUnit_Framework_TestCase
         pets {
           __typename
           name
-          barks
+          woofs
           meows
         }
       }
@@ -167,12 +169,12 @@ class UnionInterfaceTest extends \PHPUnit_Framework_TestCase
                 'name' => 'John',
                 'pets' => [
                     ['__typename' => 'Cat', 'name' => 'Garfield', 'meows' => false],
-                    ['__typename' => 'Dog', 'name' => 'Odie', 'barks' => true]
+                    ['__typename' => 'Dog', 'name' => 'Odie', 'woofs' => true]
                 ]
             ]
         ];
 
-        $this->assertEquals($expected, Executor::execute($this->schema, $this->john, $ast));
+        $this->assertEquals($expected, Executor::execute($this->schema, $ast, $this->john)->toArray());
     }
 
     public function testExecutesUnionTypesWithInlineFragments()
@@ -186,7 +188,7 @@ class UnionInterfaceTest extends \PHPUnit_Framework_TestCase
           __typename
           ... on Dog {
             name
-            barks
+            woofs
           }
           ... on Cat {
             name
@@ -201,12 +203,12 @@ class UnionInterfaceTest extends \PHPUnit_Framework_TestCase
                 'name' => 'John',
                 'pets' => [
                     ['__typename' => 'Cat', 'name' => 'Garfield', 'meows' => false],
-                    ['__typename' => 'Dog', 'name' => 'Odie', 'barks' => true]
+                    ['__typename' => 'Dog', 'name' => 'Odie', 'woofs' => true]
                 ]
 
             ]
         ];
-        $this->assertEquals($expected, Executor::execute($this->schema, $this->john, $ast));
+        $this->assertEquals($expected, Executor::execute($this->schema, $ast, $this->john)->toArray());
     }
 
     public function testExecutesUsingInterfaceTypes()
@@ -219,7 +221,7 @@ class UnionInterfaceTest extends \PHPUnit_Framework_TestCase
         friends {
           __typename
           name
-          barks
+          woofs
           meows
         }
       }
@@ -230,12 +232,12 @@ class UnionInterfaceTest extends \PHPUnit_Framework_TestCase
                 'name' => 'John',
                 'friends' => [
                     ['__typename' => 'Person', 'name' => 'Liz'],
-                    ['__typename' => 'Dog', 'name' => 'Odie', 'barks' => true]
+                    ['__typename' => 'Dog', 'name' => 'Odie', 'woofs' => true]
                 ]
             ]
         ];
 
-        $this->assertEquals($expected, Executor::execute($this->schema, $this->john, $ast));
+        $this->assertEquals($expected, Executor::execute($this->schema, $ast, $this->john)->toArray());
     }
 
     public function testExecutesInterfaceTypesWithInlineFragments()
@@ -249,7 +251,7 @@ class UnionInterfaceTest extends \PHPUnit_Framework_TestCase
           __typename
           name
           ... on Dog {
-            barks
+            woofs
           }
           ... on Cat {
             meows
@@ -263,12 +265,12 @@ class UnionInterfaceTest extends \PHPUnit_Framework_TestCase
                 'name' => 'John',
                 'friends' => [
                     ['__typename' => 'Person', 'name' => 'Liz'],
-                    ['__typename' => 'Dog', 'name' => 'Odie', 'barks' => true]
+                    ['__typename' => 'Dog', 'name' => 'Odie', 'woofs' => true]
                 ]
             ]
         ];
 
-        $this->assertEquals($expected, Executor::execute($this->schema, $this->john, $ast));
+        $this->assertEquals($expected, Executor::execute($this->schema, $ast, $this->john)->toArray());
     }
 
     public function testAllowsFragmentConditionsToBeAbstractTypes()
@@ -285,7 +287,7 @@ class UnionInterfaceTest extends \PHPUnit_Framework_TestCase
         __typename
         ... on Dog {
           name
-          barks
+          woofs
         }
         ... on Cat {
           name
@@ -297,7 +299,7 @@ class UnionInterfaceTest extends \PHPUnit_Framework_TestCase
         __typename
         name
         ... on Dog {
-          barks
+          woofs
         }
         ... on Cat {
           meows
@@ -311,54 +313,15 @@ class UnionInterfaceTest extends \PHPUnit_Framework_TestCase
                 'name' => 'John',
                 'pets' => [
                     ['__typename' => 'Cat', 'name' => 'Garfield', 'meows' => false],
-                    ['__typename' => 'Dog', 'name' => 'Odie', 'barks' => true]
+                    ['__typename' => 'Dog', 'name' => 'Odie', 'woofs' => true]
                 ],
                 'friends' => [
                     ['__typename' => 'Person', 'name' => 'Liz'],
-                    ['__typename' => 'Dog', 'name' => 'Odie', 'barks' => true]
+                    ['__typename' => 'Dog', 'name' => 'Odie', 'woofs' => true]
                 ]
             ]
         ];
 
-        $this->assertEquals($expected, Executor::execute($this->schema, $this->john, $ast));
-    }
-}
-
-
-class Dog
-{
-    public $name;
-    public $barks;
-
-    function __construct($name, $barks)
-    {
-        $this->name = $name;
-        $this->barks = $barks;
-    }
-}
-
-class Cat
-{
-    public $name;
-    public $meows;
-
-    function __construct($name, $meows)
-    {
-        $this->name = $name;
-        $this->meows = $meows;
-    }
-}
-
-class Person
-{
-    public $name;
-    public $pets;
-    public $friends;
-
-    function __construct($name, $pets = null, $friends = null)
-    {
-        $this->name = $name;
-        $this->pets = $pets;
-        $this->friends = $friends;
+        $this->assertEquals($expected, Executor::execute($this->schema, $ast, $this->john)->toArray());
     }
 }
diff --git a/tests/Executor/InputObjectTest.php b/tests/Executor/VariablesTest.php
similarity index 59%
rename from tests/Executor/InputObjectTest.php
rename to tests/Executor/VariablesTest.php
index 4f3ed19..6cde735 100644
--- a/tests/Executor/InputObjectTest.php
+++ b/tests/Executor/VariablesTest.php
@@ -1,17 +1,21 @@
 <?php
 namespace GraphQL\Executor;
 
+require_once __DIR__ . '/TestClasses.php';
+
+use GraphQL\Error;
 use GraphQL\FormattedError;
 use GraphQL\Language\Parser;
 use GraphQL\Language\SourceLocation;
 use GraphQL\Schema;
 use GraphQL\Type\Definition\InputObjectType;
 use GraphQL\Type\Definition\ObjectType;
+use GraphQL\Type\Definition\ScalarType;
 use GraphQL\Type\Definition\Type;
 
-class InputObjectTest extends \PHPUnit_Framework_TestCase
+class VariablesTest extends \PHPUnit_Framework_TestCase
 {
-    // Execute: Handles input objects
+    // Execute: Handles inputs
     // Handles objects and nullability
 
     public function testUsingInlineStructs()
@@ -29,9 +33,9 @@ class InputObjectTest extends \PHPUnit_Framework_TestCase
             'fieldWithObjectInput' => '{"a":"foo","b":["bar"],"c":"baz"}'
           ]
         ];
-        $this->assertEquals($expected, Executor::execute($this->schema(), null, $ast));
+        $this->assertEquals($expected, Executor::execute($this->schema(), $ast)->toArray());
 
-        // properly coerces single value to array:
+        // properly parses single value to list:
         $doc = '
         {
           fieldWithObjectInput(input: {a: "foo", b: "bar", c: "baz"})
@@ -40,7 +44,23 @@ class InputObjectTest extends \PHPUnit_Framework_TestCase
         $ast = Parser::parse($doc);
         $expected = ['data' => ['fieldWithObjectInput' => '{"a":"foo","b":["bar"],"c":"baz"}']];
 
-        $this->assertEquals($expected, Executor::execute($this->schema(), null, $ast));
+        $this->assertEquals($expected, Executor::execute($this->schema(), $ast)->toArray());
+    }
+
+    public function testDoesNotUseIncorrectValue()
+    {
+        $doc = '
+        {
+          fieldWithObjectInput(input: ["foo", "bar", "baz"])
+        }
+        ';
+        $ast = Parser::parse($doc);
+        $result = Executor::execute($this->schema(), $ast)->toArray();
+
+        $expected = [
+            'data' => ['fieldWithObjectInput' => null]
+        ];
+        $this->assertEquals($expected, $result);
     }
 
     public function testUsingVariables()
@@ -57,46 +77,100 @@ class InputObjectTest extends \PHPUnit_Framework_TestCase
 
         $this->assertEquals(
             ['data' => ['fieldWithObjectInput' => '{"a":"foo","b":["bar"],"c":"baz"}']],
-            Executor::execute($schema, null, $ast, null, $params)
+            Executor::execute($schema, $ast, null, $params)->toArray()
         );
 
-        // properly coerces single value to array:
+        // uses default value when not provided:
+        $withDefaultsAST = Parser::parse('
+          query q($input: TestInputObject = {a: "foo", b: ["bar"], c: "baz"}) {
+            fieldWithObjectInput(input: $input)
+          }
+        ');
+
+        $result = Executor::execute($this->schema(), $withDefaultsAST)->toArray();
+        $expected = [
+            'data' => ['fieldWithObjectInput' => '{"a":"foo","b":["bar"],"c":"baz"}']
+        ];
+        $this->assertEquals($expected, $result);
+
+
+        // properly parses single value to array:
         $params = ['input' => ['a' => 'foo', 'b' => 'bar', 'c' => 'baz']];
         $this->assertEquals(
             ['data' => ['fieldWithObjectInput' => '{"a":"foo","b":["bar"],"c":"baz"}']],
-            Executor::execute($schema, null, $ast, null, $params)
+            Executor::execute($schema, $ast, null, $params)->toArray()
         );
 
+        // executes with complex scalar input:
+        $params = [ 'input' => [ 'c' => 'foo', 'd' => 'SerializedValue' ] ];
+        $result = Executor::execute($schema, $ast, null, $params)->toArray();
+        $expected = [
+          'data' => [
+            'fieldWithObjectInput' => '{"c":"foo","d":"DeserializedValue"}'
+          ]
+        ];
+        $this->assertEquals($expected, $result);
+
         // errors on null for nested non-null:
         $params = ['input' => ['a' => 'foo', 'b' => 'bar', 'c' => null]];
-        $expected = [
-            'data' => null,
-            'errors' => [
-                new FormattedError(
-                    'Variable $input expected value of type ' .
-                    'TestInputObject but got: ' .
-                    '{"a":"foo","b":"bar","c":null}.',
-                    [new SourceLocation(2, 17)]
-                )
-            ]
-        ];
+        $expected = FormattedError::create(
+            'Variable $input expected value of type ' .
+            'TestInputObject but got: ' .
+            '{"a":"foo","b":"bar","c":null}.',
+            [new SourceLocation(2, 17)]
+        );
 
-        $this->assertEquals($expected, Executor::execute($schema, null, $ast, null, $params));
+        try {
+            Executor::execute($schema, $ast, null, $params);
+            $this->fail('Expected exception not thrown');
+        } catch (Error $err) {
+            $this->assertEquals($expected, Error::formatError($err));
+        }
+
+        // errors on incorrect type:
+        $params = [ 'input' => 'foo bar' ];
+
+        try {
+            Executor::execute($schema, $ast, null, $params);
+            $this->fail('Expected exception not thrown');
+        } catch (Error $error) {
+            $expected = FormattedError::create(
+                'Variable $input expected value of type TestInputObject but got: "foo bar".',
+                [new SourceLocation(2, 17)]
+            );
+            $this->assertEquals($expected, Error::formatError($error));
+        }
 
         // errors on omission of nested non-null:
         $params = ['input' => ['a' => 'foo', 'b' => 'bar']];
-        $expected = [
-            'data' => null,
-            'errors' => [
-                new FormattedError(
-                    'Variable $input expected value of type ' .
-                    'TestInputObject but got: {"a":"foo","b":"bar"}.',
-                    [new SourceLocation(2, 17)]
-                )
-            ]
-        ];
 
-        $this->assertEquals($expected, Executor::execute($schema, null, $ast, null, $params));
+        try {
+            Executor::execute($schema, $ast, null, $params);
+            $this->fail('Expected exception not thrown');
+        } catch (Error $e) {
+            $expected = FormattedError::create(
+                'Variable $input expected value of type ' .
+                'TestInputObject but got: {"a":"foo","b":"bar"}.',
+                [new SourceLocation(2, 17)]
+            );
+
+            $this->assertEquals($expected, Error::formatError($e));
+        }
+
+        // errors on addition of unknown input field
+        $params = ['input' => [ 'a' => 'foo', 'b' => 'bar', 'c' => 'baz', 'd' => 'dog' ]];
+
+        try {
+            Executor::execute($schema, $ast, null, $params);
+            $this->fail('Expected exception not thrown');
+        } catch (Error $e) {
+            $expected = FormattedError::create(
+                'Variable $input expected value of type TestInputObject but ' .
+                'got: {"a":"foo","b":"bar","c":"baz","d":"dog"}.',
+                [new SourceLocation(2, 17)]
+            );
+            $this->assertEquals($expected, Error::formatError($e));
+        }
     }
 
 
@@ -110,10 +184,10 @@ class InputObjectTest extends \PHPUnit_Framework_TestCase
         ';
         $ast = Parser::parse($doc);
         $expected = [
-            'data' => ['fieldWithNullableStringInput' => 'null']
+            'data' => ['fieldWithNullableStringInput' => null]
         ];
 
-        $this->assertEquals($expected, Executor::execute($this->schema(), null, $ast));
+        $this->assertEquals($expected, Executor::execute($this->schema(), $ast)->toArray());
     }
 
     public function testAllowsNullableInputsToBeOmittedInAVariable()
@@ -124,9 +198,9 @@ class InputObjectTest extends \PHPUnit_Framework_TestCase
       }
         ';
         $ast = Parser::parse($doc);
-        $expected = ['data' => ['fieldWithNullableStringInput' => 'null']];
+        $expected = ['data' => ['fieldWithNullableStringInput' => null]];
 
-        $this->assertEquals($expected, Executor::execute($this->schema(), null, $ast));
+        $this->assertEquals($expected, Executor::execute($this->schema(), $ast)->toArray());
     }
 
     public function testAllowsNullableInputsToBeOmittedInAnUnlistedVariable()
@@ -137,8 +211,8 @@ class InputObjectTest extends \PHPUnit_Framework_TestCase
       }
       ';
         $ast = Parser::parse($doc);
-        $expected = ['data' => ['fieldWithNullableStringInput' => 'null']];
-        $this->assertEquals($expected, Executor::execute($this->schema(), null, $ast));
+        $expected = ['data' => ['fieldWithNullableStringInput' => null]];
+        $this->assertEquals($expected, Executor::execute($this->schema(), $ast)->toArray());
     }
 
     public function testAllowsNullableInputsToBeSetToNullInAVariable()
@@ -149,21 +223,9 @@ class InputObjectTest extends \PHPUnit_Framework_TestCase
       }
         ';
         $ast = Parser::parse($doc);
-        $expected = ['data' => ['fieldWithNullableStringInput' => 'null']];
+        $expected = ['data' => ['fieldWithNullableStringInput' => null]];
 
-        $this->assertEquals($expected, Executor::execute($this->schema(), null, $ast, null, ['value' => null]));
-    }
-
-    public function testAllowsNullableInputsToBeSetToNullDirectly()
-    {
-        $doc = '
-      {
-        fieldWithNullableStringInput(input: null)
-      }
-        ';
-        $ast = Parser::parse($doc);
-        $expected = ['data' => ['fieldWithNullableStringInput' => 'null']];
-        $this->assertEquals($expected, Executor::execute($this->schema(), null, $ast));
+        $this->assertEquals($expected, Executor::execute($this->schema(), $ast, null, ['value' => null])->toArray());
     }
 
     public function testAllowsNullableInputsToBeSetToAValueInAVariable()
@@ -175,7 +237,7 @@ class InputObjectTest extends \PHPUnit_Framework_TestCase
         ';
         $ast = Parser::parse($doc);
         $expected = ['data' => ['fieldWithNullableStringInput' => '"a"']];
-        $this->assertEquals($expected, Executor::execute($this->schema(), null, $ast, null, ['value' => 'a']));
+        $this->assertEquals($expected, Executor::execute($this->schema(), $ast, null, ['value' => 'a'])->toArray());
     }
 
     public function testAllowsNullableInputsToBeSetToAValueDirectly()
@@ -187,7 +249,7 @@ class InputObjectTest extends \PHPUnit_Framework_TestCase
         ';
         $ast = Parser::parse($doc);
         $expected = ['data' => ['fieldWithNullableStringInput' => '"a"']];
-        $this->assertEquals($expected, Executor::execute($this->schema(), null, $ast));
+        $this->assertEquals($expected, Executor::execute($this->schema(), $ast)->toArray());
     }
 
