diff --git a/src/Executor/Executor.php b/src/Executor/Executor.php
index 1052841..3872fd8 100644
--- a/src/Executor/Executor.php
+++ b/src/Executor/Executor.php
@@ -2,6 +2,7 @@
 namespace GraphQL\Executor;
 
 use GraphQL\Error\Error;
+use GraphQL\Error\InvariantViolation;
 use GraphQL\Language\AST\Document;
 use GraphQL\Language\AST\Field;
 use GraphQL\Language\AST\FragmentDefinition;
@@ -11,13 +12,12 @@ 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;
+use GraphQL\Type\Definition\LeafType;
 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\DefinitionContainer;
 use GraphQL\Type\Introspection;
@@ -619,7 +619,7 @@ class Executor
                 $result
             );
             if ($completed === null) {
-                throw new \UnexpectedValueException(
+                throw new InvariantViolation(
                     'Cannot return null for non-nullable field ' . $info->parentType . '.' . $info->fieldName . '.'
                 );
             }
@@ -638,8 +638,7 @@ class Executor
 
         // If field type is Scalar or Enum, serialize to a valid value, returning
         // null if serialization is not possible.
-        if ($returnType instanceof ScalarType ||
-            $returnType instanceof EnumType) {
+        if ($returnType instanceof LeafType) {
             return self::completeLeafValue($returnType, $result);
         }
 
@@ -745,6 +744,7 @@ class Executor
             $runtimeType = $exeContext->schema->getType($runtimeType);
         }
 
+        // FIXME: Executor should never face with DefinitionContainer - it should be unwrapped in Schema
         if ($runtimeType instanceof DefinitionContainer) {
             $runtimeType = $runtimeType->getDefinition();
         }
@@ -801,19 +801,18 @@ class Executor
      * Complete a Scalar or Enum by serializing to a valid value, returning
      * null if serialization is not possible.
      *
-     * @param Type $returnType
+     * @param LeafType $returnType
      * @param $result
      * @return mixed
      * @throws \Exception
      */
-    private static function completeLeafValue(Type $returnType, &$result)
+    private static function completeLeafValue(LeafType $returnType, &$result)
     {
-        Utils::invariant(method_exists($returnType, 'serialize'), 'Missing serialize method on type');
         $serializedResult = $returnType->serialize($result);
 
         if ($serializedResult === null) {
-            throw new \UnexpectedValueException(
-                'Expected a value of type "'. $returnType . '" but received: ' . Utils::printSafe($result)
+            throw new InvariantViolation(
+                'Expected a value of type "'. Utils::printSafe($returnType) . '" but received: ' . Utils::printSafe($result)
             );
         }
         return $serializedResult;
diff --git a/src/Executor/Values.php b/src/Executor/Values.php
index b43e82a..e6b2986 100644
--- a/src/Executor/Values.php
+++ b/src/Executor/Values.php
@@ -3,6 +3,7 @@ namespace GraphQL\Executor;
 
 
 use GraphQL\Error\Error;
+use GraphQL\Error\InvariantViolation;
 use GraphQL\Language\AST\Argument;
 use GraphQL\Language\AST\VariableDefinition;
 use GraphQL\Language\Printer;
@@ -11,6 +12,7 @@ use GraphQL\Type\Definition\EnumType;
 use GraphQL\Type\Definition\FieldArgument;
 use GraphQL\Type\Definition\InputObjectType;
 use GraphQL\Type\Definition\InputType;
+use GraphQL\Type\Definition\LeafType;
 use GraphQL\Type\Definition\ListOfType;
 use GraphQL\Type\Definition\NonNull;
 use GraphQL\Type\Definition\ScalarType;
@@ -199,22 +201,20 @@ class Values
             return $errors;
         }
 
-        Utils::invariant(
-            $type instanceof ScalarType || $type instanceof EnumType,
-            'Must be input type'
-        );
-
-        // Scalar/Enum input checks to ensure the type can parse the value to
-        // a non-null value.
-        $parseResult = $type->parseValue($value);
-        if (null === $parseResult) {
-            $v = json_encode($value);
-            return [
-                "Expected type \"{$type->name}\", found $v."
-            ];
+        if ($type instanceof LeafType) {
+            // Scalar/Enum input checks to ensure the type can parse the value to
+            // a non-null value.
+            $parseResult = $type->parseValue($value);
+            if (null === $parseResult) {
+                $v = json_encode($value);
+                return [
+                    "Expected type \"{$type->name}\", found $v."
+                ];
+            }
+            return [];
         }
 
-        return [];
+        throw new InvariantViolation('Must be input type');
     }
 
     /**
diff --git a/src/Type/Definition/FloatType.php b/src/Type/Definition/FloatType.php
index 522d507..ddc1ece 100644
--- a/src/Type/Definition/FloatType.php
+++ b/src/Type/Definition/FloatType.php
@@ -1,6 +1,7 @@
 <?php
 namespace GraphQL\Type\Definition;
 
