diff --git a/src/Executor/ExecutionResult.php b/src/Executor/ExecutionResult.php
index dbc3591..6ac7d41 100644
--- a/src/Executor/ExecutionResult.php
+++ b/src/Executor/ExecutionResult.php
@@ -6,6 +6,7 @@ namespace GraphQL\Executor;
 
 use GraphQL\Error\Error;
 use GraphQL\Error\FormattedError;
+use JsonSerializable;
 use function array_map;
 
 /**
@@ -16,7 +17,7 @@ use function array_map;
  * Could be converted to [spec-compliant](https://facebook.github.io/graphql/#sec-Response-Format)
  * serializable array using `toArray()`
  */
-class ExecutionResult implements \JsonSerializable
+class ExecutionResult implements JsonSerializable
 {
     /**
      * Data collected from resolvers during query execution
@@ -77,8 +78,9 @@ class ExecutionResult implements \JsonSerializable
      *    // ... other keys
      * );
      *
-     * @api
      * @return self
+     *
+     * @api
      */
     public function setErrorFormatter(callable $errorFormatter)
     {
@@ -97,8 +99,9 @@ class ExecutionResult implements \JsonSerializable
      *     return array_map($formatter, $errors);
      * }
      *
-     * @api
      * @return self
+     *
+     * @api
      */
     public function setErrorsHandler(callable $handler)
     {
@@ -125,16 +128,18 @@ class ExecutionResult implements \JsonSerializable
      * $debug argument must be either bool (only adds "debugMessage" to result) or sum of flags from
      * GraphQL\Error\Debug
      *
-     * @api
      * @param bool|int $debug
+     *
      * @return mixed[]
+     *
+     * @api
      */
     public function toArray($debug = false)
     {
         $result = [];
 
         if (! empty($this->errors)) {
-            $errorsHandler = $this->errorsHandler ?: function (array $errors, callable $formatter) {
+            $errorsHandler = $this->errorsHandler ?: static function (array $errors, callable $formatter) {
                 return array_map($formatter, $errors);
             };
 
diff --git a/src/Executor/Executor.php b/src/Executor/Executor.php
index 511939e..41d2eab 100644
--- a/src/Executor/Executor.php
+++ b/src/Executor/Executor.php
@@ -4,7 +4,9 @@ declare(strict_types=1);
 
 namespace GraphQL\Executor;
 
+use ArrayAccess;
 use ArrayObject;
+use Exception;
 use GraphQL\Error\Error;
 use GraphQL\Error\InvariantViolation;
 use GraphQL\Error\Warning;
@@ -33,12 +35,18 @@ use GraphQL\Type\Introspection;
 use GraphQL\Type\Schema;
 use GraphQL\Utils\TypeInfo;
 use GraphQL\Utils\Utils;
+use RuntimeException;
+use SplObjectStorage;
+use stdClass;
+use Throwable;
+use Traversable;
 use function array_keys;
 use function array_merge;
 use function array_reduce;
 use function array_values;
 use function get_class;
 use function is_array;
+use function is_callable;
 use function is_object;
 use function is_string;
 use function sprintf;
@@ -52,7 +60,7 @@ class Executor
     private static $UNDEFINED;
 
     /** @var callable|string[] */
-    private static $defaultFieldResolver = [__CLASS__, 'defaultFieldResolver'];
+    private static $defaultFieldResolver = [self::class, 'defaultFieldResolver'];
 
     /** @var PromiseAdapter */
     private static $promiseAdapter;
@@ -60,7 +68,7 @@ class Executor
     /** @var ExecutionContext */
     private $exeContext;
 
-    /** @var \SplObjectStorage */
+    /** @var SplObjectStorage */
     private $subFieldCache;
 
     private function __construct(ExecutionContext $context)
@@ -70,13 +78,13 @@ class Executor
         }
 
         $this->exeContext    = $context;
-        $this->subFieldCache = new \SplObjectStorage();
+        $this->subFieldCache = new SplObjectStorage();
     }
 