 
@@ -201,16 +263,16 @@ class InputObjectTest extends \PHPUnit_Framework_TestCase
         }
         ';
         $ast = Parser::parse($doc);
-        $expected = [
-            'data' => null,
-            'errors' => [
-                new FormattedError(
-                    'Variable $value expected value of type String! but got: null.',
-                    [new SourceLocation(2, 31)]
-                )
-            ]
-        ];
-        $this->assertEquals($expected, Executor::execute($this->schema(), null, $ast));
+        try {
+            Executor::execute($this->schema(), $ast);
+            $this->fail('Expected exception not thrown');
+        } catch (Error $e) {
+            $expected = FormattedError::create(
+                'Variable $value expected value of type String! but got: null.',
+                [new SourceLocation(2, 31)]
+            );
+            $this->assertEquals($expected, Error::formatError($e));
+        }
     }
 
     public function testDoesNotAllowNonNullableInputsToBeSetToNullInAVariable()
@@ -222,16 +284,17 @@ class InputObjectTest extends \PHPUnit_Framework_TestCase
         }
         ';
         $ast = Parser::parse($doc);
-        $expected = [
-            'data' => null,
-            'errors' => [
-                new FormattedError(
-                    'Variable $value expected value of type String! but got: null.',
-                    [new SourceLocation(2, 31)]
-                )
-            ]
-        ];
-        $this->assertEquals($expected, Executor::execute($this->schema(), null, $ast, null, ['value' => null]));
+
+        try {
+            Executor::execute($this->schema(), $ast, null, ['value' => null]);
+            $this->fail('Expected exception not thrown');
+        } catch (Error $e) {
+            $expected = FormattedError::create(
+                'Variable $value expected value of type String! but got: null.',
+                [new SourceLocation(2, 31)]
+            );
+            $this->assertEquals($expected, Error::formatError($e));
+        }
     }
 
     public function testAllowsNonNullableInputsToBeSetToAValueInAVariable()
@@ -243,7 +306,7 @@ class InputObjectTest extends \PHPUnit_Framework_TestCase
         ';
         $ast = Parser::parse($doc);
         $expected = ['data' => ['fieldWithNonNullableStringInput' => '"a"']];
-        $this->assertEquals($expected, Executor::execute($this->schema(), null, $ast, null, ['value' => 'a']));
+        $this->assertEquals($expected, Executor::execute($this->schema(), $ast, null, ['value' => 'a'])->toArray());
     }
 
     public function testAllowsNonNullableInputsToBeSetToAValueDirectly()
@@ -256,7 +319,7 @@ class InputObjectTest extends \PHPUnit_Framework_TestCase
         $ast = Parser::parse($doc);
         $expected = ['data' => ['fieldWithNonNullableStringInput' => '"a"']];
 
-        $this->assertEquals($expected, Executor::execute($this->schema(), null, $ast));
+        $this->assertEquals($expected, Executor::execute($this->schema(), $ast)->toArray());
     }
 
     public function testPassesAlongNullForNonNullableInputsIfExplcitlySetInTheQuery()
@@ -267,8 +330,8 @@ class InputObjectTest extends \PHPUnit_Framework_TestCase
       }
         ';
         $ast = Parser::parse($doc);
-        $expected = ['data' => ['fieldWithNonNullableStringInput' => 'null']];
-        $this->assertEquals($expected, Executor::execute($this->schema(), null, $ast));
+        $expected = ['data' => ['fieldWithNonNullableStringInput' => null]];
+        $this->assertEquals($expected, Executor::execute($this->schema(), $ast)->toArray());
     }
 
     // Handles lists and nullability
@@ -280,9 +343,9 @@ class InputObjectTest extends \PHPUnit_Framework_TestCase
         }
         ';
         $ast = Parser::parse($doc);
-        $expected = ['data' => ['list' => 'null']];
+        $expected = ['data' => ['list' => null]];
 
-        $this->assertEquals($expected, Executor::execute($this->schema(), null, $ast, null, ['input' => null]));
+        $this->assertEquals($expected, Executor::execute($this->schema(), $ast, null, ['input' => null])->toArray());
     }
 
     public function testAllowsListsToContainValues()
@@ -294,7 +357,7 @@ class InputObjectTest extends \PHPUnit_Framework_TestCase
         ';
         $ast = Parser::parse($doc);
         $expected = ['data' => ['list' => '["A"]']];
-        $this->assertEquals($expected, Executor::execute($this->schema(), null, $ast, null, ['input' => ['A']]));
+        $this->assertEquals($expected, Executor::execute($this->schema(), $ast, null, ['input' => ['A']])->toArray());
     }
 
     public function testAllowsListsToContainNull()
@@ -306,7 +369,7 @@ class InputObjectTest extends \PHPUnit_Framework_TestCase
         ';
         $ast = Parser::parse($doc);
         $expected = ['data' => ['list' => '["A",null,"B"]']];
-        $this->assertEquals($expected, Executor::execute($this->schema(), null, $ast, null, ['input' => ['A',null,'B']]));
+        $this->assertEquals($expected, Executor::execute($this->schema(), $ast, null, ['input' => ['A',null,'B']])->toArray());
     }
 
     public function testDoesNotAllowNonNullListsToBeNull()
@@ -317,17 +380,17 @@ class InputObjectTest extends \PHPUnit_Framework_TestCase
         }
         ';
         $ast = Parser::parse($doc);
-        $expected = [
-            'data' => null,
-            'errors' => [
-                new FormattedError(
-                    'Variable $input expected value of type [String]! but got: null.',
-                    [new SourceLocation(2, 17)]
-                )
-            ]
-        ];
+        $expected = FormattedError::create(
+            'Variable $input expected value of type [String]! but got: null.',
+            [new SourceLocation(2, 17)]
+        );
 
-        $this->assertEquals($expected, Executor::execute($this->schema(), null, $ast, null, ['input' => null]));
+        try {
+            $this->assertEquals($expected, Executor::execute($this->schema(), $ast, null, ['input' => null])->toArray());
+            $this->fail('Expected exception not thrown');
+        } catch (Error $e) {
+            $this->assertEquals($expected, Error::formatError($e));
+        }
     }
 
     public function testAllowsNonNullListsToContainValues()
@@ -339,7 +402,7 @@ class InputObjectTest extends \PHPUnit_Framework_TestCase
         ';
         $ast = Parser::parse($doc);
         $expected = ['data' => ['nnList' => '["A"]']];
-        $this->assertEquals($expected, Executor::execute($this->schema(), null, $ast, null, ['input' => 'A']));
+        $this->assertEquals($expected, Executor::execute($this->schema(), $ast, null, ['input' => 'A'])->toArray());
     }
 
     public function testAllowsNonNullListsToContainNull()
@@ -352,7 +415,7 @@ class InputObjectTest extends \PHPUnit_Framework_TestCase
         $ast = Parser::parse($doc);
         $expected = ['data' => ['nnList' => '["A",null,"B"]']];
 
-        $this->assertEquals($expected, Executor::execute($this->schema(), null, $ast, null, ['input' => ['A',null,'B']]));
+        $this->assertEquals($expected, Executor::execute($this->schema(), $ast, null, ['input' => ['A',null,'B']])->toArray());
     }
 
     public function testAllowsListsOfNonNullsToBeNull()
@@ -363,8 +426,8 @@ class InputObjectTest extends \PHPUnit_Framework_TestCase
         }
         ';
         $ast = Parser::parse($doc);
-        $expected = ['data' => ['listNN' => 'null']];
-        $this->assertEquals($expected, Executor::execute($this->schema(), null, $ast, null, ['input' => null]));
+        $expected = ['data' => ['listNN' => null]];
+        $this->assertEquals($expected, Executor::execute($this->schema(), $ast, null, ['input' => null])->toArray());
     }
 
     public function testAllowsListsOfNonNullsToContainValues()
@@ -377,7 +440,7 @@ class InputObjectTest extends \PHPUnit_Framework_TestCase
         $ast = Parser::parse($doc);
         $expected = ['data' => ['listNN' => '["A"]']];
 
-        $this->assertEquals($expected, Executor::execute($this->schema(), null, $ast, null, ['input' => 'A']));
+        $this->assertEquals($expected, Executor::execute($this->schema(), $ast, null, ['input' => 'A'])->toArray());
     }
 
     public function testDoesNotAllowListsOfNonNullsToContainNull()
@@ -388,16 +451,17 @@ class InputObjectTest extends \PHPUnit_Framework_TestCase
         }
         ';
         $ast = Parser::parse($doc);
-        $expected = [
-            'data' => null,
-            'errors' => [
-                new FormattedError(
-                    'Variable $input expected value of type [String!] but got: ["A",null,"B"].',
-                    [new SourceLocation(2, 17)]
-                )
-            ]
-        ];
-        $this->assertEquals($expected, Executor::execute($this->schema(), null, $ast, null, ['input' => ['A', null, 'B']]));
+        $expected = FormattedError::create(
+            'Variable $input expected value of type [String!] but got: ["A",null,"B"].',
+            [new SourceLocation(2, 17)]
+        );
+
+        try {
+            Executor::execute($this->schema(), $ast, null, ['input' => ['A', null, 'B']]);
+            $this->fail('Expected exception not thrown');
+        } catch (Error $e) {
+            $this->assertEquals($expected, Error::formatError($e));
+        }
     }
 
     public function testDoesNotAllowNonNullListsOfNonNullsToBeNull()
@@ -408,16 +472,15 @@ class InputObjectTest extends \PHPUnit_Framework_TestCase
         }
         ';
         $ast = Parser::parse($doc);
-        $expected = [
-            'data' => null,
-            'errors' => [
-                new FormattedError(
-                    'Variable $input expected value of type [String!]! but got: null.',
-                    [new SourceLocation(2, 17)]
-                )
-            ]
-        ];
-        $this->assertEquals($expected, Executor::execute($this->schema(), null, $ast, null, ['input' => null]));
+        $expected = FormattedError::create(
+            'Variable $input expected value of type [String!]! but got: null.',
+            [new SourceLocation(2, 17)]
+        );
+        try {
+            Executor::execute($this->schema(), $ast, null, ['input' => null]);
+        } catch (Error $e) {
+            $this->assertEquals($expected, Error::formatError($e));
+        }
     }
 
     public function testAllowsNonNullListsOfNonNullsToContainValues()
@@ -429,7 +492,7 @@ class InputObjectTest extends \PHPUnit_Framework_TestCase
         ';
         $ast = Parser::parse($doc);
         $expected = ['data' => ['nnListNN' => '["A"]']];
-        $this->assertEquals($expected, Executor::execute($this->schema(), null, $ast, null, ['input' => ['A']]));
+        $this->assertEquals($expected, Executor::execute($this->schema(), $ast, null, ['input' => ['A']])->toArray());
     }
 
     public function testDoesNotAllowNonNullListsOfNonNullsToContainNull()
@@ -440,27 +503,29 @@ class InputObjectTest extends \PHPUnit_Framework_TestCase
         }
         ';
         $ast = Parser::parse($doc);
-        $expected = [
-            'data' => null,
-            'errors' => [
-                new FormattedError(
-                    'Variable $input expected value of type [String!]! but got: ["A",null,"B"].',
-                    [new SourceLocation(2, 17)]
-                )
-            ]
-        ];
-        $this->assertEquals($expected, Executor::execute($this->schema(), null, $ast, null, ['input' => ['A',null,'B']]));
+        $expected = FormattedError::create(
+            'Variable $input expected value of type [String!]! but got: ["A",null,"B"].',
+            [new SourceLocation(2, 17)]
+        );
+        try {
+            Executor::execute($this->schema(), $ast, null, ['input' => ['A', null, 'B']]);
+        } catch (Error $e) {
+            $this->assertEquals($expected, Error::formatError($e));
+        }
     }
 
 
     public function schema()
     {
+        $ComplexScalarType = ComplexScalar::create();
+
         $TestInputObject = new InputObjectType([
             'name' => 'TestInputObject',
             'fields' => [
                 'a' => ['type' => Type::string()],
                 'b' => ['type' => Type::listOf(Type::string())],
-                'c' => ['type' => Type::nonNull(Type::string())]
+                'c' => ['type' => Type::nonNull(Type::string())],
+                'd' => ['type' => $ComplexScalarType],
             ]
         ]);
 
@@ -471,49 +536,56 @@ class InputObjectTest extends \PHPUnit_Framework_TestCase
                     'type' => Type::string(),
                     'args' => ['input' => ['type' => $TestInputObject]],
                     'resolve' => function ($_, $args) {
-                        return json_encode($args['input']);
+                        return isset($args['input']) ? json_encode($args['input']) : null;
                     }
                 ],
                 'fieldWithNullableStringInput' => [
                     'type' => Type::string(),
                     'args' => ['input' => ['type' => Type::string()]],
                     'resolve' => function ($_, $args) {
-                        return json_encode($args['input']);
+                        return isset($args['input']) ?  json_encode($args['input']) : null;
                     }
                 ],
                 'fieldWithNonNullableStringInput' => [
                     'type' => Type::string(),
                     'args' => ['input' => ['type' => Type::nonNull(Type::string())]],
                     'resolve' => function ($_, $args) {
-                        return json_encode($args['input']);
+                        return isset($args['input']) ? json_encode($args['input']) : null;
+                    }
+                ],
+                'fieldWithDefaultArgumentValue' => [
+                    'type' => Type::string(),
+                    'args' => [ 'input' => [ 'type' => Type::string(), 'defaultValue' => 'Hello World' ]],
+                    'resolve' => function($_, $args) {
+                        return isset($args['input']) ? json_encode($args['input']) : null;
                     }
                 ],
                 'list' => [
                     'type' => Type::string(),
                     'args' => ['input' => ['type' => Type::listOf(Type::string())]],
                     'resolve' => function ($_, $args) {
-                        return json_encode($args['input']);
+                        return isset($args['input']) ? json_encode($args['input']) : null;
                     }
                 ],
                 'nnList' => [
                     'type' => Type::string(),
                     'args' => ['input' => ['type' => Type::nonNull(Type::listOf(Type::string()))]],
                     'resolve' => function ($_, $args) {
-                        return json_encode($args['input']);
+                        return isset($args['input']) ? json_encode($args['input']) : null;
                     }
                 ],
                 'listNN' => [
                     'type' => Type::string(),
                     'args' => ['input' => ['type' => Type::listOf(Type::nonNull(Type::string()))]],
                     'resolve' => function ($_, $args) {
-                        return json_encode($args['input']);
+                        return isset($args['input']) ? json_encode($args['input']) : null;
                     }
                 ],
                 'nnListNN' => [
                     'type' => Type::string(),
                     'args' => ['input' => ['type' => Type::nonNull(Type::listOf(Type::nonNull(Type::string())))]],
                     'resolve' => function ($_, $args) {
-                        return json_encode($args['input']);
+                        return isset($args['input']) ? json_encode($args['input']) : null;
                     }
                 ],
             ]
diff --git a/tests/StarWarsSchema.php b/tests/StarWarsSchema.php
index cae03a9..07f400f 100644
--- a/tests/StarWarsSchema.php
+++ b/tests/StarWarsSchema.php
@@ -245,7 +245,7 @@ class StarWarsSchema
                         ]
                     ],
                     'resolve' => function ($root, $args) {
-                        return StarWarsData::getHero($args['episode']);
+                        return StarWarsData::getHero(isset($args['episode']) ? $args['episode'] : null);
                     },
                 ],
                 'human' => [
diff --git a/tests/StarWarsValidationTest.php b/tests/StarWarsValidationTest.php
index a2b76a0..357bb1c 100644
--- a/tests/StarWarsValidationTest.php
+++ b/tests/StarWarsValidationTest.php
@@ -28,8 +28,8 @@ class StartWarsValidationTest extends \PHPUnit_Framework_TestCase
           appearsIn
         }
       ';
-        $result = $this->validationResult($query);
-        $this->assertEquals(true, $result['isValid']);
+        $errors = $this->validationErrors($query);
+        $this->assertEquals(true, empty($errors));
     }
 
     public function testThatNonExistentFieldsAreInvalid()
@@ -42,7 +42,8 @@ class StartWarsValidationTest extends \PHPUnit_Framework_TestCase
           }
         }
         ';
-        $this->assertEquals(false, $this->validationResult($query)['isValid']);
+        $errors = $this->validationErrors($query);
+        $this->assertEquals(false, empty($errors));
     }
 
     public function testRequiresFieldsOnObjects()
@@ -52,7 +53,9 @@ class StartWarsValidationTest extends \PHPUnit_Framework_TestCase
           hero
         }
         ';
-        $this->assertEquals(false, $this->validationResult($query)['isValid']);
+
+        $errors = $this->validationErrors($query);
+        $this->assertEquals(false, empty($errors));
     }
 
     public function testDisallowsFieldsOnScalars()
@@ -67,7 +70,8 @@ class StartWarsValidationTest extends \PHPUnit_Framework_TestCase
           }
         }
         ';
-        $this->assertEquals(false, $this->validationResult($query)['isValid']);
+        $errors = $this->validationErrors($query);
+        $this->assertEquals(false, empty($errors));
     }
 
     public function testDisallowsObjectFieldsOnInterfaces()
@@ -80,7 +84,8 @@ class StartWarsValidationTest extends \PHPUnit_Framework_TestCase
           }
         }
         ';
-        $this->assertEquals(false, $this->validationResult($query)['isValid']);
+        $errors = $this->validationErrors($query);
+        $this->assertEquals(false, empty($errors));
     }
 
     public function testAllowsObjectFieldsInFragments()
@@ -97,7 +102,8 @@ class StartWarsValidationTest extends \PHPUnit_Framework_TestCase
           primaryFunction
         }
         ';
-        $this->assertEquals(true, $this->validationResult($query)['isValid']);
+        $errors = $this->validationErrors($query);
+        $this->assertEquals(true, empty($errors));
     }
 
     public function testAllowsObjectFieldsInInlineFragments()
@@ -112,13 +118,14 @@ class StartWarsValidationTest extends \PHPUnit_Framework_TestCase
           }
         }
         ';
-        $this->assertEquals(true, $this->validationResult($query)['isValid']);
+        $errors = $this->validationErrors($query);
+        $this->assertEquals(true, empty($errors));
     }
 
     /**
      * Helper function to test a query and the expected response.
      */
-    private function validationResult($query)
+    private function validationErrors($query)
     {
         $ast = Parser::parse($query);
         return DocumentValidator::validate(StarWarsSchema::build(), $ast);
diff --git a/tests/Type/DefinitionTest.php b/tests/Type/DefinitionTest.php
index a9800da..a06d23a 100644
--- a/tests/Type/DefinitionTest.php
+++ b/tests/Type/DefinitionTest.php
@@ -6,7 +6,6 @@ use GraphQL\Type\Definition\Config;
 use GraphQL\Type\Definition\EnumType;
 use GraphQL\Type\Definition\InputObjectType;
 use GraphQL\Type\Definition\InterfaceType;
-use GraphQL\Type\Definition\IntType;
 use GraphQL\Type\Definition\ListOfType;
 use GraphQL\Type\Definition\NonNull;
 use GraphQL\Type\Definition\ObjectType;
@@ -67,7 +66,10 @@ class DefinitionTest extends \PHPUnit_Framework_TestCase
 
     public function setUp()
     {
-        $this->objectType = new ObjectType(['name' => 'Object']);
+        $this->objectType = new ObjectType([
+            'name' => 'Object',
+            'isTypeOf' => function() {return true;}
+        ]);
         $this->interfaceType = new InterfaceType(['name' => 'Interface']);
         $this->unionType = new UnionType(['name' => 'Union', 'types' => [$this->objectType]]);
         $this->enumType = new EnumType(['name' => 'Enum']);
@@ -176,20 +178,83 @@ class DefinitionTest extends \PHPUnit_Framework_TestCase
         $this->assertSame('writeArticle', $writeMutation->name);
     }
 