+use GraphQL\Error\InvariantViolation;
 use GraphQL\Language\AST\FloatValue;
 use GraphQL\Language\AST\IntValue;
 use GraphQL\Utils;
@@ -49,14 +50,14 @@ values as specified by
     private function coerceFloat($value)
     {
         if ($value === '') {
-            throw new \UnexpectedValueException(
+            throw new InvariantViolation(
                 'Float cannot represent non numeric value: (empty string)'
             );
         }
         if (is_numeric($value) || $value === true || $value === false) {
             return (float)$value;
         }
-        throw new \UnexpectedValueException('Float cannot represent non numeric value: ' . Utils::printSafe($value));
+        throw new InvariantViolation('Float cannot represent non numeric value: ' . Utils::printSafe($value));
     }
 
     /**
diff --git a/src/Type/Definition/IntType.php b/src/Type/Definition/IntType.php
index b36cca8..2d713e8 100644
--- a/src/Type/Definition/IntType.php
+++ b/src/Type/Definition/IntType.php
@@ -1,6 +1,7 @@
 <?php
 namespace GraphQL\Type\Definition;
 
+use GraphQL\Error\InvariantViolation;
 use GraphQL\Language\AST\IntValue;
 use GraphQL\Language\AST\Value;
 use GraphQL\Utils;
@@ -56,7 +57,7 @@ values. Int can represent values between -(2^31) and 2^31 - 1. ';
     private function coerceInt($value)
     {
         if ($value === '') {
-            throw new \UnexpectedValueException(
+            throw new InvariantViolation(
                 'Int cannot represent non 32-bit signed integer value: (empty string)'
             );
         }
@@ -66,7 +67,7 @@ values. Int can represent values between -(2^31) and 2^31 - 1. ';
         if (is_numeric($value) && $value <= self::MAX_INT && $value >= self::MIN_INT) {
             return (int) $value;
         }
-        throw new \UnexpectedValueException(
+        throw new InvariantViolation(
             'Int cannot represent non 32-bit signed integer value: ' . Utils::printSafe($value)
         );
     }
diff --git a/src/Type/Definition/ListOfType.php b/src/Type/Definition/ListOfType.php
index 3e37a54..f4f892f 100644
--- a/src/Type/Definition/ListOfType.php
+++ b/src/Type/Definition/ListOfType.php
@@ -33,7 +33,8 @@ class ListOfType extends Type implements WrappingType, OutputType, InputType
      */
     public function toString()
     {
-        $str = $this->ofType instanceof Type ? $this->ofType->toString() : (string) $this->ofType;
+        $type = Type::resolve($this->ofType);
+        $str = $type instanceof Type ? $type->toString() : (string) $type;
         return '[' . $str . ']';
     }
 
diff --git a/src/Type/Definition/Type.php b/src/Type/Definition/Type.php
index 4349b78..52a1f63 100644
--- a/src/Type/Definition/Type.php
+++ b/src/Type/Definition/Type.php
@@ -187,7 +187,6 @@ abstract class Type
 
     /**
      * @param $type
-     * @deprecated in favor of defining ObjectType 'fields' as closure (vs defining closure per field type)
      * @return mixed
      */
     public static function resolve($type)
diff --git a/src/Utils/TypeInfo.php b/src/Utils/TypeInfo.php
index 740b1e6..e7bd67f 100644
--- a/src/Utils/TypeInfo.php
+++ b/src/Utils/TypeInfo.php
@@ -156,7 +156,7 @@ class TypeInfo
             return $innerType ? new NonNull($innerType) : null;
         }
 
-        Utils::invariant($inputTypeAst && $inputTypeAst->kind === Node::NAMED_TYPE, 'Must be a named type');
+        Utils::invariant($inputTypeAst && $inputTypeAst instanceof NamedType, 'Must be a named type');
         return $schema->getType($inputTypeAst->name->value);
     }
 
@@ -180,11 +180,7 @@ class TypeInfo
             return $typeMeta;
         }
         $typeNameMeta = Introspection::typeNameMetaFieldDef();
-        if ($name === $typeNameMeta->name &&
-            ($parentType instanceof ObjectType ||
-            $parentType instanceof InterfaceType ||
-            $parentType instanceof UnionType)
-        ) {
+        if ($name === $typeNameMeta->name && $parentType instanceof CompositeType) {
             return $typeNameMeta;
         }
         if ($parentType instanceof ObjectType ||
diff --git a/src/Validator/DocumentValidator.php b/src/Validator/DocumentValidator.php
index 1566344..c55a5b0 100644
--- a/src/Validator/DocumentValidator.php
+++ b/src/Validator/DocumentValidator.php
@@ -2,6 +2,7 @@
 namespace GraphQL\Validator;
 
 use GraphQL\Error\Error;
+use GraphQL\Error\InvariantViolation;
 use GraphQL\Language\AST\ListValue;
 use GraphQL\Language\AST\Document;
 use GraphQL\Language\AST\FragmentSpread;
@@ -12,12 +13,11 @@ use GraphQL\Language\Printer;
 use GraphQL\Language\Visitor;
 use GraphQL\Language\VisitorOperation;
 use GraphQL\Schema;
-use GraphQL\Type\Definition\EnumType;
 use GraphQL\Type\Definition\InputObjectType;
 use GraphQL\Type\Definition\InputType;
+use GraphQL\Type\Definition\LeafType;
 use GraphQL\Type\Definition\ListOfType;
 use GraphQL\Type\Definition\NonNull;
-use GraphQL\Type\Definition\ScalarType;
 use GraphQL\Type\Definition\Type;
 use GraphQL\Utils;
 use GraphQL\Utils\TypeInfo;
@@ -228,21 +228,20 @@ class DocumentValidator
             return $errors;
         }
 