     /**
      * Custom default resolve function
      *
-     * @throws \Exception
+     * @throws Exception
      */
     public static function setDefaultFieldResolver(callable $fn)
     {
@@ -89,13 +97,14 @@ class Executor
      * Always returns ExecutionResult and never throws. All errors which occur during operation
      * execution are collected in `$result->errors`.
      *
-     * @api
-     * @param mixed|null                $rootValue
-     * @param mixed[]|null              $contextValue
-     * @param mixed[]|\ArrayAccess|null $variableValues
-     * @param string|null               $operationName
+     * @param mixed|null               $rootValue
+     * @param mixed[]|null             $contextValue
+     * @param mixed[]|ArrayAccess|null $variableValues
+     * @param string|null              $operationName
      *
      * @return ExecutionResult|Promise
+     *
+     * @api
      */
     public static function execute(
         Schema $schema,
@@ -146,12 +155,14 @@ class Executor
      *
      * Useful for async PHP platforms.
      *
-     * @api
      * @param mixed[]|null $rootValue
      * @param mixed[]|null $contextValue
      * @param mixed[]|null $variableValues
      * @param string|null  $operationName
+     *
      * @return Promise
+     *
+     * @api
      */
     public static function promiseToExecute(
         PromiseAdapter $promiseAdapter,
@@ -187,10 +198,10 @@ class Executor
      * Constructs an ExecutionContext object from the arguments passed to
      * execute, which we will pass throughout the other execution methods.
      *
-     * @param mixed[]              $rootValue
-     * @param mixed[]              $contextValue
-     * @param mixed[]|\Traversable $rawVariableValues
-     * @param string|null          $operationName
+     * @param mixed[]             $rootValue
+     * @param mixed[]             $contextValue
+     * @param mixed[]|Traversable $rawVariableValues
+     * @param string|null         $operationName
      *
      * @return ExecutionContext|Error[]
      */
@@ -297,7 +308,8 @@ class Executor
     }
 
     /**
-     * @param mixed|null|Promise $data
+     * @param mixed|Promise|null $data
+     *
      * @return ExecutionResult|Promise
      */
     private function buildResponse($data)
@@ -317,12 +329,13 @@ class Executor
      * Implements the "Evaluating operations" section of the spec.
      *
      * @param  mixed[] $rootValue
-     * @return Promise|\stdClass|mixed[]
+     *
+     * @return Promise|stdClass|mixed[]
      */
     private function executeOperation(OperationDefinitionNode $operation, $rootValue)
     {
         $type   = $this->getOperationRootType($this->exeContext->schema, $operation);
-        $fields = $this->collectFields($type, $operation->selectionSet, new \ArrayObject(), new \ArrayObject());
+        $fields = $this->collectFields($type, $operation->selectionSet, new ArrayObject(), new ArrayObject());
 
         $path = [];
 
@@ -357,6 +370,7 @@ class Executor
      * Extracts the root type of the operation from the schema.
      *
      * @return ObjectType
+     *
      * @throws Error
      */
     private function getOperationRootType(Schema $schema, OperationDefinitionNode $operation)
@@ -411,7 +425,7 @@ class Executor
      * @param ArrayObject $fields
      * @param ArrayObject $visitedFragmentNames
      *
-     * @return \ArrayObject
+     * @return ArrayObject
      */
     private function collectFields(
         ObjectType $runtimeType,
@@ -428,7 +442,7 @@ class Executor
                     }
                     $name = self::getFieldEntryKey($selection);
                     if (! isset($fields[$name])) {
-                        $fields[$name] = new \ArrayObject();
+                        $fields[$name] = new ArrayObject();
                     }
                     $fields[$name][] = $selection;
                     break;
@@ -475,6 +489,7 @@ class Executor
      * directives, where @skip has higher precedence than @include.
      *
      * @param FragmentSpreadNode|FieldNode|InlineFragmentNode $node
+     *
      * @return bool
      */
     private function shouldIncludeNode($node)
@@ -521,6 +536,7 @@ class Executor
      * Determines if a fragment is applicable to the given type.
      *
      * @param FragmentDefinitionNode|InlineFragmentNode $fragment
+     *
      * @return bool
      */
     private function doesFragmentConditionMatch(
@@ -551,7 +567,8 @@ class Executor
      * @param mixed[]     $sourceValue
      * @param mixed[]     $path
      * @param ArrayObject $fields
-     * @return Promise|\stdClass|mixed[]
+     *
+     * @return Promise|stdClass|mixed[]
      */
     private function executeFieldsSerially(ObjectType $parentType, $sourceValue, $path, $fields)
     {
@@ -567,7 +584,7 @@ class Executor
                 }
                 $promise = $this->getPromise($result);
                 if ($promise) {
-                    return $promise->then(function ($resolvedResult) use ($responseName, $results) {
+                    return $promise->then(static function ($resolvedResult) use ($responseName, $results) {
                         $results[$responseName] = $resolvedResult;
                         return $results;
                     });
@@ -578,7 +595,7 @@ class Executor
             []
         );
         if ($this->isPromise($result)) {
-            return $result->then(function ($resolvedResults) {
+            return $result->then(static function ($resolvedResults) {
                 return self::fixResultsIfEmptyArray($resolvedResults);
             });
         }
@@ -595,7 +612,7 @@ class Executor
      * @param FieldNode[] $fieldNodes
      * @param mixed[]     $path
      *
-     * @return mixed[]|\Exception|mixed|null
+     * @return mixed[]|Exception|mixed|null
      */
     private function resolveField(ObjectType $parentType, $source, $fieldNodes, $path)
     {
@@ -705,7 +722,8 @@ class Executor
      * @param mixed           $source
      * @param mixed           $context
      * @param ResolveInfo     $info
-     * @return \Throwable|Promise|mixed
+     *
+     * @return Throwable|Promise|mixed
      */
     private function resolveOrError($fieldDef, $fieldNode, $resolveFn, $source, $context, $info)
     {
@@ -719,9 +737,9 @@ class Executor
             );
 