+    public function testIncludesNestedInputObjectInTheMap()
+    {
+        $nestedInputObject = new InputObjectType([
+            'name' => 'NestedInputObject',
+            'fields' => ['value' => ['type' => Type::string()]]
+        ]);
+        $someInputObject = new InputObjectType([
+            'name' => 'SomeInputObject',
+            'fields' => ['nested' => ['type' => $nestedInputObject]]
+        ]);
+        $someMutation = new ObjectType([
+            'name' => 'SomeMutation',
+            'fields' => [
+                'mutateSomething' => [
+                    'type' => $this->blogArticle,
+                    'args' => ['input' => ['type' => $someInputObject]]
+                ]
+            ]
+        ]);
+
+        $schema = new Schema($this->blogQuery, $someMutation);
+        $this->assertSame($nestedInputObject, $schema->getType('NestedInputObject'));
+    }
+
     public function testIncludesInterfaceSubtypesInTheTypeMap()
     {
         $someInterface = new InterfaceType([
             'name' => 'SomeInterface',
-            'fields' => []
+            'fields' => [
+                'f' => ['type' => Type::int()]
+            ]
         ]);
 
         $someSubtype = new ObjectType([
             'name' => 'SomeSubtype',
-            'fields' => [],
-            'interfaces' => [$someInterface]
+            'fields' => [
+                'f' => ['type' => Type::int()]
+            ],
+            'interfaces' => [$someInterface],
+            'isTypeOf' => function() {return true;}
         ]);
 
-        $schema = new Schema($someInterface);
+        $schema = new Schema(new ObjectType([
+            'name' => 'Query',
+            'fields' => [
+                'iface' => ['type' => $someInterface]
+            ]
+        ]));
+        $this->assertSame($someSubtype, $schema->getType('SomeSubtype'));
+    }
+
+    public function testIncludesInterfacesThunkSubtypesInTheTypeMap()
+    {
+        // includes interfaces' thunk subtypes in the type map
+        $someInterface = new InterfaceType([
+            'name' => 'SomeInterface',
+            'fields' => [
+                'f' => ['type' => Type::int()]
+            ]
+        ]);
+
+        $someSubtype = new ObjectType([
+            'name' => 'SomeSubtype',
+            'fields' => [
+                'f' => ['type' => Type::int()]
+            ],
+            'interfaces' => function() use ($someInterface) { return [$someInterface]; },
+            'isTypeOf' => function() {return true;}
+        ]);
+
+        $schema = new Schema(new ObjectType([
+            'name' => 'Query',
+            'fields' => [
+                'iface' => ['type' => $someInterface]
+            ]
+        ]));
+
         $this->assertSame($someSubtype, $schema->getType('SomeSubtype'));
     }
 
diff --git a/tests/Type/IntrospectionTest.php b/tests/Type/IntrospectionTest.php
index 5af9b04..c9f28bc 100644
--- a/tests/Type/IntrospectionTest.php
+++ b/tests/Type/IntrospectionTest.php
@@ -9,7 +9,7 @@ use GraphQL\Type\Definition\EnumType;
 use GraphQL\Type\Definition\InputObjectType;
 use GraphQL\Type\Definition\ObjectType;
 use GraphQL\Type\Definition\Type;
-use GraphQL\Validator\Messages;
+use GraphQL\Validator\Rules\ProvidedNonNullArguments;
 
 class IntrospectionTest extends \PHPUnit_Framework_TestCase
 {
@@ -20,137 +20,57 @@ class IntrospectionTest extends \PHPUnit_Framework_TestCase
             'fields' => []
         ]));
 