-        Utils::invariant(
-            $type instanceof ScalarType || $type instanceof EnumType,
-            'Must be input type'
-        );
+        if ($type instanceof LeafType) {
+            // Scalar/Enum input checks to ensure the type can parse the value to
+            // a non-null value.
+            $parseResult = $type->parseLiteral($valueAST);
 
-        // Scalar/Enum input checks to ensure the type can parse the value to
-        // a non-null value.
-        $parseResult = $type->parseLiteral($valueAST);
+            if (null === $parseResult) {
+                $printed = Printer::doPrint($valueAST);
+                return [ "Expected type \"{$type->name}\", found $printed." ];
+            }
 
-        if (null === $parseResult) {
-            $printed = Printer::doPrint($valueAST);
-            return [ "Expected type \"{$type->name}\", found $printed." ];
+            return [];
         }
 
-        return [];
+        throw new InvariantViolation('Must be input type');
     }
 
     /**
diff --git a/tests/Type/ScalarSerializationTest.php b/tests/Type/ScalarSerializationTest.php
index cd95daf..e0be2fa 100644
--- a/tests/Type/ScalarSerializationTest.php
+++ b/tests/Type/ScalarSerializationTest.php
@@ -1,6 +1,7 @@
 <?php
 namespace GraphQL\Tests\Type;
 
+use GraphQL\Error\InvariantViolation;
 use GraphQL\Type\Definition\Type;
 
 class ScalarSerializationTest extends \PHPUnit_Framework_TestCase
@@ -27,28 +28,28 @@ class ScalarSerializationTest extends \PHPUnit_Framework_TestCase
         try {
             $intType->serialize(9876504321);
             $this->fail('Expected exception was not thrown');
-        } catch (\UnexpectedValueException $e) {
+        } catch (InvariantViolation $e) {
             $this->assertEquals('Int cannot represent non 32-bit signed integer value: 9876504321', $e->getMessage());
         }
 
         try {
             $intType->serialize(-9876504321);
             $this->fail('Expected exception was not thrown');
-        } catch (\UnexpectedValueException $e) {
+        } catch (InvariantViolation $e) {
             $this->assertEquals('Int cannot represent non 32-bit signed integer value: -9876504321', $e->getMessage());
         }
 
         try {
             $intType->serialize(1e100);
             $this->fail('Expected exception was not thrown');
-        } catch (\UnexpectedValueException $e) {
+        } catch (InvariantViolation $e) {
             $this->assertEquals('Int cannot represent non 32-bit signed integer value: 1.0E+100', $e->getMessage());
         }
 
         try {
             $intType->serialize(-1e100);
             $this->fail('Expected exception was not thrown');
-        } catch (\UnexpectedValueException $e) {
+        } catch (InvariantViolation $e) {
             $this->assertEquals('Int cannot represent non 32-bit signed integer value: -1.0E+100', $e->getMessage());
         }
 
@@ -57,14 +58,14 @@ class ScalarSerializationTest extends \PHPUnit_Framework_TestCase
         try {
             $intType->serialize('one');
             $this->fail('Expected exception was not thrown');
-        } catch (\UnexpectedValueException $e) {
+        } catch (InvariantViolation $e) {
             $this->assertEquals('Int cannot represent non 32-bit signed integer value: one', $e->getMessage());
         }
 
         try {
             $intType->serialize('');
             $this->fail('Expected exception was not thrown');
-        } catch (\UnexpectedValueException $e) {
+        } catch (InvariantViolation $e) {
             $this->assertEquals('Int cannot represent non 32-bit signed integer value: (empty string)', $e->getMessage());
         }
 
@@ -91,14 +92,14 @@ class ScalarSerializationTest extends \PHPUnit_Framework_TestCase
         try {
             $floatType->serialize('one');
             $this->fail('Expected exception was not thrown');
-        } catch (\UnexpectedValueException $e) {
+        } catch (InvariantViolation $e) {
             $this->assertEquals('Float cannot represent non numeric value: one', $e->getMessage());
         }
 
         try {
             $floatType->serialize('');
             $this->fail('Expected exception was not thrown');
-        } catch (\UnexpectedValueException $e) {
+        } catch (InvariantViolation $e) {
             $this->assertEquals('Float cannot represent non numeric value: (empty string)', $e->getMessage());
         }