             return $resolveFn($source, $args, $context, $info);
-        } catch (\Exception $error) {
+        } catch (Exception $error) {
             return $error;
-        } catch (\Throwable $error) {
+        } catch (Throwable $error) {
             return $error;
         }
     }
@@ -733,6 +751,7 @@ class Executor
      * @param FieldNode[] $fieldNodes
      * @param string[]    $path
      * @param mixed       $result
+     *
      * @return mixed[]|Promise|null
      */
     private function completeValueCatchingError(
@@ -796,7 +815,9 @@ class Executor
      * @param FieldNode[] $fieldNodes
      * @param string[]    $path
      * @param mixed       $result
+     *
      * @return mixed[]|mixed|Promise|null
+     *
      * @throws Error
      */
     public function completeValueWithLocatedError(
@@ -829,9 +850,9 @@ class Executor
             }
 
             return $completed;
-        } catch (\Exception $error) {
+        } catch (Exception $error) {
             throw Error::createLocatedError($error, $fieldNodes, $path);
-        } catch (\Throwable $error) {
+        } catch (Throwable $error) {
             throw Error::createLocatedError($error, $fieldNodes, $path);
         }
     }
@@ -860,9 +881,11 @@ class Executor
      * @param FieldNode[] $fieldNodes
      * @param string[]    $path
      * @param mixed       $result
+     *
      * @return mixed[]|mixed|Promise|null
+     *
      * @throws Error
-     * @throws \Throwable
+     * @throws Throwable
      */
     private function completeValue(
         Type $returnType,
@@ -880,7 +903,7 @@ class Executor
             });
         }
 
-        if ($result instanceof \Exception || $result instanceof \Throwable) {
+        if ($result instanceof Exception || $result instanceof Throwable) {
             throw $result;
         }
 
@@ -949,11 +972,12 @@ class Executor
             return $this->completeObjectValue($returnType, $fieldNodes, $info, $path, $result);
         }
 
-        throw new \RuntimeException(sprintf('Cannot complete value of unexpected type "%s".', $returnType));
+        throw new RuntimeException(sprintf('Cannot complete value of unexpected type "%s".', $returnType));
     }
 