-        $request = <<<'EOD'
-      query IntrospectionTestQuery {
-        schemaType: __type(name: "__Schema") {
-          name
-        }
-        queryRootType: __type(name: "QueryRoot") {
-          name
-        }
-        __schema {
-          __typename
-          types {
-            __typename
-            kind
-            name
-            fields {
-              __typename
-              name
-              args {
-                __typename
-                name
-                type { ...TypeRef }
-                defaultValue
-              }
-              type {
-                ...TypeRef
-              }
-              isDeprecated
-              deprecationReason
-            }
-            interfaces {
-              ...TypeRef
-            }
-            enumValues {
-              __typename
-              name
-              isDeprecated
-              deprecationReason
-            }
-          }
-          directives {
-            __typename
-            name
-            type { ...TypeRef }
-            onOperation
-            onFragment
-            onField
-          }
-        }
-      }
-
-      fragment TypeRef on __Type {
-        __typename
-        kind
-        name
-        ofType {
-          __typename
-          kind
-          name
-          ofType {
-            __typename
-            kind
-            name
-            ofType {
-              __typename
-              kind
-              name
-            }
-          }
-        }
-      }
-EOD;
-
-        $expected = array(
+        $request = Introspection::getIntrospectionQuery(false);
+        $expected = array (
             'data' =>
-                array(
-                    'schemaType' =>
-                        array(
-                            'name' => '__Schema',
-                        ),
-                    'queryRootType' =>
-                        array(
-                            'name' => 'QueryRoot',
-                        ),
+                array (
                     '__schema' =>
-                        array(
-                            '__typename' => '__Schema',
+                        array (
+                            'mutationType' => NULL,
+                            'queryType' =>
+                                array (
+                                    'name' => 'QueryRoot',
+                                ),
                             'types' =>
-                                array(
+                                array (
                                     0 =>
-                                        array(
-                                            '__typename' => '__Type',
+                                        array (
                                             'kind' => 'OBJECT',
                                             'name' => 'QueryRoot',
-                                            'fields' =>
-                                                array(
-                                                ),
+                                            'inputFields' => NULL,
                                             'interfaces' =>
-                                                array(),
+                                                array (
+                                                ),
                                             'enumValues' => NULL,
+                                            'possibleTypes' => NULL,
+                                            'fields' => []
                                         ),
                                     1 =>
-                                        array(
-                                            '__typename' => '__Type',
+                                        array (
                                             'kind' => 'OBJECT',
                                             'name' => '__Schema',
                                             'fields' =>
-                                                array(
+                                                array (
                                                     0 =>
-                                                        array(
-                                                            '__typename' => '__Field',
+                                                        array (
                                                             'name' => 'types',
                                                             'args' =>
-                                                                array(),
+                                                                array (
+                                                                ),
                                                             'type' =>
-                                                                array(
-                                                                    '__typename' => '__Type',
+                                                                array (
                                                                     'kind' => 'NON_NULL',
                                                                     'name' => NULL,
                                                                     'ofType' =>
-                                                                        array(
-                                                                            '__typename' => '__Type',
+                                                                        array (
                                                                             'kind' => 'LIST',
                                                                             'name' => NULL,
                                                                             'ofType' =>
-                                                                                array(
-                                                                                    '__typename' => '__Type',
+                                                                                array (
                                                                                     'kind' => 'NON_NULL',
                                                                                     'name' => NULL,
                                                                                     'ofType' =>
-                                                                                        array(
-                                                                                            '__typename' => '__Type',
+                                                                                        array (
                                                                                             'kind' => 'OBJECT',
                                                                                             'name' => '__Type',
                                                                                         ),
@@ -161,19 +81,17 @@ EOD;
                                                             'deprecationReason' => NULL,
                                                         ),
                                                     1 =>
-                                                        array(
-                                                            '__typename' => '__Field',
+                                                        array (
                                                             'name' => 'queryType',
                                                             'args' =>
-                                                                array(),
+                                                                array (
+                                                                ),
                                                             'type' =>
-                                                                array(
-                                                                    '__typename' => '__Type',
+                                                                array (
                                                                     'kind' => 'NON_NULL',
                                                                     'name' => NULL,
                                                                     'ofType' =>
-                                                                        array(
-                                                                            '__typename' => '__Type',
+                                                                        array (
                                                                             'kind' => 'OBJECT',
                                                                             'name' => '__Type',
                                                                             'ofType' => NULL,
@@ -183,14 +101,13 @@ EOD;
                                                             'deprecationReason' => NULL,
                                                         ),
                                                     2 =>
-                                                        array(
-                                                            '__typename' => '__Field',
+                                                        array (
                                                             'name' => 'mutationType',
                                                             'args' =>
-                                                                array(),
+                                                                array (
+                                                                ),
                                                             'type' =>
-                                                                array(
-                                                                    '__typename' => '__Type',
+                                                                array (
                                                                     'kind' => 'OBJECT',
                                                                     'name' => '__Type',
                                                                     'ofType' => NULL,
@@ -199,29 +116,25 @@ EOD;
                                                             'deprecationReason' => NULL,
                                                         ),
                                                     3 =>
-                                                        array(
-                                                            '__typename' => '__Field',
+                                                        array (
                                                             'name' => 'directives',
                                                             'args' =>
-                                                                array(),
+                                                                array (
+                                                                ),
                                                             'type' =>
-                                                                array(
-                                                                    '__typename' => '__Type',
+                                                                array (
                                                                     'kind' => 'NON_NULL',
                                                                     'name' => NULL,
                                                                     'ofType' =>
-                                                                        array(
-                                                                            '__typename' => '__Type',
+                                                                        array (
                                                                             'kind' => 'LIST',
                                                                             'name' => NULL,
                                                                             'ofType' =>
-                                                                                array(
-                                                                                    '__typename' => '__Type',
+                                                                                array (
                                                                                     'kind' => 'NON_NULL',
                                                                                     'name' => NULL,
                                                                                     'ofType' =>
-                                                                                        array(
-                                                                                            '__typename' => '__Type',
+                                                                                        array (
                                                                                             'kind' => 'OBJECT',
                                                                                             'name' => '__Directive',
                                                                                         ),
@@ -232,31 +145,31 @@ EOD;
                                                             'deprecationReason' => NULL,
                                                         ),
                                                 ),
+                                            'inputFields' => NULL,
                                             'interfaces' =>
-                                                array(),
+                                                array (
+                                                ),
                                             'enumValues' => NULL,
+                                            'possibleTypes' => NULL,
                                         ),
                                     2 =>
-                                        array(
-                                            '__typename' => '__Type',
+                                        array (
                                             'kind' => 'OBJECT',
                                             'name' => '__Type',
                                             'fields' =>
-                                                array(
+                                                array (
                                                     0 =>
-                                                        array(
-                                                            '__typename' => '__Field',
+                                                        array (
                                                             'name' => 'kind',
                                                             'args' =>
-                                                                array(),
+                                                                array (
+                                                                ),
                                                             'type' =>
-                                                                array(
-                                                                    '__typename' => '__Type',
+                                                                array (
                                                                     'kind' => 'NON_NULL',
                                                                     'name' => NULL,
                                                                     'ofType' =>
-                                                                        array(
-                                                                            '__typename' => '__Type',
+                                                                        array (
                                                                             'kind' => 'ENUM',
                                                                             'name' => '__TypeKind',
                                                                             'ofType' => NULL,
@@ -266,14 +179,13 @@ EOD;
                                                             'deprecationReason' => NULL,
                                                         ),
                                                     1 =>
-                                                        array(
-                                                            '__typename' => '__Field',
+                                                        array (
                                                             'name' => 'name',
                                                             'args' =>
-                                                                array(),
+                                                                array (
+                                                                ),
                                                             'type' =>
-                                                                array(
-                                                                    '__typename' => '__Type',
+                                                                array (
                                                                     'kind' => 'SCALAR',
                                                                     'name' => 'String',
                                                                     'ofType' => NULL,
@@ -282,14 +194,13 @@ EOD;
                                                             'deprecationReason' => NULL,
                                                         ),
                                                     2 =>
-                                                        array(
-                                                            '__typename' => '__Field',
+                                                        array (
                                                             'name' => 'description',
                                                             'args' =>
-                                                                array(),
+                                                                array (
+                                                                ),
                                                             'type' =>
-                                                                array(
-                                                                    '__typename' => '__Type',
+                                                                array (
                                                                     'kind' => 'SCALAR',
                                                                     'name' => 'String',
                                                                     'ofType' => NULL,
@@ -298,18 +209,15 @@ EOD;
                                                             'deprecationReason' => NULL,
                                                         ),
                                                     3 =>
-                                                        array(
-                                                            '__typename' => '__Field',
+                                                        array (
                                                             'name' => 'fields',
                                                             'args' =>
-                                                                array(
+                                                                array (
                                                                     0 =>
-                                                                        array(
-                                                                            '__typename' => '__InputValue',
+                                                                        array (
                                                                             'name' => 'includeDeprecated',
                                                                             'type' =>
-                                                                                array(
-                                                                                    '__typename' => '__Type',
+                                                                                array (
                                                                                     'kind' => 'SCALAR',
                                                                                     'name' => 'Boolean',
                                                                                     'ofType' => NULL,
@@ -318,18 +226,15 @@ EOD;
                                                                         ),
                                                                 ),
                                                             'type' =>
-                                                                array(
-                                                                    '__typename' => '__Type',
+                                                                array (
                                                                     'kind' => 'LIST',
                                                                     'name' => NULL,
                                                                     'ofType' =>
-                                                                        array(
-                                                                            '__typename' => '__Type',
+                                                                        array (
                                                                             'kind' => 'NON_NULL',
                                                                             'name' => NULL,
                                                                             'ofType' =>
-                                                                                array(
-                                                                                    '__typename' => '__Type',
+                                                                                array (
                                                                                     'kind' => 'OBJECT',
                                                                                     'name' => '__Field',
                                                                                     'ofType' => NULL,
@@ -340,24 +245,21 @@ EOD;
                                                             'deprecationReason' => NULL,
                                                         ),
                                                     4 =>
-                                                        array(
-                                                            '__typename' => '__Field',
+                                                        array (
                                                             'name' => 'interfaces',
                                                             'args' =>
-                                                                array(),
+                                                                array (
+                                                                ),
                                                             'type' =>
-                                                                array(
-                                                                    '__typename' => '__Type',
+                                                                array (
                                                                     'kind' => 'LIST',
                                                                     'name' => NULL,
                                                                     'ofType' =>
-                                                                        array(
-                                                                            '__typename' => '__Type',
+                                                                        array (
                                                                             'kind' => 'NON_NULL',
                                                                             'name' => NULL,
                                                                             'ofType' =>
-                                                                                array(
-                                                                                    '__typename' => '__Type',
+                                                                                array (
                                                                                     'kind' => 'OBJECT',
                                                                                     'name' => '__Type',
                                                                                     'ofType' => NULL,
@@ -368,24 +270,21 @@ EOD;
                                                             'deprecationReason' => NULL,
                                                         ),
                                                     5 =>
-                                                        array(
-                                                            '__typename' => '__Field',
+                                                        array (
                                                             'name' => 'possibleTypes',
                                                             'args' =>
-                                                                array(),
+                                                                array (
+                                                                ),
                                                             'type' =>
-                                                                array(
-                                                                    '__typename' => '__Type',
+                                                                array (
                                                                     'kind' => 'LIST',
                                                                     'name' => NULL,
                                                                     'ofType' =>
-                                                                        array(
-                                                                            '__typename' => '__Type',
+                                                                        array (
                                                                             'kind' => 'NON_NULL',
                                                                             'name' => NULL,
                                                                             'ofType' =>
-                                                                                array(
-                                                                                    '__typename' => '__Type',
+                                                                                array (
                                                                                     'kind' => 'OBJECT',
                                                                                     'name' => '__Type',
                                                                                     'ofType' => NULL,
@@ -396,38 +295,32 @@ EOD;
                                                             'deprecationReason' => NULL,
                                                         ),
                                                     6 =>
-                                                        array(
-                                                            '__typename' => '__Field',
+                                                        array (
                                                             'name' => 'enumValues',
                                                             'args' =>
-                                                                array(
+                                                                array (
                                                                     0 =>
-                                                                        array(
-                                                                            '__typename' => '__InputValue',
-                                                                            'defaultValue' => 'false',
+                                                                        array (
                                                                             'name' => 'includeDeprecated',
                                                                             'type' =>
-                                                                                array(
-                                                                                    '__typename' => '__Type',
+                                                                                array (
                                                                                     'kind' => 'SCALAR',
                                                                                     'name' => 'Boolean',
                                                                                     'ofType' => NULL,
                                                                                 ),
+                                                                            'defaultValue' => 'false',
                                                                         ),
                                                                 ),
                                                             'type' =>
-                                                                array(
-                                                                    '__typename' => '__Type',
+                                                                array (
                                                                     'kind' => 'LIST',
                                                                     'name' => NULL,
                                                                     'ofType' =>
-                                                                        array(
-                                                                            '__typename' => '__Type',
+                                                                        array (
                                                                             'kind' => 'NON_NULL',
                                                                             'name' => NULL,
                                                                             'ofType' =>
-                                                                                array(
-                                                                                    '__typename' => '__Type',
+                                                                                array (
                                                                                     'kind' => 'OBJECT',
                                                                                     'name' => '__EnumValue',
                                                                                     'ofType' => NULL,
@@ -438,24 +331,21 @@ EOD;
                                                             'deprecationReason' => NULL,
                                                         ),
                                                     7 =>
-                                                        array(
-                                                            '__typename' => '__Field',
+                                                        array (
                                                             'name' => 'inputFields',
                                                             'args' =>
-                                                                array(),
+                                                                array (
+                                                                ),
                                                             'type' =>
-                                                                array(
-                                                                    '__typename' => '__Type',
+                                                                array (
                                                                     'kind' => 'LIST',
                                                                     'name' => NULL,
                                                                     'ofType' =>
-                                                                        array(
-                                                                            '__typename' => '__Type',
+                                                                        array (
                                                                             'kind' => 'NON_NULL',
                                                                             'name' => NULL,
                                                                             'ofType' =>
-                                                                                array(
-                                                                                    '__typename' => '__Type',
+                                                                                array (
                                                                                     'kind' => 'OBJECT',
                                                                                     'name' => '__InputValue',
                                                                                     'ofType' => NULL,
@@ -466,14 +356,13 @@ EOD;
                                                             'deprecationReason' => NULL,
                                                         ),
                                                     8 =>
-                                                        array(
-                                                            '__typename' => '__Field',
+                                                        array (
                                                             'name' => 'ofType',
                                                             'args' =>
-                                                                array(),
+                                                                array (
+                                                                ),
                                                             'type' =>
-                                                                array(
-                                                                    '__typename' => '__Type',
+                                                                array (
                                                                     'kind' => 'OBJECT',
                                                                     'name' => '__Type',
                                                                     'ofType' => NULL,
@@ -482,116 +371,111 @@ EOD;
                                                             'deprecationReason' => NULL,
                                                         ),
                                                 ),
+                                            'inputFields' => NULL,
                                             'interfaces' =>
-                                                array(),
+                                                array (
+                                                ),
                                             'enumValues' => NULL,
+                                            'possibleTypes' => NULL,
                                         ),
                                     3 =>
-                                        array(
-                                            '__typename' => '__Type',
+                                        array (
                                             'kind' => 'ENUM',
                                             'name' => '__TypeKind',
                                             'fields' => NULL,
+                                            'inputFields' => NULL,
                                             'interfaces' => NULL,
                                             'enumValues' =>
-                                                array(
+                                                array (
                                                     0 =>
-                                                        array(
-                                                            '__typename' => '__EnumValue',
+                                                        array (
                                                             'name' => 'SCALAR',
                                                             'isDeprecated' => false,
                                                             'deprecationReason' => NULL,
                                                         ),
                                                     1 =>
-                                                        array(
-                                                            '__typename' => '__EnumValue',
+                                                        array (
                                                             'name' => 'OBJECT',
                                                             'isDeprecated' => false,
                                                             'deprecationReason' => NULL,
                                                         ),
                                                     2 =>
-                                                        array(
-                                                            '__typename' => '__EnumValue',
+                                                        array (
                                                             'name' => 'INTERFACE',
                                                             'isDeprecated' => false,
                                                             'deprecationReason' => NULL,
                                                         ),
                                                     3 =>
-                                                        array(
-                                                            '__typename' => '__EnumValue',
+                                                        array (
                                                             'name' => 'UNION',
                                                             'isDeprecated' => false,
                                                             'deprecationReason' => NULL,
                                                         ),
                                                     4 =>
-                                                        array(
-                                                            '__typename' => '__EnumValue',
+                                                        array (
                                                             'name' => 'ENUM',
                                                             'isDeprecated' => false,
                                                             'deprecationReason' => NULL,
                                                         ),
                                                     5 =>
-                                                        array(
-                                                            '__typename' => '__EnumValue',
+                                                        array (
                                                             'name' => 'INPUT_OBJECT',
                                                             'isDeprecated' => false,
                                                             'deprecationReason' => NULL,
                                                         ),
                                                     6 =>
-                                                        array(
-                                                            '__typename' => '__EnumValue',
+                                                        array (
                                                             'name' => 'LIST',
                                                             'isDeprecated' => false,
                                                             'deprecationReason' => NULL,
                                                         ),
                                                     7 =>
-                                                        array(
-                                                            '__typename' => '__EnumValue',
+                                                        array (
                                                             'name' => 'NON_NULL',
                                                             'isDeprecated' => false,
                                                             'deprecationReason' => NULL,
                                                         ),
                                                 ),
+                                            'possibleTypes' => NULL,
                                         ),
                                     4 =>
-                                        array(
-                                            '__typename' => '__Type',
+                                        array (
                                             'kind' => 'SCALAR',
                                             'name' => 'String',
                                             'fields' => NULL,
+                                            'inputFields' => NULL,
                                             'interfaces' => NULL,
                                             'enumValues' => NULL,
+                                            'possibleTypes' => NULL,
                                         ),
                                     5 =>
-                                        array(
-                                            '__typename' => '__Type',
+                                        array (
                                             'kind' => 'SCALAR',
                                             'name' => 'Boolean',
                                             'fields' => NULL,
+                                            'inputFields' => NULL,
                                             'interfaces' => NULL,
                                             'enumValues' => NULL,
+                                            'possibleTypes' => NULL,
                                         ),
                                     6 =>
-                                        array(
-                                            '__typename' => '__Type',
+                                        array (
                                             'kind' => 'OBJECT',
                                             'name' => '__Field',
                                             'fields' =>
-                                                array(
+                                                array (
                                                     0 =>
-                                                        array(
-                                                            '__typename' => '__Field',
+                                                        array (
                                                             'name' => 'name',
                                                             'args' =>
-                                                                array(),
+                                                                array (
+                                                                ),
                                                             'type' =>
-                                                                array(
-                                                                    '__typename' => '__Type',
+                                                                array (
                                                                     'kind' => 'NON_NULL',
                                                                     'name' => NULL,
                                                                     'ofType' =>
-                                                                        array(
-                                                                            '__typename' => '__Type',
+                                                                        array (
                                                                             'kind' => 'SCALAR',
                                                                             'name' => 'String',
                                                                             'ofType' => NULL,
@@ -601,14 +485,13 @@ EOD;
                                                             'deprecationReason' => NULL,
                                                         ),
                                                     1 =>
-                                                        array(
-                                                            '__typename' => '__Field',
+                                                        array (
                                                             'name' => 'description',
                                                             'args' =>
-                                                                array(),
+                                                                array (
+                                                                ),
                                                             'type' =>
-                                                                array(
-                                                                    '__typename' => '__Type',
+                                                                array (
                                                                     'kind' => 'SCALAR',
                                                                     'name' => 'String',
                                                                     'ofType' => NULL,
@@ -617,29 +500,25 @@ EOD;
                                                             'deprecationReason' => NULL,
                                                         ),
                                                     2 =>
-                                                        array(
-                                                            '__typename' => '__Field',
+                                                        array (
                                                             'name' => 'args',
                                                             'args' =>
-                                                                array(),
+                                                                array (
+                                                                ),
                                                             'type' =>
-                                                                array(
-                                                                    '__typename' => '__Type',
+                                                                array (
                                                                     'kind' => 'NON_NULL',
                                                                     'name' => NULL,
                                                                     'ofType' =>
-                                                                        array(
-                                                                            '__typename' => '__Type',
+                                                                        array (
                                                                             'kind' => 'LIST',
                                                                             'name' => NULL,
                                                                             'ofType' =>
-                                                                                array(
-                                                                                    '__typename' => '__Type',
+                                                                                array (
                                                                                     'kind' => 'NON_NULL',
                                                                                     'name' => NULL,
                                                                                     'ofType' =>
-                                                                                        array(
-                                                                                            '__typename' => '__Type',
+                                                                                        array (
                                                                                             'kind' => 'OBJECT',
                                                                                             'name' => '__InputValue',
                                                                                         ),
@@ -650,19 +529,17 @@ EOD;
                                                             'deprecationReason' => NULL,
                                                         ),
                                                     3 =>
-                                                        array(
-                                                            '__typename' => '__Field',
+                                                        array (
                                                             'name' => 'type',
                                                             'args' =>
-                                                                array(),
+                                                                array (
+                                                                ),
                                                             'type' =>
-                                                                array(
-                                                                    '__typename' => '__Type',
+                                                                array (
                                                                     'kind' => 'NON_NULL',
                                                                     'name' => NULL,
                                                                     'ofType' =>
-                                                                        array(
-                                                                            '__typename' => '__Type',
+                                                                        array (
                                                                             'kind' => 'OBJECT',
                                                                             'name' => '__Type',
                                                                             'ofType' => NULL,
@@ -672,19 +549,17 @@ EOD;
                                                             'deprecationReason' => NULL,
                                                         ),
                                                     4 =>
-                                                        array(
-                                                            '__typename' => '__Field',
+                                                        array (
                                                             'name' => 'isDeprecated',
                                                             'args' =>
-                                                                array(),
+                                                                array (
+                                                                ),
                                                             'type' =>
-                                                                array(
-                                                                    '__typename' => '__Type',
+                                                                array (
                                                                     'kind' => 'NON_NULL',
                                                                     'name' => NULL,
                                                                     'ofType' =>
-                                                                        array(
-                                                                            '__typename' => '__Type',
+                                                                        array (
                                                                             'kind' => 'SCALAR',
                                                                             'name' => 'Boolean',
                                                                             'ofType' => NULL,
@@ -694,14 +569,13 @@ EOD;
                                                             'deprecationReason' => NULL,
                                                         ),
                                                     5 =>
-                                                        array(
-                                                            '__typename' => '__Field',
+                                                        array (
                                                             'name' => 'deprecationReason',
                                                             'args' =>
-                                                                array(),
+                                                                array (
+                                                                ),
                                                             'type' =>
-                                                                array(
-                                                                    '__typename' => '__Type',
+                                                                array (
                                                                     'kind' => 'SCALAR',
                                                                     'name' => 'String',
                                                                     'ofType' => NULL,
@@ -710,31 +584,31 @@ EOD;
                                                             'deprecationReason' => NULL,
                                                         ),
                                                 ),
+                                            'inputFields' => NULL,
                                             'interfaces' =>
-                                                array(),
+                                                array (
+                                                ),
                                             'enumValues' => NULL,
+                                            'possibleTypes' => NULL,
                                         ),
                                     7 =>
-                                        array(
-                                            '__typename' => '__Type',
+                                        array (
                                             'kind' => 'OBJECT',
                                             'name' => '__InputValue',
                                             'fields' =>
-                                                array(
+                                                array (
                                                     0 =>
-                                                        array(
-                                                            '__typename' => '__Field',
+                                                        array (
                                                             'name' => 'name',
                                                             'args' =>
-                                                                array(),
+                                                                array (
+                                                                ),
                                                             'type' =>
-                                                                array(
-                                                                    '__typename' => '__Type',
+                                                                array (
                                                                     'kind' => 'NON_NULL',
                                                                     'name' => NULL,
                                                                     'ofType' =>
-                                                                        array(
-                                                                            '__typename' => '__Type',
+                                                                        array (
                                                                             'kind' => 'SCALAR',
                                                                             'name' => 'String',
                                                                             'ofType' => NULL,
@@ -744,14 +618,13 @@ EOD;
                                                             'deprecationReason' => NULL,
                                                         ),
                                                     1 =>
-                                                        array(
-                                                            '__typename' => '__Field',
+                                                        array (
                                                             'name' => 'description',
                                                             'args' =>
-                                                                array(),
+                                                                array (
+                                                                ),
                                                             'type' =>
-                                                                array(
-                                                                    '__typename' => '__Type',
+                                                                array (
                                                                     'kind' => 'SCALAR',
                                                                     'name' => 'String',
                                                                     'ofType' => NULL,
@@ -760,19 +633,17 @@ EOD;
                                                             'deprecationReason' => NULL,
                                                         ),
                                                     2 =>
-                                                        array(
-                                                            '__typename' => '__Field',
+                                                        array (
                                                             'name' => 'type',
                                                             'args' =>
-                                                                array(),
+                                                                array (
+                                                                ),
                                                             'type' =>
-                                                                array(
-                                                                    '__typename' => '__Type',
+                                                                array (
                                                                     'kind' => 'NON_NULL',
                                                                     'name' => NULL,
                                                                     'ofType' =>
-                                                                        array(
-                                                                            '__typename' => '__Type',
+                                                                        array (
                                                                             'kind' => 'OBJECT',
                                                                             'name' => '__Type',
                                                                             'ofType' => NULL,
@@ -782,14 +653,13 @@ EOD;
                                                             'deprecationReason' => NULL,
                                                         ),
                                                     3 =>
-                                                        array(
-                                                            '__typename' => '__Field',
+                                                        array (
                                                             'name' => 'defaultValue',
                                                             'args' =>
-                                                                array(),
+                                                                array (
+                                                                ),
                                                             'type' =>
-                                                                array(
-                                                                    '__typename' => '__Type',
+                                                                array (
                                                                     'kind' => 'SCALAR',
                                                                     'name' => 'String',
                                                                     'ofType' => NULL,
@@ -798,31 +668,31 @@ EOD;
                                                             'deprecationReason' => NULL,
                                                         ),
                                                 ),
+                                            'inputFields' => NULL,
                                             'interfaces' =>
-                                                array(),
+                                                array (
+                                                ),
                                             'enumValues' => NULL,
+                                            'possibleTypes' => NULL,
                                         ),
                                     8 =>
-                                        array(
-                                            '__typename' => '__Type',
+                                        array (
                                             'kind' => 'OBJECT',
                                             'name' => '__EnumValue',
                                             'fields' =>
-                                                array(
+                                                array (
                                                     0 =>
-                                                        array(
-                                                            '__typename' => '__Field',
+                                                        array (
                                                             'name' => 'name',
                                                             'args' =>
-                                                                array(),
+                                                                array (
+                                                                ),
                                                             'type' =>
-                                                                array(
-                                                                    '__typename' => '__Type',
+                                                                array (
                                                                     'kind' => 'NON_NULL',
                                                                     'name' => NULL,
                                                                     'ofType' =>
-                                                                        array(
-                                                                            '__typename' => '__Type',
+                                                                        array (
                                                                             'kind' => 'SCALAR',
                                                                             'name' => 'String',
                                                                             'ofType' => NULL,
@@ -832,14 +702,13 @@ EOD;
                                                             'deprecationReason' => NULL,
                                                         ),
                                                     1 =>
-                                                        array(
-                                                            '__typename' => '__Field',
+                                                        array (
                                                             'name' => 'description',
                                                             'args' =>
-                                                                array(),
+                                                                array (
+                                                                ),
                                                             'type' =>
-                                                                array(
-                                                                    '__typename' => '__Type',
+                                                                array (
                                                                     'kind' => 'SCALAR',
                                                                     'name' => 'String',
                                                                     'ofType' => NULL,
@@ -848,19 +717,17 @@ EOD;
                                                             'deprecationReason' => NULL,
                                                         ),
                                                     2 =>
-                                                        array(
-                                                            '__typename' => '__Field',
+                                                        array (
                                                             'name' => 'isDeprecated',
                                                             'args' =>
-                                                                array(),
+                                                                array (
+                                                                ),
                                                             'type' =>
-                                                                array(
-                                                                    '__typename' => '__Type',
+                                                                array (
                                                                     'kind' => 'NON_NULL',
                                                                     'name' => NULL,
                                                                     'ofType' =>
-                                                                        array(
-                                                                            '__typename' => '__Type',
+                                                                        array (
                                                                             'kind' => 'SCALAR',
                                                                             'name' => 'Boolean',
                                                                             'ofType' => NULL,
@@ -870,14 +737,13 @@ EOD;
                                                             'deprecationReason' => NULL,
                                                         ),
                                                     3 =>
-                                                        array(
-                                                            '__typename' => '__Field',
+                                                        array (
                                                             'name' => 'deprecationReason',
                                                             'args' =>
-                                                                array(),
+                                                                array (
+                                                                ),
                                                             'type' =>
-                                                                array(
-                                                                    '__typename' => '__Type',
+                                                                array (
                                                                     'kind' => 'SCALAR',
                                                                     'name' => 'String',
                                                                     'ofType' => NULL,
@@ -886,42 +752,47 @@ EOD;
                                                             'deprecationReason' => NULL,
                                                         ),
                                                 ),
+                                            'inputFields' => NULL,
                                             'interfaces' =>
-                                                array(),
+                                                array (
+                                                ),
                                             'enumValues' => NULL,
+                                            'possibleTypes' => NULL,
                                         ),
                                     9 =>
-                                        array(
-                                            '__typename' => '__Type',
+                                        array (
                                             'kind' => 'OBJECT',
                                             'name' => '__Directive',
                                             'fields' =>
-                                                array(
+                                                array (
                                                     0 =>
-                                                        array(
-                                                            '__typename' => '__Field',
+                                                        array (
                                                             'name' => 'name',
                                                             'args' =>
-                                                                array(),
+                                                                array (
+                                                                ),
                                                             'type' =>
-                                                                array(
-                                                                    '__typename' => '__Type',
-                                                                    'kind' => 'SCALAR',
-                                                                    'name' => 'String',
-                                                                    'ofType' => NULL,
+                                                                array (
+                                                                    'kind' => 'NON_NULL',
+                                                                    'name' => NULL,
+                                                                    'ofType' =>
+                                                                        array (
+                                                                            'kind' => 'SCALAR',
+                                                                            'name' => 'String',
+                                                                            'ofType' => NULL,
+                                                                        ),
                                                                 ),
                                                             'isDeprecated' => false,
                                                             'deprecationReason' => NULL,
                                                         ),
                                                     1 =>
-                                                        array(
-                                                            '__typename' => '__Field',
+                                                        array (
                                                             'name' => 'description',
                                                             'args' =>
-                                                                array(),
+                                                                array (
+                                                                ),
                                                             'type' =>
-                                                                array(
-                                                                    '__typename' => '__Type',
+                                                                array (
                                                                     'kind' => 'SCALAR',
                                                                     'name' => 'String',
                                                                     'ofType' => NULL,
@@ -930,149 +801,192 @@ EOD;
                                                             'deprecationReason' => NULL,
                                                         ),
                                                     2 =>
-                                                        array(
-                                                            '__typename' => '__Field',
-                                                            'name' => 'type',
+                                                        array (
+                                                            'name' => 'args',
                                                             'args' =>
-                                                                array(),
+                                                                array (
+                                                                ),
                                                             'type' =>
-                                                                array(
-                                                                    '__typename' => '__Type',
-                                                                    'kind' => 'OBJECT',
-                                                                    'name' => '__Type',
-                                                                    'ofType' => NULL,
+                                                                array (
+                                                                    'kind' => 'NON_NULL',
+                                                                    'name' => NULL,
+                                                                    'ofType' =>
+                                                                        array (
+                                                                            'kind' => 'LIST',
+                                                                            'name' => NULL,
+                                                                            'ofType' =>
+                                                                                array (
+                                                                                    'kind' => 'NON_NULL',
+                                                                                    'name' => NULL,
+                                                                                    'ofType' =>
+                                                                                        array (
+                                                                                            'kind' => 'OBJECT',
+                                                                                            'name' => '__InputValue',
+                                                                                        ),
+                                                                                ),
+                                                                        ),
                                                                 ),
                                                             'isDeprecated' => false,
                                                             'deprecationReason' => NULL,
                                                         ),
                                                     3 =>
-                                                        array(
-                                                            '__typename' => '__Field',
+                                                        array (
                                                             'name' => 'onOperation',
                                                             'args' =>
-                                                                array(),
+                                                                array (
+                                                                ),
                                                             'type' =>
-                                                                array(
-                                                                    '__typename' => '__Type',
-                                                                    'kind' => 'SCALAR',
-                                                                    'name' => 'Boolean',
-                                                                    'ofType' => NULL,
+                                                                array (
+                                                                    'kind' => 'NON_NULL',
+                                                                    'name' => NULL,
+                                                                    'ofType' =>
+                                                                        array (
+                                                                            'kind' => 'SCALAR',
+                                                                            'name' => 'Boolean',
+                                                                            'ofType' => NULL,
+                                                                        ),
                                                                 ),
                                                             'isDeprecated' => false,
                                                             'deprecationReason' => NULL,
                                                         ),
                                                     4 =>
-                                                        array(
-                                                            '__typename' => '__Field',
+                                                        array (
                                                             'name' => 'onFragment',
                                                             'args' =>
-                                                                array(),
+                                                                array (
+                                                                ),
                                                             'type' =>
-                                                                array(
-                                                                    '__typename' => '__Type',
-                                                                    'kind' => 'SCALAR',
-                                                                    'name' => 'Boolean',
-                                                                    'ofType' => NULL,
+                                                                array (
+                                                                    'kind' => 'NON_NULL',
+                                                                    'name' => NULL,
+                                                                    'ofType' =>
+                                                                        array (
+                                                                            'kind' => 'SCALAR',
+                                                                            'name' => 'Boolean',
+                                                                            'ofType' => NULL,
+                                                                        ),
                                                                 ),
                                                             'isDeprecated' => false,
                                                             'deprecationReason' => NULL,
                                                         ),
                                                     5 =>
-                                                        array(
-                                                            '__typename' => '__Field',
+                                                        array (
                                                             'name' => 'onField',
                                                             'args' =>
-                                                                array(),
+                                                                array (
+                                                                ),
                                                             'type' =>
-                                                                array(
-                                                                    '__typename' => '__Type',
-                                                                    'kind' => 'SCALAR',
-                                                                    'name' => 'Boolean',
-                                                                    'ofType' => NULL,
+                                                                array (
+                                                                    'kind' => 'NON_NULL',
+                                                                    'name' => NULL,
+                                                                    'ofType' =>
+                                                                        array (
+                                                                            'kind' => 'SCALAR',
+                                                                            'name' => 'Boolean',
+                                                                            'ofType' => NULL,
+                                                                        ),
                                                                 ),
                                                             'isDeprecated' => false,
                                                             'deprecationReason' => NULL,
                                                         ),
                                                 ),
+                                            'inputFields' => NULL,
                                             'interfaces' =>
-                                                array(),
+                                                array (
+                                                ),
                                             'enumValues' => NULL,
+                                            'possibleTypes' => NULL,
                                         ),
                                     10 => [
-                                        '__typename' => '__Type',
                                         'kind' => 'SCALAR',
                                         'name' => 'ID',
                                         'fields' => null,
+                                        'inputFields' => null,
                                         'interfaces' => null,
                                         'enumValues' => null,
+                                        'possibleTypes' => null
                                     ],
                                     11 => [
-                                        '__typename' => '__Type',
                                         'kind' => 'SCALAR',
                                         'name' => 'Float',
                                         'fields' => null,
+                                        'inputFields' => null,
                                         'interfaces' => null,
                                         'enumValues' => null,
+                                        'possibleTypes' => null
                                     ],
                                     12 => [
-                                        '__typename' => '__Type',
                                         'kind' => 'SCALAR',
                                         'name' => 'Int',
                                         'fields' => null,
+                                        'inputFields' => null,
                                         'interfaces' => null,
                                         'enumValues' => null,
-                                    ]
+                                        'possibleTypes' => null
+                                    ],
                                 ),
                             'directives' =>
-                                array(
+                                array (
                                     0 =>
-                                        array(
-                                            '__typename' => '__Directive',
-                                            'name' => 'if',
-                                            'type' =>
-                                                array(
-                                                    '__typename' => '__Type',
-                                                    'kind' => 'NON_NULL',
-                                                    'name' => NULL,
-                                                    'ofType' =>
-                                                        array(
-                                                            '__typename' => '__Type',
-                                                            'kind' => 'SCALAR',
-                                                            'name' => 'Boolean',
-                                                            'ofType' => NULL,
+                                        array (
+                                            'name' => 'include',
+                                            'args' =>
+                                                array (
+                                                    0 =>
+                                                        array (
+                                                            'defaultValue' => NULL,
+                                                            'name' => 'if',
+                                                            'type' =>
+                                                                array (
+                                                                    'kind' => 'NON_NULL',
+                                                                    'name' => NULL,
+                                                                    'ofType' =>
+                                                                        array (
+                                                                            'kind' => 'SCALAR',
+                                                                            'name' => 'Boolean',
+                                                                            'ofType' => NULL,
+                                                                        ),
+                                                                ),
                                                         ),
                                                 ),
                                             'onOperation' => false,
-                                            'onFragment' => false,
+                                            'onFragment' => true,
                                             'onField' => true,
                                         ),
                                     1 =>
-                                        array(
-                                            '__typename' => '__Directive',
-                                            'name' => 'unless',
-                                            'type' =>
-                                                array(
-                                                    '__typename' => '__Type',
-                                                    'kind' => 'NON_NULL',
-                                                    'name' => NULL,
-                                                    'ofType' =>
-                                                        array(
-                                                            '__typename' => '__Type',
-                                                            'kind' => 'SCALAR',
-                                                            'name' => 'Boolean',
-                                                            'ofType' => NULL,
+                                        array (
+                                            'name' => 'skip',
+                                            'args' =>
+                                                array (
+                                                    0 =>
+                                                        array (
+                                                            'defaultValue' => NULL,
+                                                            'name' => 'if',
+                                                            'type' =>
+                                                                array (
+                                                                    'kind' => 'NON_NULL',
+                                                                    'name' => NULL,
+                                                                    'ofType' =>
+                                                                        array (
+                                                                            'kind' => 'SCALAR',
+                                                                            'name' => 'Boolean',
+                                                                            'ofType' => NULL,
+                                                                        ),
+                                                                ),
                                                         ),
                                                 ),
                                             'onOperation' => false,
-                                            'onFragment' => false,
+                                            'onFragment' => true,
                                             'onField' => true,
                                         ),
                                 ),
                         ),
-                )
+                ),
         );
 
