diff --git a/src/Type/Definition/Type.php b/src/Type/Definition/Type.php
index b483d7f..f4a1467 100644
--- a/src/Type/Definition/Type.php
+++ b/src/Type/Definition/Type.php
@@ -1,6 +1,7 @@
 <?php
 namespace GraphQL\Type\Definition;
 
+use GraphQL\Error\InvariantViolation;
 use GraphQL\Utils;
 
 /*
@@ -191,14 +192,19 @@ abstract class Type
     public static function resolve($type)
     {
         if (is_callable($type)) {
+            trigger_error(
+                'Passing type as closure is deprecated (see https://github.com/webonyx/graphql-php/issues/35 for alternatives)',
+                E_USER_DEPRECATED
+            );
             $type = $type();
         }
 
-        Utils::invariant(
-            $type instanceof Type,
-            'Expecting instance of ' . __CLASS__ . ' (or callable returning instance of that type), got "%s"',
-            Utils::getVariableType($type)
-        );
+        if (!$type instanceof Type) {
+            throw new InvariantViolation(sprintf(
+                'Expecting instance of ' . __CLASS__ . ', got "%s"',
+                Utils::getVariableType($type)
+            ));
+        }
         return $type;
     }
 
diff --git a/src/Type/Introspection.php b/src/Type/Introspection.php
index 68b225d..19041f6 100644
--- a/src/Type/Introspection.php
+++ b/src/Type/Introspection.php
@@ -498,7 +498,7 @@ EOD;
                             }
                         ],
                         'possibleTypes' => [
-                            'type' => Type::listOf(Type::nonNull([__CLASS__, '_type'])),
+                            'type' => Type::listOf(Type::nonNull(self::_type())),
                             'resolve' => function ($type, $args, $context, ResolveInfo $info) {
                                 if ($type instanceof InterfaceType || $type instanceof UnionType) {
                                     return $info->schema->getPossibleTypes($type);
diff --git a/tests/Executor/ExecutorSchemaTest.php b/tests/Executor/ExecutorSchemaTest.php
index ff57ebb..4da1af6 100644
--- a/tests/Executor/ExecutorSchemaTest.php
+++ b/tests/Executor/ExecutorSchemaTest.php
@@ -29,20 +29,20 @@ class ExecutorSchemaTest extends \PHPUnit_Framework_TestCase
 
         $BlogAuthor = new ObjectType([
             'name' => 'Author',
-            'fields' => [
-                'id' => ['type' => Type::string()],
-                'name' => ['type' => Type::string()],
-                'pic' => [
-                    'args' => ['width' => ['type' => Type::int()], 'height' => ['type' => Type::int()]],
-                    'type' => $BlogImage,
-                    'resolve' => function ($obj, $args) {
-                        return $obj['pic']($args['width'], $args['height']);
-                    }
-                ],
-                'recentArticle' => ['type' => function () use (&$BlogArticle) {
-                    return $BlogArticle;
-                }]
-            ]
+            'fields' => function() use (&$BlogArticle, &$BlogImage) {
+                return [
+                    'id' => ['type' => Type::string()],
+                    'name' => ['type' => Type::string()],
+                    'pic' => [
+                        'args' => ['width' => ['type' => Type::int()], 'height' => ['type' => Type::int()]],
+                        'type' => $BlogImage,
+                        'resolve' => function ($obj, $args) {
+                            return $obj['pic']($args['width'], $args['height']);
+                        }
+                    ],
+                    'recentArticle' => $BlogArticle
+                ];
+            }
         ]);
 
         $BlogArticle = new ObjectType([
diff --git a/tests/Executor/ExecutorTest.php b/tests/Executor/ExecutorTest.php
index 6bd7c77..07daaa3 100644
--- a/tests/Executor/ExecutorTest.php
+++ b/tests/Executor/ExecutorTest.php
@@ -116,23 +116,25 @@ class ExecutorTest extends \PHPUnit_Framework_TestCase
         $deepDataType = null;
         $dataType = new ObjectType([
             'name' => 'DataType',
-            'fields' => [
-                'a' => [ 'type' => Type::string() ],
-                'b' => [ 'type' => Type::string() ],
-                'c' => [ 'type' => Type::string() ],
-                'd' => [ 'type' => Type::string() ],
-                'e' => [ 'type' => Type::string() ],
-                'f' => [ 'type' => Type::string() ],
-                'pic' => [
-                    'args' => [ 'size' => ['type' => Type::int() ] ],
-                    'type' => Type::string(),
-                    'resolve' => function($obj, $args) {
-                        return $obj['pic']($args['size']);
-                    }
-                ],
-                'promise' => ['type' => function() use (&$dataType) {return $dataType;}],
-                'deep' => [ 'type' => function() use(&$deepDataType) {return $deepDataType; }],
-            ]
+            'fields' => function() use (&$dataType, &$deepDataType) {
+                return [
+                    'a' => [ 'type' => Type::string() ],
+                    'b' => [ 'type' => Type::string() ],
+                    'c' => [ 'type' => Type::string() ],
+                    'd' => [ 'type' => Type::string() ],
+                    'e' => [ 'type' => Type::string() ],
+                    'f' => [ 'type' => Type::string() ],
+                    'pic' => [
+                        'args' => [ 'size' => ['type' => Type::int() ] ],
+                        'type' => Type::string(),
+                        'resolve' => function($obj, $args) {
+                            return $obj['pic']($args['size']);
+                        }
+                    ],
+                    'promise' => ['type' => $dataType],
+                    'deep' => ['type' => $deepDataType],
+                ];
+            }
         ]);
 