     /**
      * @param mixed $value
+     *
      * @return bool
      */
     private function isPromise($value)
@@ -966,6 +990,7 @@ class Executor
      * otherwise returns null.
      *
      * @param mixed $value
+     *
      * @return Promise|null
      */
     private function getPromise($value)
@@ -998,14 +1023,15 @@ class Executor
      *
      * @param mixed[]            $values
      * @param Promise|mixed|null $initialValue
+     *
      * @return mixed[]
      */
-    private function promiseReduce(array $values, \Closure $callback, $initialValue)
+    private function promiseReduce(array $values, callable $callback, $initialValue)
     {
         return array_reduce($values, function ($previous, $value) use ($callback) {
             $promise = $this->getPromise($previous);
             if ($promise) {
-                return $promise->then(function ($resolved) use ($callback, $value) {
+                return $promise->then(static function ($resolved) use ($callback, $value) {
                     return $callback($resolved, $value);
                 });
             }
@@ -1020,14 +1046,16 @@ class Executor
      * @param FieldNode[] $fieldNodes
      * @param mixed[]     $path
      * @param mixed       $result
+     *
      * @return mixed[]|Promise
-     * @throws \Exception
+     *
+     * @throws Exception
      */
     private function completeListValue(ListOfType $returnType, $fieldNodes, ResolveInfo $info, $path, &$result)
     {
         $itemType = $returnType->getWrappedType();
         Utils::invariant(
-            is_array($result) || $result instanceof \Traversable,
+            is_array($result) || $result instanceof Traversable,
             'User Error: expected iterable, but did not find one for field ' . $info->parentType . '.' . $info->fieldName . '.'
         );
         $containsPromise = false;
@@ -1051,20 +1079,22 @@ class Executor
      * Complete a Scalar or Enum by serializing to a valid value, throwing if serialization is not possible.
      *
      * @param  mixed $result
+     *
      * @return mixed
-     * @throws \Exception
+     *
+     * @throws Exception
      */
     private function completeLeafValue(LeafType $returnType, &$result)
     {
         try {
             return $returnType->serialize($result);
-        } catch (\Exception $error) {
+        } catch (Exception $error) {
             throw new InvariantViolation(
                 'Expected a value of type "' . Utils::printSafe($returnType) . '" but received: ' . Utils::printSafe($result),
                 0,
                 $error
             );
-        } catch (\Throwable $error) {
+        } catch (Throwable $error) {
             throw new InvariantViolation(
                 'Expected a value of type "' . Utils::printSafe($returnType) . '" but received: ' . Utils::printSafe($result),
                 0,
@@ -1080,7 +1110,9 @@ class Executor
      * @param FieldNode[] $fieldNodes
      * @param mixed[]     $path
      * @param mixed[]     $result
+     *
      * @return mixed
+     *
      * @throws Error
      */
     private function completeAbstractValue(AbstractType $returnType, $fieldNodes, ResolveInfo $info, $path, &$result)
@@ -1142,6 +1174,7 @@ class Executor
      *
      * @param mixed|null $value
      * @param mixed|null $context
+     *
      * @return ObjectType|Promise|null
      */
     private function defaultTypeResolver($value, $context, ResolveInfo $info, AbstractType $abstractType)
@@ -1190,7 +1223,7 @@ class Executor
 
         if (! empty($promisedIsTypeOfResults)) {
             return $this->exeContext->promises->all($promisedIsTypeOfResults)
-                ->then(function ($isTypeOfResults) use ($possibleTypes) {
+                ->then(static function ($isTypeOfResults) use ($possibleTypes) {
                     foreach ($isTypeOfResults as $index => $result) {
                         if ($result) {
                             return $possibleTypes[$index];
@@ -1210,7 +1243,9 @@ class Executor
      * @param FieldNode[] $fieldNodes
      * @param mixed[]     $path
      * @param mixed       $result
-     * @return mixed[]|Promise|\stdClass
+     *
+     * @return mixed[]|Promise|stdClass
+     *
      * @throws Error
      */
     private function completeObjectValue(ObjectType $returnType, $fieldNodes, ResolveInfo $info, $path, &$result)
@@ -1260,6 +1295,7 @@ class Executor
     /**
      * @param mixed[]     $result
      * @param FieldNode[] $fieldNodes
+     *
      * @return Error
      */
     private function invalidReturnTypeError(
@@ -1277,7 +1313,9 @@ class Executor
      * @param FieldNode[] $fieldNodes
      * @param mixed[]     $path
      * @param mixed[]     $result
-     * @return mixed[]|Promise|\stdClass
+     *
+     * @return mixed[]|Promise|stdClass
+     *
      * @throws Error
      */
     private function collectAndExecuteSubfields(
@@ -1294,12 +1332,12 @@ class Executor
     private function collectSubFields(ObjectType $returnType, $fieldNodes) : ArrayObject
     {
         if (! isset($this->subFieldCache[$returnType])) {
-            $this->subFieldCache[$returnType] = new \SplObjectStorage();
+            $this->subFieldCache[$returnType] = new SplObjectStorage();
         }
         if (! isset($this->subFieldCache[$returnType][$fieldNodes])) {
             // Collect sub-fields to execute to complete this value.
-            $subFieldNodes        = new \ArrayObject();
-            $visitedFragmentNames = new \ArrayObject();
+            $subFieldNodes        = new ArrayObject();
+            $visitedFragmentNames = new ArrayObject();
 