-        $this->assertEquals($expected, GraphQL::execute($emptySchema, $request));
+        $actual = GraphQL::execute($emptySchema, $request);
+
+        $this->assertEquals($expected, $actual);
     }
 
     function testIntrospectsOnInputObject()
@@ -1445,7 +1359,9 @@ EOD;
     ';
         $expected = [
             'errors' => [
-                new FormattedError(Messages::missingArgMessage('__type', 'name', 'String!'), [new SourceLocation(3, 9)])
+                FormattedError::create(
+                    ProvidedNonNullArguments::missingFieldArgMessage('__type', 'name', 'String!'), [new SourceLocation(3, 9)]
+                )
             ]
         ];
         $this->assertEquals($expected, GraphQL::execute($schema, $request));
diff --git a/tests/Type/ScalarCoercionTest.php b/tests/Type/ScalarCoercionTest.php
deleted file mode 100644
index f3257d1..0000000
--- a/tests/Type/ScalarCoercionTest.php
+++ /dev/null
@@ -1,63 +0,0 @@
-<?php
-namespace GraphQL\Type;
-
-use GraphQL\Type\Definition\Type;
-
-class ScalarCoercionTest extends \PHPUnit_Framework_TestCase
-{
-    public function testCoercesOutputInt()
-    {
-        $intType = Type::int();
-
-        $this->assertSame(1, $intType->coerce(1));
-        $this->assertSame(0, $intType->coerce(0));
-        $this->assertSame(0, $intType->coerce(0.1));
-        $this->assertSame(1, $intType->coerce(1.1));
-        $this->assertSame(-1, $intType->coerce(-1.1));
-        $this->assertSame(100000, $intType->coerce(1e5));
-        $this->assertSame(null, $intType->coerce(1e100));
-        $this->assertSame(null, $intType->coerce(-1e100));
-        $this->assertSame(-1, $intType->coerce('-1.1'));
-        $this->assertSame(null, $intType->coerce('one'));
-        $this->assertSame(0, $intType->coerce(false));
-    }
-
-    public function testCoercesOutputFloat()
-    {
-        $floatType = Type::float();
-
-        $this->assertSame(1.0, $floatType->coerce(1));
-        $this->assertSame(-1.0, $floatType->coerce(-1));
-        $this->assertSame(0.1, $floatType->coerce(0.1));
-        $this->assertSame(1.1, $floatType->coerce(1.1));
-        $this->assertSame(-1.1, $floatType->coerce(-1.1));
-        $this->assertSame(null, $floatType->coerce('one'));
-        $this->assertSame(0.0, $floatType->coerce(false));
-        $this->assertSame(1.0, $floatType->coerce(true));
-    }
-
-    public function testCoercesOutputStrings()
-    {
-        $stringType = Type::string();
-
-        $this->assertSame('string', $stringType->coerce('string'));
-        $this->assertSame('1', $stringType->coerce(1));
-        $this->assertSame('-1.1', $stringType->coerce(-1.1));
-        $this->assertSame('true', $stringType->coerce(true));
-        $this->assertSame('false', $stringType->coerce(false));
-    }
-
-    public function testCoercesOutputBoolean()
-    {
-        $boolType = Type::boolean();
-
-        $this->assertSame(true, $boolType->coerce('string'));
-        $this->assertSame(false, $boolType->coerce(''));
-        $this->assertSame(true, $boolType->coerce(1));
-        $this->assertSame(false, $boolType->coerce(0));
-        $this->assertSame(true, $boolType->coerce(true));
-        $this->assertSame(false, $boolType->coerce(false));
-
-        // TODO: how should it behaive on '0'?
-    }
-}
\ No newline at end of file
diff --git a/tests/Type/ScalarSerializationTest.php b/tests/Type/ScalarSerializationTest.php
new file mode 100644
index 0000000..80ef84b
--- /dev/null
+++ b/tests/Type/ScalarSerializationTest.php
@@ -0,0 +1,63 @@
+<?php
+namespace GraphQL\Type;
+
+use GraphQL\Type\Definition\Type;
+
+class ScalarSerializationTest extends \PHPUnit_Framework_TestCase
+{
+    public function testCoercesOutputInt()
+    {
+        $intType = Type::int();
+
+        $this->assertSame(1, $intType->serialize(1));
+        $this->assertSame(0, $intType->serialize(0));
+        $this->assertSame(0, $intType->serialize(0.1));
+        $this->assertSame(1, $intType->serialize(1.1));
+        $this->assertSame(-1, $intType->serialize(-1.1));
+        $this->assertSame(100000, $intType->serialize(1e5));
+        $this->assertSame(null, $intType->serialize(1e100));
+        $this->assertSame(null, $intType->serialize(-1e100));
+        $this->assertSame(-1, $intType->serialize('-1.1'));
+        $this->assertSame(null, $intType->serialize('one'));
+        $this->assertSame(0, $intType->serialize(false));
+    }
+
+    public function testCoercesOutputFloat()
+    {
+        $floatType = Type::float();
+
+        $this->assertSame(1.0, $floatType->serialize(1));
+        $this->assertSame(-1.0, $floatType->serialize(-1));
+        $this->assertSame(0.1, $floatType->serialize(0.1));
+        $this->assertSame(1.1, $floatType->serialize(1.1));
+        $this->assertSame(-1.1, $floatType->serialize(-1.1));
+        $this->assertSame(null, $floatType->serialize('one'));
+        $this->assertSame(0.0, $floatType->serialize(false));
+        $this->assertSame(1.0, $floatType->serialize(true));
+    }
+
+    public function testCoercesOutputStrings()
+    {
+        $stringType = Type::string();
+
+        $this->assertSame('string', $stringType->serialize('string'));
+        $this->assertSame('1', $stringType->serialize(1));
+        $this->assertSame('-1.1', $stringType->serialize(-1.1));
+        $this->assertSame('true', $stringType->serialize(true));
+        $this->assertSame('false', $stringType->serialize(false));
+    }
+
+    public function testCoercesOutputBoolean()
+    {
+        $boolType = Type::boolean();
+
+        $this->assertSame(true, $boolType->serialize('string'));
+        $this->assertSame(false, $boolType->serialize(''));
+        $this->assertSame(true, $boolType->serialize(1));
+        $this->assertSame(false, $boolType->serialize(0));
+        $this->assertSame(true, $boolType->serialize(true));
+        $this->assertSame(false, $boolType->serialize(false));
+
+        // TODO: how should it behaive on '0'?
+    }
+}
\ No newline at end of file
diff --git a/tests/Type/SchemaValidatorTest.php b/tests/Type/SchemaValidatorTest.php
index cb08cc9..402f964 100644
--- a/tests/Type/SchemaValidatorTest.php
+++ b/tests/Type/SchemaValidatorTest.php
@@ -29,10 +29,8 @@ class SchemaValidatorTest extends \PHPUnit_Framework_TestCase
     public function testPassesOnTheIntrospectionSchema()
     {
         $schema = new Schema(Introspection::_schema());
-        $validationResult = SchemaValidator::validate($schema);
-
-        $this->assertSame(true, $validationResult->isValid);
-        $this->assertSame(null, $validationResult->errors);
+        $errors = SchemaValidator::validate($schema);
+        $this->assertEmpty($errors);
     }
 
 
@@ -65,12 +63,11 @@ class SchemaValidatorTest extends \PHPUnit_Framework_TestCase
             $schema = new Schema($someOutputType);
             $validationResult = SchemaValidator::validate($schema, [SchemaValidator::noInputTypesAsOutputFieldsRule()]);
 
-            $this->assertSame(false, $validationResult->isValid);
-            $this->assertSame(1, count($validationResult->errors));
+            $this->assertSame(1, count($validationResult));
             $this->assertSame(
                 'Field SomeOutputType.sneaky is of type SomeInputType, which is an ' .
                 'input type, but field types must be output types!',
-                $validationResult->errors[0]->message
+                $validationResult[0]->message
             );
         }
     }
@@ -93,17 +90,17 @@ class SchemaValidatorTest extends \PHPUnit_Framework_TestCase
         ]);
 
         $schema = new Schema($someOutputType);
-        $validationResult = SchemaValidator::validate($schema, [$rule]);
-        $this->assertSame(true, $validationResult->isValid);
+        $errors = SchemaValidator::validate($schema, [$rule]);
+        $this->assertEmpty($errors);
     }
 
-    private function checkValidationResult($validationResult, $operationType)
+    private function checkValidationResult($validationErrors, $operationType)
     {
-        $this->assertEquals(false, $validationResult->isValid);
-        $this->assertEquals(1, count($validationResult->errors));
+        $this->assertNotEmpty($validationErrors, "Should not validate");
+        $this->assertEquals(1, count($validationErrors));
         $this->assertEquals(
             "Schema $operationType type SomeInputType must be an object type!",
-            $validationResult->errors[0]->message
+            $validationErrors[0]->message
         );
     }
 
@@ -197,8 +194,8 @@ class SchemaValidatorTest extends \PHPUnit_Framework_TestCase
     private function assertAcceptingFieldArgOfType($fieldArgType)
     {
         $schema = $this->schemaWithFieldArgOfType($fieldArgType);
-        $validationResult = SchemaValidator::validate($schema, [SchemaValidator::noOutputTypesAsInputArgsRule()]);
-        $this->assertSame(true, $validationResult->isValid);
+        $errors = SchemaValidator::validate($schema, [SchemaValidator::noOutputTypesAsInputArgsRule()]);
+        $this->assertEmpty($errors);
     }
 
     private function schemaWithFieldArgOfType($argType)
@@ -223,14 +220,13 @@ class SchemaValidatorTest extends \PHPUnit_Framework_TestCase
         return new Schema($queryType);
     }
 
-    private function expectRejectionBecauseFieldIsNotInputType($validationResult, $fieldTypeName)
+    private function expectRejectionBecauseFieldIsNotInputType($errors, $fieldTypeName)
     {
-        $this->assertSame(false, $validationResult->isValid);
-        $this->assertSame(1, count($validationResult->errors));
+        $this->assertSame(1, count($errors));
         $this->assertSame(
             "Input field SomeIncorrectInputType.val has type $fieldTypeName, " .
             "which is not an input type!",
-            $validationResult->errors[0]->message
+            $errors[0]->message
         );
     }
 
@@ -245,40 +241,9 @@ class SchemaValidatorTest extends \PHPUnit_Framework_TestCase
 
     public function testRejectsWhenAPossibleTypeDoesNotImplementTheInterface()
     {
-        // rejects when a possible type does not implement the interface
-        $InterfaceType = new InterfaceType([
-            'name' => 'InterfaceType',
-            'fields' => []
-        ]);
-
-        $SubType = new ObjectType([
-            'name' => 'SubType',
-            'fields' => [],
-            'interfaces' => []
-        ]);
-
-        InterfaceType::addImplementationToInterfaces($SubType, [$InterfaceType]);
-
-        // Sanity check.
-        $this->assertEquals(1, count($InterfaceType->getPossibleTypes()));
-        $this->assertEquals($SubType, $InterfaceType->getPossibleTypes()[0]);
-
-
-        $schema = new Schema($InterfaceType);
-        $validationResult = SchemaValidator::validate(
-            $schema,
-            [SchemaValidator::interfacePossibleTypesMustImplementTheInterfaceRule()]
-        );
-        $this->assertSame(false, $validationResult->isValid);
-        $this->assertSame(1, count($validationResult->errors));
-        $this->assertSame(
-            'SubType is a possible type of interface InterfaceType but does not ' .
-            'implement it!',
-            $validationResult->errors[0]->message
-        );
+        // TODO: Validation for interfaces / implementors
     }
 