             foreach ($fieldNodes as $fieldNode) {
                 if (! isset($fieldNode->selectionSet)) {
@@ -1325,7 +1363,8 @@ class Executor
      * @param mixed|null  $source
      * @param mixed[]     $path
      * @param ArrayObject $fields
-     * @return Promise|\stdClass|mixed[]
+     *
+     * @return Promise|stdClass|mixed[]
      */
     private function executeFields(ObjectType $parentType, $source, $path, $fields)
     {
@@ -1361,12 +1400,13 @@ class Executor
      * @see https://github.com/webonyx/graphql-php/issues/59
      *
      * @param mixed[] $results
-     * @return \stdClass|mixed[]
+     *
+     * @return stdClass|mixed[]
      */
     private static function fixResultsIfEmptyArray($results)
     {
         if ($results === []) {
-            return new \stdClass();
+            return new stdClass();
         }
 
         return $results;
@@ -1380,6 +1420,7 @@ class Executor
      * any promises.
      *
      * @param (string|Promise)[] $assoc
+     *
      * @return mixed
      */
     private function promiseForAssocArray(array $assoc)
@@ -1389,7 +1430,7 @@ class Executor
 
         $promise = $this->exeContext->promises->all($valuesAndPromises);
 
-        return $promise->then(function ($values) use ($keys) {
+        return $promise->then(static function ($values) use ($keys) {
             $resolvedResults = [];
             foreach ($values as $i => $value) {
                 $resolvedResults[$keys[$i]] = $value;
@@ -1403,6 +1444,7 @@ class Executor
      * @param string|ObjectType|null $runtimeTypeOrName
      * @param FieldNode[]            $fieldNodes
      * @param mixed                  $result
+     *
      * @return ObjectType
      */
     private function ensureValidRuntimeType(
@@ -1471,7 +1513,7 @@ class Executor
         $fieldName = $info->fieldName;
         $property  = null;
 
-        if (is_array($source) || $source instanceof \ArrayAccess) {
+        if (is_array($source) || $source instanceof ArrayAccess) {
             if (isset($source[$fieldName])) {
                 $property = $source[$fieldName];
             }
@@ -1481,6 +1523,6 @@ class Executor
             }
         }
 
-        return $property instanceof \Closure ? $property($source, $args, $context, $info) : $property;
+        return is_callable($property) ? $property($source, $args, $context, $info) : $property;
     }
 }
diff --git a/src/Executor/Promise/Adapter/ReactPromiseAdapter.php b/src/Executor/Promise/Adapter/ReactPromiseAdapter.php
index 4feb3c0..446d904 100644
--- a/src/Executor/Promise/Adapter/ReactPromiseAdapter.php
+++ b/src/Executor/Promise/Adapter/ReactPromiseAdapter.php
@@ -80,12 +80,12 @@ class ReactPromiseAdapter implements PromiseAdapter
         // TODO: rework with generators when PHP minimum required version is changed to 5.5+
         $promisesOrValues = Utils::map(
             $promisesOrValues,
-            function ($item) {
+            static function ($item) {
                 return $item instanceof Promise ? $item->adoptedPromise : $item;
             }
         );
 
-        $promise = all($promisesOrValues)->then(function ($values) use ($promisesOrValues) {
+        $promise = all($promisesOrValues)->then(static function ($values) use ($promisesOrValues) {
             $orderedResults = [];
 
             foreach ($promisesOrValues as $key => $value) {
diff --git a/src/Executor/Promise/Adapter/SyncPromise.php b/src/Executor/Promise/Adapter/SyncPromise.php
index fd0f3ba..4d1f96e 100644
--- a/src/Executor/Promise/Adapter/SyncPromise.php
+++ b/src/Executor/Promise/Adapter/SyncPromise.php
@@ -4,8 +4,10 @@ declare(strict_types=1);
 
 namespace GraphQL\Executor\Promise\Adapter;
 
+use Exception;
 use GraphQL\Executor\ExecutionResult;
 use GraphQL\Utils\Utils;
+use SplQueue;
 use Throwable;
 use function is_object;
 use function method_exists;
@@ -22,7 +24,7 @@ class SyncPromise
     const FULFILLED = 'fulfilled';
     const REJECTED  = 'rejected';
 
-    /** @var \SplQueue */
+    /** @var SplQueue */
     public static $queue;
 
     /** @var string */
@@ -33,6 +35,7 @@ class SyncPromise
 
     /**
      * Promises created in `then` method of this promise and awaiting for resolution of this promise
+     *
      * @var mixed[][]
      */
     private $waiting = [];
@@ -51,7 +54,7 @@ class SyncPromise
         switch ($this->state) {
             case self::PENDING:
                 if ($value === $this) {
-                    throw new \Exception('Cannot resolve promise with self');
+                    throw new Exception('Cannot resolve promise with self');
                 }
                 if (is_object($value) && method_exists($value, 'then')) {
                     $value->then(
@@ -72,11 +75,11 @@ class SyncPromise
                 break;
             case self::FULFILLED:
                 if ($this->result !== $value) {
-                    throw new \Exception('Cannot change value of fulfilled promise');
+                    throw new Exception('Cannot change value of fulfilled promise');
                 }
                 break;
             case self::REJECTED:
-                throw new \Exception('Cannot resolve rejected promise');
+                throw new Exception('Cannot resolve rejected promise');
         }
 
         return $this;
@@ -84,8 +87,8 @@ class SyncPromise
 
     public function reject($reason)
     {
-        if (! $reason instanceof \Exception && ! $reason instanceof \Throwable) {
-            throw new \Exception('SyncPromise::reject() has to be called with an instance of \Throwable');
+        if (! $reason instanceof Exception && ! $reason instanceof Throwable) {
+            throw new Exception('SyncPromise::reject() has to be called with an instance of \Throwable');
         }
 
         switch ($this->state) {
@@ -96,11 +99,11 @@ class SyncPromise
                 break;
             case self::REJECTED:
                 if ($reason !== $this->result) {
-                    throw new \Exception('Cannot change rejection reason');
+                    throw new Exception('Cannot change rejection reason');
                 }
                 break;
             case self::FULFILLED:
-                throw new \Exception('Cannot reject fulfilled promise');
+                throw new Exception('Cannot reject fulfilled promise');
         }
 
         return $this;
@@ -116,14 +119,14 @@ class SyncPromise
         foreach ($this->waiting as $descriptor) {
             self::getQueue()->enqueue(function () use ($descriptor) {
                 /** @var $promise self */
-                list($promise, $onFulfilled, $onRejected) = $descriptor;
+                [$promise, $onFulfilled, $onRejected] = $descriptor;
 
                 if ($this->state === self::FULFILLED) {
                     try {
                         $promise->resolve($onFulfilled ? $onFulfilled($this->result) : $this->result);
-                    } catch (\Exception $e) {
+                    } catch (Exception $e) {
                         $promise->reject($e);
-                    } catch (\Throwable $e) {
+                    } catch (Throwable $e) {
                         $promise->reject($e);
                     }
                 } elseif ($this->state === self::REJECTED) {
@@ -133,9 +136,9 @@ class SyncPromise
                         } else {
                             $promise->reject($this->result);
                         }
-                    } catch (\Exception $e) {
+                    } catch (Exception $e) {
                         $promise->reject($e);
-                    } catch (\Throwable $e) {
+                    } catch (Throwable $e) {
                         $promise->reject($e);
                     }
                 }
@@ -146,7 +149,7 @@ class SyncPromise
 
     public static function getQueue()
     {
-        return self::$queue ?: self::$queue = new \SplQueue();
+        return self::$queue ?: self::$queue = new SplQueue();
     }
 
     public function then(?callable $onFulfilled = null, ?callable $onRejected = null)
diff --git a/src/Executor/Promise/Adapter/SyncPromiseAdapter.php b/src/Executor/Promise/Adapter/SyncPromiseAdapter.php
index 3ec9246..2df02af 100644
--- a/src/Executor/Promise/Adapter/SyncPromiseAdapter.php
+++ b/src/Executor/Promise/Adapter/SyncPromiseAdapter.php
@@ -4,12 +4,14 @@ declare(strict_types=1);
 
 namespace GraphQL\Executor\Promise\Adapter;
 