-
     private function assertAcceptingAnInterfaceWithANormalSubtype($rule)
     {
         $interfaceType = new InterfaceType([
@@ -294,8 +259,8 @@ class SchemaValidatorTest extends \PHPUnit_Framework_TestCase
 
         $schema = new Schema($interfaceType, $subType);
 
-        $validationResult = SchemaValidator::validate($schema, [$rule]);
-        $this->assertSame(true, $validationResult->isValid);
+        $errors = SchemaValidator::validate($schema, [$rule]);
+        $this->assertEmpty($errors);
     }
 
 
@@ -337,28 +302,12 @@ class SchemaValidatorTest extends \PHPUnit_Framework_TestCase
         // Another sanity check.
         $this->assertSame($subType, $schema->getType('SubType'));
 
-        $validationResult = SchemaValidator::validate($schema, [SchemaValidator::typesInterfacesMustShowThemAsPossibleRule()]);
-        $this->assertSame(false, $validationResult->isValid);
-        $this->assertSame(1, count($validationResult->errors));
+        $errors = SchemaValidator::validate($schema, [SchemaValidator::typesInterfacesMustShowThemAsPossibleRule()]);
+        $this->assertSame(1, count($errors));
         $this->assertSame(
             'SubType implements interface InterfaceType, but InterfaceType does ' .
             'not list it as possible!',
-            $validationResult->errors[0]->message
+            $errors[0]->message
         );
-
-
-/*
-
-    var validationResult = validateSchema(
-      schema,
-      [TypesInterfacesMustShowThemAsPossible]
-    );
-    expect(validationResult.isValid).to.equal(false);
-    expect(validationResult.errors.length).to.equal(1);
-    expect(validationResult.errors[0].message).to.equal(
-      'SubType implements interface InterfaceType, but InterfaceType does ' +
-      'not list it as possible!'
-    );
- */
     }
 }
diff --git a/tests/Validator/ArgumentsOfCorrectTypeTest.php b/tests/Validator/ArgumentsOfCorrectTypeTest.php
index e8c962f..74e5181 100644
--- a/tests/Validator/ArgumentsOfCorrectTypeTest.php
+++ b/tests/Validator/ArgumentsOfCorrectTypeTest.php
@@ -9,7 +9,7 @@ class ArgumentsOfCorrectTypeTest extends TestCase
 {
     function missingArg($fieldName, $argName, $typeName, $line, $column)
     {
-        return new FormattedError(
+        return FormattedError::create(
             Messages::missingArgMessage($fieldName, $argName, $typeName),
             [new SourceLocation($line, $column)]
         );
@@ -17,8 +17,8 @@ class ArgumentsOfCorrectTypeTest extends TestCase
 
     function badValue($argName, $typeName, $value, $line, $column)
     {
-        return new FormattedError(
-            Messages::badValueMessage($argName, $typeName, $value),
+        return FormattedError::create(
+            ArgumentsOfCorrectType::badValueMessage($argName, $typeName, $value),
             [new SourceLocation($line, $column)]
         );
     }
@@ -632,33 +632,6 @@ class ArgumentsOfCorrectTypeTest extends TestCase
         ]);
     }
 
-    public function testMissingOneNonNullableArgument()
-    {
-        $this->expectFailsRule(new ArgumentsOfCorrectType, '
-        {
-          complicatedArgs {
-            multipleReqs(req2: 2)
-          }
-        }
-        ', [
-            $this->missingArg('multipleReqs', 'req1', 'Int!', 4, 13)
-        ]);
-    }
-
-    public function testMissingMultipleNonNullableArguments()
-    {
-        $this->expectFailsRule(new ArgumentsOfCorrectType, '
-        {
-          complicatedArgs {
-            multipleReqs
-          }
-        }
-        ', [
-            $this->missingArg('multipleReqs', 'req1', 'Int!', 4, 13),
-            $this->missingArg('multipleReqs', 'req2', 'Int!', 4, 13),
-        ]);
-    }
-
     public function testIncorrectValueAndMissingArgument()
     {
         $this->expectFailsRule(new ArgumentsOfCorrectType, '
@@ -668,7 +641,6 @@ class ArgumentsOfCorrectTypeTest extends TestCase
           }
         }
         ', [
-            $this->missingArg('multipleReqs', 'req2', 'Int!', 4, 13),
             $this->badValue('req1', 'Int!', '"one"', 4, 32),
         ]);
     }
diff --git a/tests/Validator/DefaultValuesOfCorrectTypeTest.php b/tests/Validator/DefaultValuesOfCorrectTypeTest.php
index 3bfddec..06a22d6 100644
--- a/tests/Validator/DefaultValuesOfCorrectTypeTest.php
+++ b/tests/Validator/DefaultValuesOfCorrectTypeTest.php
@@ -93,7 +93,7 @@ class DefaultValuesOfCorrectTypeTest extends TestCase
 
     private function defaultForNonNullArg($varName, $typeName, $guessTypeName, $line, $column)
     {
-        return new FormattedError(
+        return FormattedError::create(
             Messages::defaultForNonNullArgMessage($varName, $typeName, $guessTypeName),
             [ new SourceLocation($line, $column) ]
         );
@@ -101,7 +101,7 @@ class DefaultValuesOfCorrectTypeTest extends TestCase
 
     private function badValue($varName, $typeName, $val, $line, $column)
     {
-        return new FormattedError(
+        return FormattedError::create(
             Messages::badValueForDefaultArgMessage($varName, $typeName, $val),
             [ new SourceLocation($line, $column) ]
         );
diff --git a/tests/Validator/FieldsOnCorrectTypeTest.php b/tests/Validator/FieldsOnCorrectTypeTest.php
index b723750..bb1c61e 100644
--- a/tests/Validator/FieldsOnCorrectTypeTest.php
+++ b/tests/Validator/FieldsOnCorrectTypeTest.php
@@ -56,6 +56,15 @@ class FieldsOnCorrectTypeTest extends TestCase
         ');
     }
 
+    public function testIgnoresFieldsOnUnknownType()
+    {
+        $this->expectPassesRule(new FieldsOnCorrectType, '
+      fragment unknownSelection on UnknownType {
+        unknownField
+      }
+        ');
+    }
+
     public function testFieldNotDefinedOnFragment()
     {
         $this->expectFailsRule(new FieldsOnCorrectType, '
@@ -184,7 +193,7 @@ class FieldsOnCorrectTypeTest extends TestCase
 
     private function undefinedField($field, $type, $line, $column)
     {
-        return new FormattedError(
+        return FormattedError::create(
             Messages::undefinedFieldMessage($field, $type),
             [new SourceLocation($line, $column)]
         );
diff --git a/tests/Validator/FragmentsOnCompositeTypesTest.php b/tests/Validator/FragmentsOnCompositeTypesTest.php
index aaa15d3..1f1a261 100644
--- a/tests/Validator/FragmentsOnCompositeTypesTest.php
+++ b/tests/Validator/FragmentsOnCompositeTypesTest.php
@@ -86,8 +86,8 @@ class FragmentsOnCompositeTypesTest extends TestCase
         }
       }
         ',
-        [new FormattedError(
-            Messages::inlineFragmentOnNonCompositeErrorMessage('String'),
+        [FormattedError::create(
+            FragmentsOnCompositeTypes::inlineFragmentOnNonCompositeErrorMessage('String'),
             [new SourceLocation(3, 16)]
         )]
         );
@@ -95,8 +95,8 @@ class FragmentsOnCompositeTypesTest extends TestCase
 
     private function error($fragName, $typeName, $line, $column)
     {
-        return new FormattedError(
-            Messages::fragmentOnNonCompositeErrorMessage($fragName, $typeName),
+        return FormattedError::create(
+            FragmentsOnCompositeTypes::fragmentOnNonCompositeErrorMessage($fragName, $typeName),
             [ new SourceLocation($line, $column) ]
         );
     }
diff --git a/tests/Validator/KnownArgumentNamesTest.php b/tests/Validator/KnownArgumentNamesTest.php
index 7357169..fafd944 100644
--- a/tests/Validator/KnownArgumentNamesTest.php
+++ b/tests/Validator/KnownArgumentNamesTest.php
@@ -26,6 +26,15 @@ class KnownArgumentNamesTest extends TestCase
         ');
     }
 
+    public function testIgnoresArgsOfUnknownFields()
+    {
+        $this->expectPassesRule(new KnownArgumentNames, '
+      fragment argOnUnknownField on Dog {
+        unknownField(unknownArg: SIT)
+      }
+        ');
+    }
+
     public function testMultipleArgsInReverseOrderAreKnown()
     {
         $this->expectPassesRule(new KnownArgumentNames, '
@@ -62,6 +71,26 @@ class KnownArgumentNamesTest extends TestCase
         ');
     }
 
+    public function testDirectiveArgsAreKnown()
+    {
+        $this->expectPassesRule(new KnownArgumentNames, '
+      {
+        dog @skip(if: true)
+      }
+        ');
+    }
+
+    public function testUndirectiveArgsAreInvalid()
+    {
+        $this->expectFailsRule(new KnownArgumentNames, '
+      {
+        dog @skip(unless: true)
+      }
+        ', [
+            $this->unknownDirectiveArg('unless', 'skip', 3, 19),
+        ]);
+    }
+
     public function testInvalidArgName()
     {
         $this->expectFailsRule(new KnownArgumentNames, '
@@ -106,28 +135,18 @@ class KnownArgumentNamesTest extends TestCase
         ]);
     }
 
-    public function testArgsMayBeOnObjectButNotInterface()
-    {
-        $this->expectFailsRule(new KnownArgumentNames, '
-      fragment nameSometimesHasArg on Being {
-        name(surname: true)
-        ... on Human {
-          name(surname: true)
-        }
-        ... on Dog {
-          name(surname: true)
-        }
-      }
-        ', [
-            $this->unknownArg('surname', 'name', 'Being', 3, 14),
-            $this->unknownArg('surname', 'name', 'Dog', 8, 16)
-        ]);
-    }
-
     private function unknownArg($argName, $fieldName, $typeName, $line, $column)
     {
-        return new FormattedError(
-            Messages::unknownArgMessage($argName, $fieldName, $typeName),
+        return FormattedError::create(
+            KnownArgumentNames::unknownArgMessage($argName, $fieldName, $typeName),
+            [new SourceLocation($line, $column)]
+        );
+    }
+
+    private function unknownDirectiveArg($argName, $directiveName, $line, $column)
+    {
+        return FormattedError::create(
+            KnownArgumentNames::unknownDirectiveArgMessage($argName, $directiveName),
             [new SourceLocation($line, $column)]
         );
     }
diff --git a/tests/Validator/KnownDirectivesTest.php b/tests/Validator/KnownDirectivesTest.php
index 8983db7..6f2d4a3 100644
--- a/tests/Validator/KnownDirectivesTest.php
+++ b/tests/Validator/KnownDirectivesTest.php
@@ -26,10 +26,10 @@ class KnownDirectivesTest extends TestCase
     {
         $this->expectPassesRule(new KnownDirectives, '
       {
-        dog @if: true {
+        dog @include(if: true) {
           name
         }
-        human @unless: false {
+        human @skip(if: true) {
           name
         }
       }
@@ -40,7 +40,7 @@ class KnownDirectivesTest extends TestCase
     {
         $this->expectFailsRule(new KnownDirectives, '
       {
-        dog @unknown: "directive" {
+        dog @unknown(directive: "value") {
           name
         }
       }
@@ -53,12 +53,12 @@ class KnownDirectivesTest extends TestCase
     {
         $this->expectFailsRule(new KnownDirectives, '
       {
-        dog @unknown: "directive" {
+        dog @unknown(directive: "value") {
           name
         }
-        human @unknown: "directive" {
+        human @unknown(directive: "value") {
           name
-          pets @unknown: "directive" {
+          pets @unknown(directive: "value") {
             name
           }
         }
@@ -70,30 +70,42 @@ class KnownDirectivesTest extends TestCase
         ]);
     }
 
+    public function testWithWellPlacedDirectives()
+    {
+        $this->expectPassesRule(new KnownDirectives, '
+      query Foo {
+        name @include(if: true)
+        ...Frag @include(if: true)
+        skippedField @skip(if: true)
+        ...SkippedFrag @skip(if: true)
+      }
+        ');
+    }
+
     public function testWithMisplacedDirectives()
     {
         $this->expectFailsRule(new KnownDirectives, '
-      query Foo @if: true {
+      query Foo @include(if: true) {
         name
         ...Frag
       }
         ', [
-            $this->misplacedDirective('if', 'operation', 2, 17)
+            $this->misplacedDirective('include', 'operation', 2, 17)
         ]);
     }
 
     private function unknownDirective($directiveName, $line, $column)
     {
-        return new FormattedError(
-            Messages::unknownDirectiveMessage($directiveName),
+        return FormattedError::create(
+            KnownDirectives::unknownDirectiveMessage($directiveName),
             [ new SourceLocation($line, $column) ]
         );
     }
 
     function misplacedDirective($directiveName, $placement, $line, $column)
     {
-        return new FormattedError(
-            Messages::misplacedDirectiveMessage($directiveName, $placement),
+        return FormattedError::create(
+            KnownDirectives::misplacedDirectiveMessage($directiveName, $placement),
             [new SourceLocation($line, $column)]
         );
     }
diff --git a/tests/Validator/KnownFragmentNamesTest.php b/tests/Validator/KnownFragmentNamesTest.php
index 2ff97c0..f79857e 100644
--- a/tests/Validator/KnownFragmentNamesTest.php
+++ b/tests/Validator/KnownFragmentNamesTest.php
@@ -57,8 +57,8 @@ class KnownFragmentNamesTest extends TestCase
 
     private function undefFrag($fragName, $line, $column)
     {
-        return new FormattedError(
-            "Undefined fragment $fragName.",
+        return FormattedError::create(
+            KnownFragmentNames::unknownFragmentMessage($fragName),
             [new SourceLocation($line, $column)]
         );
     }
diff --git a/tests/Validator/KnownTypeNamesTest.php b/tests/Validator/KnownTypeNamesTest.php
index 2ffd19f..635616e 100644
--- a/tests/Validator/KnownTypeNamesTest.php
+++ b/tests/Validator/KnownTypeNamesTest.php
@@ -44,8 +44,8 @@ class KnownTypeNamesTest extends TestCase
 
     private function unknownType($typeName, $line, $column)
     {
-        return new FormattedError(
-            Messages::unknownTypeMessage($typeName),
+        return FormattedError::create(
+            KnownTypeNames::unknownTypeMessage($typeName),
             [new SourceLocation($line, $column)]
         );
     }
diff --git a/tests/Validator/NoFragmentCyclesTest.php b/tests/Validator/NoFragmentCyclesTest.php
index 4b5daf8..8dd1d3a 100644
--- a/tests/Validator/NoFragmentCyclesTest.php
+++ b/tests/Validator/NoFragmentCyclesTest.php
@@ -2,9 +2,7 @@
 namespace GraphQL\Validator;
 
 use GraphQL\FormattedError;
-use GraphQL\Language\Source;
 use GraphQL\Language\SourceLocation;
-use GraphQL\Type\Definition\Config;
 use GraphQL\Validator\Rules\NoFragmentCycles;
 
 class NoFragmentCyclesTest extends TestCase
@@ -88,8 +86,8 @@ class NoFragmentCyclesTest extends TestCase
       fragment fragA on Dog { ...fragB }
       fragment fragB on Dog { ...fragA }
         ', [
-            new FormattedError(
-                Messages::cycleErrorMessage('fragA', ['fragB']),
+            FormattedError::create(
+                NoFragmentCycles::cycleErrorMessage('fragA', ['fragB']),
                 [ new SourceLocation(2, 31), new SourceLocation(3, 31) ]
             )
         ]);
@@ -101,8 +99,8 @@ class NoFragmentCyclesTest extends TestCase
       fragment fragB on Dog { ...fragA }
       fragment fragA on Dog { ...fragB }
         ', [
-            new FormattedError(
-                Messages::cycleErrorMessage('fragB', ['fragA']),
+            FormattedError::create(
+                NoFragmentCycles::cycleErrorMessage('fragB', ['fragA']),
                 [new SourceLocation(2, 31), new SourceLocation(3, 31)]
             )
         ]);
@@ -122,8 +120,8 @@ class NoFragmentCyclesTest extends TestCase
         }
       }
         ', [
-            new FormattedError(
-                Messages::cycleErrorMessage('fragA', ['fragB']),
+            FormattedError::create(
+                NoFragmentCycles::cycleErrorMessage('fragA', ['fragB']),
                 [new SourceLocation(4, 11), new SourceLocation(9, 11)]
             )
         ]);
@@ -140,8 +138,8 @@ class NoFragmentCyclesTest extends TestCase
       fragment fragZ on Dog { ...fragO }
       fragment fragO on Dog { ...fragA, ...fragX }
     ', [
-            new FormattedError(
-                Messages::cycleErrorMessage('fragA', ['fragB', 'fragC', 'fragO']),
+            FormattedError::create(
+                NoFragmentCycles::cycleErrorMessage('fragA', ['fragB', 'fragC', 'fragO']),
                 [
                     new SourceLocation(2, 31),
                     new SourceLocation(3, 31),
@@ -149,8 +147,8 @@ class NoFragmentCyclesTest extends TestCase
                     new SourceLocation(8, 31),
                 ]
             ),
-            new FormattedError(
-                Messages::cycleErrorMessage('fragX', ['fragY', 'fragZ', 'fragO']),
+            FormattedError::create(
+                NoFragmentCycles::cycleErrorMessage('fragX', ['fragY', 'fragZ', 'fragO']),
                 [
                     new SourceLocation(5, 31),
                     new SourceLocation(6, 31),
@@ -168,12 +166,12 @@ class NoFragmentCyclesTest extends TestCase
       fragment fragB on Dog { ...fragA }
       fragment fragC on Dog { ...fragA }
         ', [
-            new FormattedError(
-                'Cannot spread fragment fragA within itself via fragB.',
+            FormattedError::create(
+                NoFragmentCycles::cycleErrorMessage('fragA', ['fragB']),
                 [new SourceLocation(2, 31), new SourceLocation(3, 31)]
             ),
-            new FormattedError(
-                'Cannot spread fragment fragA within itself via fragC.',
+            FormattedError::create(
+                NoFragmentCycles::cycleErrorMessage('fragA', ['fragC']),
                 [new SourceLocation(2, 41), new SourceLocation(4, 31)]
             )
         ]);
@@ -181,8 +179,8 @@ class NoFragmentCyclesTest extends TestCase
 
     private function cycleError($fargment, $spreadNames, $line, $column)
     {
-        return new FormattedError(
-            Messages::cycleErrorMessage($fargment, $spreadNames),
+        return FormattedError::create(
+            NoFragmentCycles::cycleErrorMessage($fargment, $spreadNames),
             [new SourceLocation($line, $column)]
         );
     }
diff --git a/tests/Validator/NoUndefinedVariablesTest.php b/tests/Validator/NoUndefinedVariablesTest.php
index acf0fe9..ee4b249 100644
--- a/tests/Validator/NoUndefinedVariablesTest.php
+++ b/tests/Validator/NoUndefinedVariablesTest.php
@@ -302,16 +302,16 @@ class NoUndefinedVariablesTest extends TestCase
 
     private function undefVar($varName, $line, $column)
     {
-        return new FormattedError(
-            Messages::undefinedVarMessage($varName),
+        return FormattedError::create(
+            NoUndefinedVariables::undefinedVarMessage($varName),
             [new SourceLocation($line, $column)]
         );
     }
 
     private function undefVarByOp($varName, $l1, $c1, $opName, $l2, $c2)
     {
-        return new FormattedError(
-            Messages::undefinedVarByOpMessage($varName, $opName),
+        return FormattedError::create(
+            NoUndefinedVariables::undefinedVarByOpMessage($varName, $opName),
             [new SourceLocation($l1, $c1), new SourceLocation($l2, $c2)]
         );
     }
diff --git a/tests/Validator/NoUnusedFragmentsTest.php b/tests/Validator/NoUnusedFragmentsTest.php
index b201421..7ba79a5 100644
--- a/tests/Validator/NoUnusedFragmentsTest.php
+++ b/tests/Validator/NoUnusedFragmentsTest.php
@@ -130,10 +130,27 @@ class NoUnusedFragmentsTest extends TestCase
         ]);
     }
 