+use Exception;
 use GraphQL\Deferred;
 use GraphQL\Error\InvariantViolation;
 use GraphQL\Executor\ExecutionResult;
 use GraphQL\Executor\Promise\Promise;
 use GraphQL\Executor\Promise\PromiseAdapter;
 use GraphQL\Utils\Utils;
+use Throwable;
 use function count;
 
 /**
@@ -67,9 +69,9 @@ class SyncPromiseAdapter implements PromiseAdapter
                     'reject',
                 ]
             );
-        } catch (\Exception $e) {
+        } catch (Exception $e) {
             $promise->reject($e);
-        } catch (\Throwable $e) {
+        } catch (Throwable $e) {
             $promise->reject($e);
         }
 
@@ -111,7 +113,7 @@ class SyncPromiseAdapter implements PromiseAdapter
             if ($promiseOrValue instanceof Promise) {
                 $result[$index] = null;
                 $promiseOrValue->then(
-                    function ($value) use ($index, &$count, $total, &$result, $all) {
+                    static function ($value) use ($index, &$count, $total, &$result, $all) {
                         $result[$index] = $value;
                         $count++;
                         if ($count < $total) {
@@ -167,7 +169,6 @@ class SyncPromiseAdapter implements PromiseAdapter
 
     /**
      * Execute just before starting to run promise completion
-     *
      */
     protected function beforeWait(Promise $promise)
     {
@@ -175,7 +176,6 @@ class SyncPromiseAdapter implements PromiseAdapter
 
     /**
      * Execute while running promise completion
-     *
      */
     protected function onWait(Promise $promise)
     {
diff --git a/src/Executor/Promise/Promise.php b/src/Executor/Promise/Promise.php
index 2f060e1..5823d14 100644
--- a/src/Executor/Promise/Promise.php
+++ b/src/Executor/Promise/Promise.php
@@ -24,14 +24,13 @@ class Promise
      */
     public function __construct($adoptedPromise, PromiseAdapter $adapter)
     {
-        Utils::invariant(! $adoptedPromise instanceof self, 'Expecting promise from adapted system, got ' . __CLASS__);
+        Utils::invariant(! $adoptedPromise instanceof self, 'Expecting promise from adapted system, got ' . self::class);
 
         $this->adapter        = $adapter;
         $this->adoptedPromise = $adoptedPromise;
     }
 
     /**
-     *
      * @return Promise
      */
     public function then(?callable $onFulfilled = null, ?callable $onRejected = null)
diff --git a/src/Executor/Promise/PromiseAdapter.php b/src/Executor/Promise/PromiseAdapter.php
index 74582fa..c3d4098 100644
--- a/src/Executor/Promise/PromiseAdapter.php
+++ b/src/Executor/Promise/PromiseAdapter.php
@@ -4,6 +4,8 @@ declare(strict_types=1);
 
 namespace GraphQL\Executor\Promise;
 
+use Throwable;
+
 /**
  * Provides a means for integration of async PHP platforms ([related docs](data-fetching.md#async-php))
  */
@@ -12,18 +14,22 @@ interface PromiseAdapter
     /**
      * Return true if the value is a promise or a deferred of the underlying platform
      *
-     * @api
      * @param mixed $value
+     *
      * @return bool
+     *
+     * @api
      */
     public function isThenable($value);
 
     /**
      * Converts thenable of the underlying platform into GraphQL\Executor\Promise\Promise instance
      *
-     * @api
      * @param object $thenable
+     *
      * @return Promise
+     *
+     * @api
      */
     public function convertThenable($thenable);
 
@@ -31,9 +37,9 @@ interface PromiseAdapter
      * Accepts our Promise wrapper, extracts adopted promise out of it and executes actual `then` logic described
      * in Promises/A+ specs. Then returns new wrapped instance of GraphQL\Executor\Promise\Promise.
      *
-     * @api
-     *
      * @return Promise
+     *
+     * @api
      */
     public function then(Promise $promise, ?callable $onFulfilled = null, ?callable $onRejected = null);
 