+    public function testContainsUnknownAndUndefFragments()
+    {
+
+        $this->expectFailsRule(new NoUnusedFragments, '
+      query Foo {
+        human(id: 4) {
+          ...bar
+        }
+      }
+      fragment foo on Human {
+        name
+      }
+    ', [
+            $this->unusedFrag('foo', 7, 7),
+        ]);
+    }
+
     private function unusedFrag($fragName, $line, $column)
     {
-        return new FormattedError(
-            Messages::unusedFragMessage($fragName),
+        return FormattedError::create(
+            NoUnusedFragments::unusedFragMessage($fragName),
             [new SourceLocation($line, $column)]
         );
     }
diff --git a/tests/Validator/NoUnusedVariablesTest.php b/tests/Validator/NoUnusedVariablesTest.php
index 1e8398a..00218ec 100644
--- a/tests/Validator/NoUnusedVariablesTest.php
+++ b/tests/Validator/NoUnusedVariablesTest.php
@@ -213,8 +213,8 @@ class NoUnusedVariablesTest extends TestCase
 
     private function unusedVar($varName, $line, $column)
     {
-        return new FormattedError(
-            Messages::unusedVariableMessage($varName),
+        return FormattedError::create(
+            NoUnusedVariables::unusedVariableMessage($varName),
             [new SourceLocation($line, $column)]
         );
     }
diff --git a/tests/Validator/OverlappingFieldsCanBeMergedTest.php b/tests/Validator/OverlappingFieldsCanBeMergedTest.php
index 7d32219..d326a79 100644
--- a/tests/Validator/OverlappingFieldsCanBeMergedTest.php
+++ b/tests/Validator/OverlappingFieldsCanBeMergedTest.php
@@ -48,8 +48,8 @@ class OverlappingFieldsCanBeMergedTest extends TestCase
     {
         $this->expectPassesRule(new OverlappingFieldsCanBeMerged, '
       fragment mergeSameFieldsWithSameDirectives on Dog {
-        name @if:true
-        name @if:true
+        name @include(if: true)
+        name @include(if: true)
       }
         ');
     }
@@ -68,8 +68,8 @@ class OverlappingFieldsCanBeMergedTest extends TestCase
     {
         $this->expectPassesRule(new OverlappingFieldsCanBeMerged, '
       fragment differentDirectivesWithDifferentAliases on Dog {
-        nameIfTrue : name @if:true
-        nameIfFalse : name @if:false
+        nameIfTrue : name @include(if: true)
+        nameIfFalse : name @include(if: false)
       }
         ');
     }
@@ -82,8 +82,8 @@ class OverlappingFieldsCanBeMergedTest extends TestCase
         fido : nickname
       }
         ', [
-            new FormattedError(
-                Messages::fieldsConflictMessage('fido', 'name and nickname are different fields'),
+            FormattedError::create(
+                OverlappingFieldsCanBeMerged::fieldsConflictMessage('fido', 'name and nickname are different fields'),
                 [new SourceLocation(3, 9), new SourceLocation(4, 9)]
             )
         ]);
@@ -97,8 +97,8 @@ class OverlappingFieldsCanBeMergedTest extends TestCase
         name
       }
         ', [
-            new FormattedError(
-                Messages::fieldsConflictMessage('name', 'nickname and name are different fields'),
+            FormattedError::create(
+                OverlappingFieldsCanBeMerged::fieldsConflictMessage('name', 'nickname and name are different fields'),
                 [new SourceLocation(3, 9), new SourceLocation(4, 9)]
             )
         ]);
@@ -112,8 +112,8 @@ class OverlappingFieldsCanBeMergedTest extends TestCase
         doesKnowCommand(dogCommand: HEEL)
       }
         ', [
-            new FormattedError(
-                Messages::fieldsConflictMessage('doesKnowCommand', 'they have differing arguments'),
+            FormattedError::create(
+                OverlappingFieldsCanBeMerged::fieldsConflictMessage('doesKnowCommand', 'they have differing arguments'),
                 [new SourceLocation(3,9), new SourceLocation(4,9)]
             )
         ]);
@@ -123,27 +123,43 @@ class OverlappingFieldsCanBeMergedTest extends TestCase
     {
         $this->expectFailsRule(new OverlappingFieldsCanBeMerged, '
       fragment conflictingDirectiveArgs on Dog {
-        name @if: true
-        name @unless: false
+        name @include(if: true)
+        name @skip(if: true)
       }
         ', [
-            new FormattedError(
-                Messages::fieldsConflictMessage('name', 'they have differing directives'),
+            FormattedError::create(
+                OverlappingFieldsCanBeMerged::fieldsConflictMessage('name', 'they have differing directives'),
                 [new SourceLocation(3, 9), new SourceLocation(4, 9)]
             )
         ]);
     }
 
+    public function testConflictingDirectiveArgs()
+    {
+        $this->expectFailsRule(new OverlappingFieldsCanBeMerged, '
+      fragment conflictingDirectiveArgs on Dog {
+        name @include(if: true)
+        name @include(if: false)
+      }
+    ', [
+                FormattedError::create(
+                    OverlappingFieldsCanBeMerged::fieldsConflictMessage('name', 'they have differing directives'),
+                    [new SourceLocation(3, 9), new SourceLocation(4, 9)]
+                )
+            ]
+        );
+    }
+
     public function testConflictingArgsWithMatchingDirectives()
     {
         $this->expectFailsRule(new OverlappingFieldsCanBeMerged, '
       fragment conflictingArgsWithMatchingDirectiveArgs on Dog {
-        doesKnowCommand(dogCommand: SIT) @if:true
-        doesKnowCommand(dogCommand: HEEL) @if:true
+        doesKnowCommand(dogCommand: SIT) @include(if: true)
+        doesKnowCommand(dogCommand: HEEL) @include(if: true)
       }
         ', [
-            new FormattedError(
-                Messages::fieldsConflictMessage('doesKnowCommand', 'they have differing arguments'),
+            FormattedError::create(
+                OverlappingFieldsCanBeMerged::fieldsConflictMessage('doesKnowCommand', 'they have differing arguments'),
                 [new SourceLocation(3, 9), new SourceLocation(4, 9)]
             )
         ]);
@@ -153,12 +169,12 @@ class OverlappingFieldsCanBeMergedTest extends TestCase
     {
         $this->expectFailsRule(new OverlappingFieldsCanBeMerged, '
       fragment conflictingDirectiveArgsWithMatchingArgs on Dog {
-        doesKnowCommand(dogCommand: SIT) @if: true
-        doesKnowCommand(dogCommand: SIT) @unless: false
+        doesKnowCommand(dogCommand: SIT) @include(if: true)
+        doesKnowCommand(dogCommand: SIT) @skip(if: true)
       }
         ', [
-            new FormattedError(
-                Messages::fieldsConflictMessage('doesKnowCommand', 'they have differing directives'),
+            FormattedError::create(
+                OverlappingFieldsCanBeMerged::fieldsConflictMessage('doesKnowCommand', 'they have differing directives'),
                 [new SourceLocation(3, 9), new SourceLocation(4, 9)]
             )
         ]);
@@ -178,8 +194,8 @@ class OverlappingFieldsCanBeMergedTest extends TestCase
         x: b
       }
         ', [
-            new FormattedError(
-                Messages::fieldsConflictMessage('x', 'a and b are different fields'),
+            FormattedError::create(
+                OverlappingFieldsCanBeMerged::fieldsConflictMessage('x', 'a and b are different fields'),
                 [new SourceLocation(7, 9), new SourceLocation(10, 9)]
             )
         ]);
@@ -210,16 +226,16 @@ class OverlappingFieldsCanBeMergedTest extends TestCase
         x: b
       }
     ', [
-            new FormattedError(
-                Messages::fieldsConflictMessage('x', 'a and b are different fields'),
+            FormattedError::create(
+                OverlappingFieldsCanBeMerged::fieldsConflictMessage('x', 'a and b are different fields'),
                 [new SourceLocation(18, 9), new SourceLocation(21, 9)]
             ),
-            new FormattedError(
-                Messages::fieldsConflictMessage('x', 'a and c are different fields'),
+            FormattedError::create(
+                OverlappingFieldsCanBeMerged::fieldsConflictMessage('x', 'a and c are different fields'),
                 [new SourceLocation(18, 9), new SourceLocation(14, 11)]
             ),
-            new FormattedError(
-                Messages::fieldsConflictMessage('x', 'b and c are different fields'),
+            FormattedError::create(
+                OverlappingFieldsCanBeMerged::fieldsConflictMessage('x', 'b and c are different fields'),
                 [new SourceLocation(21, 9), new SourceLocation(14, 11)]
             )
         ]);
@@ -237,8 +253,8 @@ class OverlappingFieldsCanBeMergedTest extends TestCase
         }
       }
         ', [
-            new FormattedError(
-                Messages::fieldsConflictMessage('field', [['x', 'a and b are different fields']]),
+            FormattedError::create(
+                OverlappingFieldsCanBeMerged::fieldsConflictMessage('field', [['x', 'a and b are different fields']]),
                 [
                     new SourceLocation(3, 9),
                     new SourceLocation(6,9),
@@ -263,8 +279,8 @@ class OverlappingFieldsCanBeMergedTest extends TestCase
         }
       }
         ', [
-            new FormattedError(
-                Messages::fieldsConflictMessage('field', [
+            FormattedError::create(
+                OverlappingFieldsCanBeMerged::fieldsConflictMessage('field', [
                     ['x', 'a and b are different fields'],
                     ['y', 'c and d are different fields']
                 ]),
@@ -296,8 +312,8 @@ class OverlappingFieldsCanBeMergedTest extends TestCase
         }
       }
         ', [
-            new FormattedError(
-                Messages::fieldsConflictMessage('field', [['deepField', [['x', 'a and b are different fields']]]]),
+            FormattedError::create(
+                OverlappingFieldsCanBeMerged::fieldsConflictMessage('field', [['deepField', [['x', 'a and b are different fields']]]]),
                 [
                     new SourceLocation(3,9),
                     new SourceLocation(8,9),
@@ -329,8 +345,8 @@ class OverlappingFieldsCanBeMergedTest extends TestCase
         }
       }
         ', [
-            new FormattedError(
-                Messages::fieldsConflictMessage('deepField', [['x', 'a and b are different fields']]),
+            FormattedError::create(
+                OverlappingFieldsCanBeMerged::fieldsConflictMessage('deepField', [['x', 'a and b are different fields']]),
                 [
                     new SourceLocation(4,11),
                     new SourceLocation(7,11),
@@ -356,8 +372,8 @@ class OverlappingFieldsCanBeMergedTest extends TestCase
           }
         }
       ', [
-            new FormattedError(
-                Messages::fieldsConflictMessage('scalar', 'they return differing types Int and String'),
+            FormattedError::create(
+                OverlappingFieldsCanBeMerged::fieldsConflictMessage('scalar', 'they return differing types Int and String'),
                 [ new SourceLocation(5,15), new SourceLocation(8,15) ]
             )
         ]);
@@ -379,6 +395,55 @@ class OverlappingFieldsCanBeMergedTest extends TestCase
         ');
     }
 
+    public function testComparesDeepTypesIncludingList()
+    {
+        $this->expectFailsRuleWithSchema($this->getTestSchema(), new OverlappingFieldsCanBeMerged, '
+        {
+          connection {
+            ...edgeID
+            edges {
+              node {
+                id: name
+              }
+            }
+          }
+        }
+
+        fragment edgeID on Connection {
+          edges {
+            node {
+              id
+            }
+          }
+        }
+      ', [
+            FormattedError::create(
+                OverlappingFieldsCanBeMerged::fieldsConflictMessage('edges', [['node', [['id', 'id and name are different fields']]]]),
+                [
+                    new SourceLocation(14, 11), new SourceLocation(5, 13),
+                    new SourceLocation(15, 13), new SourceLocation(6, 15),
+                    new SourceLocation(16, 15), new SourceLocation(7, 17),
+                ]
+            )
+        ]);
+     }
+
+    public function testIgnoresUnknownTypes()
+    {
+        $this->expectPassesRuleWithSchema($this->getTestSchema(), new OverlappingFieldsCanBeMerged, '
+        {
+          boxUnion {
+            ...on UnknownType {
+              scalar
+            }
+            ...on NonNullStringBox2 {
+              scalar
+            }
+          }
+        }
+        ');
+    }
+
     private function getTestSchema()
     {
         $StringBox = new ObjectType([
@@ -411,13 +476,37 @@ class OverlappingFieldsCanBeMergedTest extends TestCase
 
         $BoxUnion = new UnionType([
             'name' => 'BoxUnion',
+            'resolveType' => function()  use ($StringBox) {return $StringBox;},
             'types' => [ $StringBox, $IntBox, $NonNullStringBox1, $NonNullStringBox2 ]
         ]);
 
+        $Connection = new ObjectType([
+            'name' => 'Connection',
+            'fields' => [
+                'edges' => [
+                    'type' => Type::listOf(new ObjectType([
+                        'name' => 'Edge',
+                        'fields' => [
+                            'node' => [
+                                'type' => new ObjectType([
+                                    'name' => 'Node',
+                                    'fields' => [
+                                        'id' => ['type' => Type::id()],
+                                        'name' => ['type' => Type::string()]
+                                    ]
+                                ])
+                            ]
+                        ]
+                    ]))
+                ]
+            ]
+        ]);
+
         $schema = new Schema(new ObjectType([
             'name' => 'QueryRoot',
             'fields' => [
-                'boxUnion' => ['type' => $BoxUnion ]
+                'boxUnion' => ['type' => $BoxUnion ],
+                'connection' => ['type' => $Connection]
             ]
         ]));
 
diff --git a/tests/Validator/PossibleFragmentSpreadsTest.php b/tests/Validator/PossibleFragmentSpreadsTest.php
index 6176e78..3e3b9c1 100644
--- a/tests/Validator/PossibleFragmentSpreadsTest.php
+++ b/tests/Validator/PossibleFragmentSpreadsTest.php
@@ -210,16 +210,16 @@ class PossibleFragmentSpreadsTest extends TestCase
 
     private function error($fragName, $parentType, $fragType, $line, $column)
     {
-        return new FormattedError(
-            Messages::typeIncompatibleSpreadMessage($fragName, $parentType, $fragType),
+        return FormattedError::create(
+            PossibleFragmentSpreads::typeIncompatibleSpreadMessage($fragName, $parentType, $fragType),
             [new SourceLocation($line, $column)]
         );
     }
 
     private function errorAnon($parentType, $fragType, $line, $column)
     {
-        return new FormattedError(
-            Messages::typeIncompatibleAnonSpreadMessage($parentType, $fragType),
+        return FormattedError::create(
+            PossibleFragmentSpreads::typeIncompatibleAnonSpreadMessage($parentType, $fragType),
             [new SourceLocation($line, $column)]
         );
     }
diff --git a/tests/Validator/ProvidedNonNullArgumentsTest.php b/tests/Validator/ProvidedNonNullArgumentsTest.php
new file mode 100644
index 0000000..cba4a06
--- /dev/null
+++ b/tests/Validator/ProvidedNonNullArgumentsTest.php
@@ -0,0 +1,217 @@
+<?php
+namespace GraphQL\Validator;
+
+use GraphQL\FormattedError;
+use GraphQL\Language\SourceLocation;
+use GraphQL\Validator\Rules\ProvidedNonNullArguments;
+
+class ProvidedNonNullArgumentsTest extends TestCase
+{
+    // Validate: Provided required arguments
+    public function testIgnoresUnknownArguments()
+    {
+        // ignores unknown arguments
+        $this->expectPassesRule(new ProvidedNonNullArguments, '
+      {
+        dog {
+          isHousetrained(unknownArgument: true)
+        }
+      }
+        ');
+    }
+
+    // Valid non-nullable value:
+    public function testArgOnOptionalArg()
+    {
+        $this->expectPassesRule(new ProvidedNonNullArguments, '
+        {
+          dog {
+            isHousetrained(atOtherHomes: true)
+          }
+        }
+        ');
+    }
+
+    public function testMultipleArgs()
+    {
+        $this->expectPassesRule(new ProvidedNonNullArguments, '
+        {
+          complicatedArgs {
+            multipleReqs(req1: 1, req2: 2)
+          }
+        }
+        ');
+    }
+
+    public function testMultipleArgsReverseOrder()
+    {
+        $this->expectPassesRule(new ProvidedNonNullArguments, '
+        {
+          complicatedArgs {
+            multipleReqs(req2: 2, req1: 1)
+          }
+        }
+        ');
+    }
+
+    public function testNoArgsOnMultipleOptional()
+    {
+        $this->expectPassesRule(new ProvidedNonNullArguments, '
+        {
+          complicatedArgs {
+            multipleOpts
+          }
+        }
+        ');
+    }
+
+    public function testOneArgOnMultipleOptional()
+    {
+        $this->expectPassesRule(new ProvidedNonNullArguments, '
+        {
+          complicatedArgs {
+            multipleOpts(opt1: 1)
+          }
+        }
+        ');
+    }
+
+    public function testSecondArgOnMultipleOptional()
+    {
+        $this->expectPassesRule(new ProvidedNonNullArguments, '
+        {
+          complicatedArgs {
+            multipleOpts(opt2: 1)
+          }
+        }
+        ');
+    }
+
+    public function testMultipleReqsOnMixedList()
+    {
+        $this->expectPassesRule(new ProvidedNonNullArguments, '
+        {
+          complicatedArgs {
+            multipleOptAndReq(req1: 3, req2: 4)
+          }
+        }
+        ');
+    }
+
+    public function testMultipleReqsAndOneOptOnMixedList()
+    {
+        $this->expectPassesRule(new ProvidedNonNullArguments, '
+        {
+          complicatedArgs {
+            multipleOptAndReq(req1: 3, req2: 4, opt1: 5)
+          }
+        }
+        ');
+    }
+
+    public function testAllReqsAndOptsOnMixedList()
+    {
+        $this->expectPassesRule(new ProvidedNonNullArguments, '
+        {
+          complicatedArgs {
+            multipleOptAndReq(req1: 3, req2: 4, opt1: 5, opt2: 6)
+          }
+        }
+        ');
+    }
+
+    // Invalid non-nullable value
+    public function testMissingOneNonNullableArgument()
+    {
+        $this->expectFailsRule(new ProvidedNonNullArguments, '
+        {
+          complicatedArgs {
+            multipleReqs(req2: 2)
+          }
+        }
+        ', [
+            $this->missingFieldArg('multipleReqs', 'req1', 'Int!', 4, 13)
+        ]);
+    }
+
+    public function testMissingMultipleNonNullableArguments()
+    {
+        $this->expectFailsRule(new ProvidedNonNullArguments, '
+        {
+          complicatedArgs {
+            multipleReqs
+          }
+        }
+        ', [
+            $this->missingFieldArg('multipleReqs', 'req1', 'Int!', 4, 13),
+            $this->missingFieldArg('multipleReqs', 'req2', 'Int!', 4, 13),
+        ]);
+    }
+
+    public function testIncorrectValueAndMissingArgument()
+    {
+        $this->expectFailsRule(new ProvidedNonNullArguments, '
+        {
+          complicatedArgs {
+            multipleReqs(req1: "one")
+          }
+        }
+        ', [
+            $this->missingFieldArg('multipleReqs', 'req2', 'Int!', 4, 13),
+        ]);
+    }
+
+    // Directive arguments
+    public function testIgnoresUnknownDirectives()
+    {
+        $this->expectPassesRule(new ProvidedNonNullArguments, '
+        {
+          dog @unknown
+        }
+        ');
+    }
+
+    public function testWithDirectivesOfValidTypes()
+    {
+        $this->expectPassesRule(new ProvidedNonNullArguments, '
+        {
+          dog @include(if: true) {
+            name
+          }
+          human @skip(if: false) {
+            name
+          }
+        }
+        ');
+    }
+
+    public function testWithDirectiveWithMissingTypes()
+    {
+        $this->expectFailsRule(new ProvidedNonNullArguments, '
+        {
+          dog @include {
+            name @skip
+          }
+        }
+        ', [
+            $this->missingDirectiveArg('include', 'if', 'Boolean!', 3, 15),
+            $this->missingDirectiveArg('skip', 'if', 'Boolean!', 4, 18)
+        ]);
+    }
+
+    private function missingFieldArg($fieldName, $argName, $typeName, $line, $column)
+    {
+        return FormattedError::create(
+            ProvidedNonNullArguments::missingFieldArgMessage($fieldName, $argName, $typeName),
+            [new SourceLocation($line, $column)]
+        );
+    }
+
+    private function missingDirectiveArg($directiveName, $argName, $typeName, $line, $column)
+    {
+        return FormattedError::create(
+            ProvidedNonNullArguments::missingDirectiveArgMessage($directiveName, $argName, $typeName),
+            [new SourceLocation($line, $column)]
+        );
+    }
+}
diff --git a/tests/Validator/ScalarLeafsTest.php b/tests/Validator/ScalarLeafsTest.php
index b8fbf5f..1fe67dd 100644
--- a/tests/Validator/ScalarLeafsTest.php
+++ b/tests/Validator/ScalarLeafsTest.php
@@ -81,10 +81,10 @@ class ScalarLeafsTest extends TestCase
     {
         $this->expectFailsRule(new ScalarLeafs, '
       fragment scalarSelectionsNotAllowedWithDirectives on Dog {
-        name @if: true { isAlsoHumanName }
+        name @include(if: true) { isAlsoHumanName }
       }
         ',
-        [$this->noScalarSubselection('name', 'String', 3, 24)]
+        [$this->noScalarSubselection('name', 'String', 3, 33)]
         );
     }
 
@@ -92,25 +92,25 @@ class ScalarLeafsTest extends TestCase
     {
         $this->expectFailsRule(new ScalarLeafs, '
       fragment scalarSelectionsNotAllowedWithDirectivesAndArgs on Dog {
-        doesKnowCommand(dogCommand: SIT) @if: true { sinceWhen }
+        doesKnowCommand(dogCommand: SIT) @include(if: true) { sinceWhen }
       }
         ',
-            [$this->noScalarSubselection('doesKnowCommand', 'Boolean', 3, 52)]
+            [$this->noScalarSubselection('doesKnowCommand', 'Boolean', 3, 61)]
         );
     }
 
     private function noScalarSubselection($field, $type, $line, $column)
     {
-        return new FormattedError(
-            Messages::noSubselectionAllowedMessage($field, $type),
+        return FormattedError::create(
+            ScalarLeafs::noSubselectionAllowedMessage($field, $type),
             [new SourceLocation($line, $column)]
         );
     }
 
     private function missingObjSubselection($field, $type, $line, $column)
     {
-        return new FormattedError(
-            Messages::requiredSubselectionMessage($field, $type),
+        return FormattedError::create(
+            ScalarLeafs::requiredSubselectionMessage($field, $type),
             [new SourceLocation($line, $column)]
         );
     }
diff --git a/tests/Validator/TestCase.php b/tests/Validator/TestCase.php
index 158237a..41a8b1b 100644
--- a/tests/Validator/TestCase.php
+++ b/tests/Validator/TestCase.php
@@ -20,17 +20,25 @@ abstract class TestCase extends \PHPUnit_Framework_TestCase
      */
     protected function getDefaultSchema()
     {
+        $FurColor = null;
+
         $Being = new InterfaceType([
             'name' => 'Being',
             'fields' => [
-                'name' => [ 'type' => Type::string() ]
+                'name' => [
+                    'type' => Type::string(),
+                    'args' => [ 'surname' => [ 'type' => Type::boolean() ] ]
+                ]
             ],
         ]);
 
         $Pet = new InterfaceType([
             'name' => 'Pet',
             'fields' => [
-                'name' => [ 'type' => Type::string() ]
+                'name' => [
+                    'type' => Type::string(),
+                    'args' => [ 'surname' => [ 'type' => Type::boolean() ] ]
+                ]
             ],
         ]);
 
@@ -45,8 +53,12 @@ abstract class TestCase extends \PHPUnit_Framework_TestCase
 
         $Dog = new ObjectType([
             'name' => 'Dog',
+            'isTypeOf' => function() {return true;},
             'fields' => [
-                'name' => ['type' => Type::string()],
+                'name' => [
+                    'type' => Type::string(),
+                    'args' => [ 'surname' => [ 'type' => Type::boolean() ] ]
+                ],
                 'nickname' => ['type' => Type::string()],
                 'barkVolume' => ['type' => Type::int()],
                 'barks' => ['type' => Type::boolean()],
@@ -66,24 +78,18 @@ abstract class TestCase extends \PHPUnit_Framework_TestCase
             'interfaces' => [$Being, $Pet]
         ]);
 
-        $FurColor = new EnumType([
-            'name' => 'FurColor',
-            'values' => [
-                'BROWN' => [ 'value' => 0 ],
-                'BLACK' => [ 'value' => 1 ],
-                'TAN' => [ 'value' => 2 ],
-                'SPOTTED' => [ 'value' => 3 ],
-            ],
-        ]);
-
         $Cat = new ObjectType([
             'name' => 'Cat',
+            'isTypeOf' => function() {return true;},
             'fields' => [
-                'name' => ['type' => Type::string()],
+                'name' => [
+                    'type' => Type::string(),
+                    'args' => [ 'surname' => [ 'type' => Type::boolean() ] ]
+                ],
                 'nickname' => ['type' => Type::string()],
                 'meows' => ['type' => Type::boolean()],
                 'meowVolume' => ['type' => Type::int()],
-                'furColor' => ['type' => $FurColor]
+                'furColor' => ['type' => function() use (&$FurColor) {return $FurColor;}]
             ],
             'interfaces' => [$Being, $Pet]
         ]);
@@ -106,22 +112,29 @@ abstract class TestCase extends \PHPUnit_Framework_TestCase
 
         $Human = $this->humanType = new ObjectType([
             'name' => 'Human',
+            'isTypeOf' => function() {return true;},
             'interfaces' => [$Being, $Intelligent],
             'fields' => [
                 'name' => [
-                    'args' => ['surname' => ['type' => Type::boolean()]],
-                    'type' => Type::string()
+                    'type' => Type::string(),
+                    'args' => ['surname' => ['type' => Type::boolean()]]
                 ],
                 'pets' => ['type' => Type::listOf($Pet)],
-                'relatives' => ['type' => function() {return Type::listOf($this->humanType); }]
+                'relatives' => ['type' => function() {return Type::listOf($this->humanType); }],
+                'iq' => ['type' => Type::int()]
             ]
         ]);
 
         $Alien = new ObjectType([
             'name' => 'Alien',
+            'isTypeOf' => function() {return true;},
             'interfaces' => [$Being, $Intelligent],
             'fields' => [
                 'iq' => ['type' => Type::int()],
+                'name' => [
+                    'type' => Type::string(),
+                    'args' => ['surname' => ['type' => Type::boolean()]]
+                ],
                 'numEyes' => ['type' => Type::int()]
             ]
         ]);
@@ -144,6 +157,16 @@ abstract class TestCase extends \PHPUnit_Framework_TestCase
             }
         ]);
 
+        $FurColor = new EnumType([
+            'name' => 'FurColor',
+            'values' => [
+                'BROWN' => [ 'value' => 0 ],
+                'BLACK' => [ 'value' => 1 ],
+                'TAN' => [ 'value' => 2 ],
+                'SPOTTED' => [ 'value' => 3 ],
+            ],
+        ]);
+
         $ComplexInput = new InputObjectType([
             'name' => 'ComplexInput',
             'fields' => [
@@ -260,19 +283,20 @@ abstract class TestCase extends \PHPUnit_Framework_TestCase
     function expectValid($schema, $rules, $queryString)
     {
         $this->assertEquals(
-            ['isValid' => true, 'errors' => null],
-            DocumentValidator::validate($schema, Parser::parse($queryString), $rules)
+            [],
+            DocumentValidator::validate($schema, Parser::parse($queryString), $rules),
+            'Should validate'
         );
     }
 
-    function expectInvalid($schema, $rules, $queryString, $errors)
+    function expectInvalid($schema, $rules, $queryString, $expectedErrors)
     {
-        $result = DocumentValidator::validate($schema, Parser::parse($queryString), $rules);
+        $errors = DocumentValidator::validate($schema, Parser::parse($queryString), $rules);
 
-        $this->assertEquals(false, $result['isValid'], 'GraphQL should not validate');
-        $this->assertEquals($errors, $result['errors']);
+        $this->assertNotEmpty($errors, 'GraphQL should not validate');
+        $this->assertEquals($expectedErrors, array_map(['GraphQL\Error', 'formatError'], $errors));
 
-        return $result;
+        return $errors;
     }
 
     function expectPassesRule($rule, $queryString)
diff --git a/tests/Validator/VariablesAreInputTypesTest.php b/tests/Validator/VariablesAreInputTypesTest.php
index 4a00571..04b76cd 100644
--- a/tests/Validator/VariablesAreInputTypesTest.php
+++ b/tests/Validator/VariablesAreInputTypesTest.php
@@ -20,20 +20,20 @@ class VariablesAreInputTypesTest extends TestCase
     public function testOutputTypesAreInvalid()
     {
         $this->expectFailsRule(new VariablesAreInputTypes, '
-      query Foo($a: Dog, $b: [[DogOrCat!]]!, $c: Pet) {
+      query Foo($a: Dog, $b: [[CatOrDog!]]!, $c: Pet) {
         field(a: $a, b: $b, c: $c)
       }
         ', [
-                new FormattedError(
-                    Messages::nonInputTypeOnVarMessage('a', 'Dog'),
+                FormattedError::create(
+                    VariablesAreInputTypes::nonInputTypeOnVarMessage('a', 'Dog'),
                     [new SourceLocation(2, 21)]
                 ),
-                new FormattedError(
-                    Messages::nonInputTypeOnVarMessage('b', '[[DogOrCat!]]!'),
+                FormattedError::create(
+                    VariablesAreInputTypes::nonInputTypeOnVarMessage('b', '[[CatOrDog!]]!'),
                     [new SourceLocation(2, 30)]
                 ),
-                new FormattedError(
-                    Messages::nonInputTypeOnVarMessage('c', 'Pet'),
+                FormattedError::create(
+                    VariablesAreInputTypes::nonInputTypeOnVarMessage('c', 'Pet'),
                     [new SourceLocation(2, 50)]
                 )
             ]
diff --git a/tests/Validator/VariablesInAllowedPositionTest.php b/tests/Validator/VariablesInAllowedPositionTest.php
index 126c012..c4accb4 100644
--- a/tests/Validator/VariablesInAllowedPositionTest.php
+++ b/tests/Validator/VariablesInAllowedPositionTest.php
@@ -177,7 +177,7 @@ class VariablesInAllowedPositionTest extends TestCase
         $this->expectPassesRule(new VariablesInAllowedPosition, '
       query Query($boolVar: Boolean!)
       {
-        dog @if: $boolVar
+        dog @include(if: $boolVar)
       }
         ');
     }
@@ -188,7 +188,7 @@ class VariablesInAllowedPositionTest extends TestCase
         $this->expectPassesRule(new VariablesInAllowedPosition, '
       query Query($boolVar: Boolean = false)
       {
-        dog @if: $boolVar
+        dog @include(if: $boolVar)
       }
         ');
     }
@@ -204,7 +204,7 @@ class VariablesInAllowedPositionTest extends TestCase
         }
       }
         ', [
-            new FormattedError(
+            FormattedError::create(
                 Messages::badVarPosMessage('intArg', 'Int', 'Int!'),
                 [new SourceLocation(5, 45)]
             )
@@ -226,7 +226,7 @@ class VariablesInAllowedPositionTest extends TestCase
         }
       }
         ', [
-            new FormattedError(
+            FormattedError::create(
                 Messages::badVarPosMessage('intArg', 'Int', 'Int!'),
                 [new SourceLocation(3, 43)]
             )
@@ -252,7 +252,7 @@ class VariablesInAllowedPositionTest extends TestCase
         }
       }
         ', [
-            new FormattedError(
+            FormattedError::create(
                 Messages::badVarPosMessage('intArg', 'Int', 'Int!'),
                 [new SourceLocation(7,43)]
             )
@@ -270,7 +270,7 @@ class VariablesInAllowedPositionTest extends TestCase
         }
       }
         ', [
-            new FormattedError(
+            FormattedError::create(
                 Messages::badVarPosMessage('stringVar', 'String', 'Boolean'),
                 [new SourceLocation(5,39)]
             )
@@ -288,7 +288,7 @@ class VariablesInAllowedPositionTest extends TestCase
         }
       }
         ', [
-            new FormattedError(
+            FormattedError::create(
                 Messages::badVarPosMessage('stringVar', 'String', '[String]'),
                 [new SourceLocation(5,45)]
             )
@@ -301,12 +301,12 @@ class VariablesInAllowedPositionTest extends TestCase
         $this->expectFailsRule(new VariablesInAllowedPosition, '
       query Query($boolVar: Boolean)
       {
-        dog @if: $boolVar
+        dog @include(if: $boolVar)
       }
         ', [
-            new FormattedError(
+            FormattedError::create(
                 Messages::badVarPosMessage('boolVar', 'Boolean', 'Boolean!'),
-                [new SourceLocation(4,18)]
+                [new SourceLocation(4,26)]
             )
         ]);
     }
@@ -317,12 +317,12 @@ class VariablesInAllowedPositionTest extends TestCase
         $this->expectFailsRule(new VariablesInAllowedPosition, '
       query Query($stringVar: String)
       {
-        dog @if: $stringVar
+        dog @include(if: $stringVar)
       }
         ', [
-            new FormattedError(
+            FormattedError::create(
                 Messages::badVarPosMessage('stringVar', 'String', 'Boolean!'),
-                [new SourceLocation(4,18)]
+                [new SourceLocation(4,26)]
             )
         ]);
     }