@@ -43,17 +49,20 @@ interface PromiseAdapter
      * Expected resolver signature:
      *     function(callable $resolve, callable $reject)
      *
-     * @api
      * @return Promise
+     *
+     * @api
      */
     public function create(callable $resolver);
 
     /**
      * Creates a fulfilled Promise for a value if the value is not a promise.
      *
-     * @api
      * @param mixed $value
+     *
      * @return Promise
+     *
+     * @api
      */
     public function createFulfilled($value = null);
 
@@ -61,9 +70,11 @@ interface PromiseAdapter
      * Creates a rejected promise for a reason if the reason is not a promise. If
      * the provided reason is a promise, then it is returned as-is.
      *
-     * @api
-     * @param \Throwable $reason
+     * @param Throwable $reason
+     *
      * @return Promise
+     *
+     * @api
      */
     public function createRejected($reason);
 
@@ -71,9 +82,11 @@ interface PromiseAdapter
      * Given an array of promises (or values), returns a promise that is fulfilled when all the
      * items in the array are fulfilled.
      *
-     * @api
      * @param Promise[]|mixed[] $promisesOrValues Promises or values.
+     *
      * @return Promise
+     *
+     * @api
      */
     public function all(array $promisesOrValues);
 }
diff --git a/src/Executor/Values.php b/src/Executor/Values.php
index 53e66cb..5550544 100644
--- a/src/Executor/Values.php
+++ b/src/Executor/Values.php
@@ -27,6 +27,7 @@ use GraphQL\Utils\AST;
 use GraphQL\Utils\TypeInfo;
 use GraphQL\Utils\Utils;
 use GraphQL\Utils\Value;
+use stdClass;
 use Throwable;
 use function array_key_exists;
 use function array_map;
@@ -41,6 +42,7 @@ class Values
      *
      * @param VariableDefinitionNode[] $varDefNodes
      * @param mixed[]                  $inputs
+     *
      * @return mixed[]
      */
     public static function getVariableValues(Schema $schema, $varDefNodes, array $inputs)
@@ -125,7 +127,7 @@ class Values
         if (isset($node->directives) && $node->directives instanceof NodeList) {
             $directiveNode = Utils::find(
                 $node->directives,
-                function (DirectiveNode $directive) use ($directiveDef) {
+                static function (DirectiveNode $directive) use ($directiveDef) {
                     return $directive->name->value === $directiveDef->name;
                 }
             );
@@ -145,7 +147,9 @@ class Values
      * @param FieldDefinition|Directive $def
      * @param FieldNode|DirectiveNode   $node
      * @param mixed[]                   $variableValues
+     *
      * @return mixed[]
+     *
      * @throws Error
      */
     public static function getArgumentValues($def, $node, $variableValues = null)
@@ -162,7 +166,7 @@ class Values
         /** @var ArgumentNode[] $argNodeMap */
         $argNodeMap = $argNodes ? Utils::keyMap(
             $argNodes,
-            function (ArgumentNode $arg) {
+            static function (ArgumentNode $arg) {
                 return $arg->name->value;
             }
         ) : [];
@@ -222,18 +226,21 @@ class Values
     /**
      * @deprecated as of 8.0 (Moved to \GraphQL\Utils\AST::valueFromAST)
      *
-     * @param ValueNode $valueNode
-     * @param null      $variables
-     * @return mixed[]|null|\stdClass
+     * @param ValueNode    $valueNode
+     * @param mixed[]|null $variables
+     *
+     * @return mixed[]|stdClass|null
      */
-    public static function valueFromAST($valueNode, InputType $type, $variables = null)
+    public static function valueFromAST($valueNode, InputType $type, ?array $variables = null)
     {
         return AST::valueFromAST($valueNode, $type, $variables);
     }
 
     /**
      * @deprecated as of 0.12 (Use coerceValue() directly for richer information)
+     *
      * @param mixed[] $value
+     *
      * @return string[]
      */
     public static function isValidPHPValue($value, InputType $type)
@@ -242,7 +249,7 @@ class Values
 
         return $errors
             ? array_map(
-                function (Throwable $error) {
+                static function (Throwable $error) {
                     return $error->getMessage();
                 },
                 $errors