         $deepDataType = new ObjectType([
@@ -170,25 +172,25 @@ class ExecutorTest extends \PHPUnit_Framework_TestCase
 
         $Type = new ObjectType([
             'name' => 'Type',
-            'fields' => [
-                'a' => ['type' => Type::string(), 'resolve' => function () {
-                    return 'Apple';
-                }],
-                'b' => ['type' => Type::string(), 'resolve' => function () {
-                    return 'Banana';
-                }],
-                'c' => ['type' => Type::string(), 'resolve' => function () {
-                    return 'Cherry';
-                }],
-                'deep' => [
-                    'type' => function () use (&$Type) {
-                        return $Type;
-                    },
-                    'resolve' => function () {
-                        return [];
-                    }
-                ]
-            ]
+            'fields' => function() use (&$Type) {
+                return [
+                    'a' => ['type' => Type::string(), 'resolve' => function () {
+                        return 'Apple';
+                    }],
+                    'b' => ['type' => Type::string(), 'resolve' => function () {
+                        return 'Banana';
+                    }],
+                    'c' => ['type' => Type::string(), 'resolve' => function () {
+                        return 'Cherry';
+                    }],
+                    'deep' => [
+                        'type' => $Type,
+                        'resolve' => function () {
+                            return [];
+                        }
+                    ]
+                ];
+            }
         ]);
         $schema = new Schema(['query' => $Type]);
         $expected = [
diff --git a/tests/Executor/NonNullTest.php b/tests/Executor/NonNullTest.php
index 6bf619d..6aa147f 100644
--- a/tests/Executor/NonNullTest.php
+++ b/tests/Executor/NonNullTest.php
@@ -58,16 +58,14 @@ class NonNullTest extends \PHPUnit_Framework_TestCase
 
         $dataType = new ObjectType([
             'name' => 'DataType',
-            'fields' => [
-                'sync' => ['type' => Type::string()],
-                'nonNullSync' => ['type' => Type::nonNull(Type::string())],
-                'nest' => ['type' => function () use (&$dataType) {
-                    return $dataType;
-                }],
-                'nonNullNest' => ['type' => function () use (&$dataType) {
-                    return Type::nonNull($dataType);
-                }]
-            ]
+            'fields' => function() use (&$dataType) {
+                return [
+                    'sync' => ['type' => Type::string()],
+                    'nonNullSync' => ['type' => Type::nonNull(Type::string())],
+                    'nest' => $dataType,
+                    'nonNullNest' => Type::nonNull($dataType)
+                ];
+            }
         ]);
 
         $this->schema = new Schema(['query' => $dataType]);
diff --git a/tests/Type/DefinitionTest.php b/tests/Type/DefinitionTest.php
index 0292ac8..e6af327 100644
--- a/tests/Type/DefinitionTest.php
+++ b/tests/Type/DefinitionTest.php
@@ -92,15 +92,17 @@ class DefinitionTest extends \PHPUnit_Framework_TestCase
 
         $this->blogAuthor = new ObjectType([
             'name' => 'Author',
-            'fields' => [
-                'id' => ['type' => Type::string()],
-                'name' => ['type' => Type::string()],
-                'pic' => [ 'type' => $this->blogImage, 'args' => [
-                    'width' => ['type' => Type::int()],
-                    'height' => ['type' => Type::int()]
-                ]],
-                'recentArticle' => ['type' => function() {return $this->blogArticle;}],
-            ],
+            'fields' => function() {
+                return [
+                    'id' => ['type' => Type::string()],
+                    'name' => ['type' => Type::string()],
+                    'pic' => [ 'type' => $this->blogImage, 'args' => [
+                        'width' => ['type' => Type::int()],
+                        'height' => ['type' => Type::int()]
+                    ]],
+                    'recentArticle' => $this->blogArticle,
+                ];
+            },
         ]);
 
         $this->blogArticle = new ObjectType([
diff --git a/tests/Type/SchemaValidatorTest.php b/tests/Type/SchemaValidatorTest.php
index b8c3e77..67bf0bf 100644
--- a/tests/Type/SchemaValidatorTest.php
+++ b/tests/Type/SchemaValidatorTest.php
@@ -398,9 +398,11 @@ class SchemaValidatorTest extends \PHPUnit_Framework_TestCase
         foreach ($kinds as $kind) {
             $someOutputType = new $kind([
                 'name' => 'SomeOutputType',
-                'fields' => [
-                    'sneaky' => ['type' => function() {return $this->someInputObjectType;}]
-                ]
+                'fields' => function() {
+                    return [
+                        'sneaky' => $this->someInputObjectType
+                    ];
+                }
             ]);
 
             $schema = new Schema(['query' => $someOutputType]);
@@ -545,9 +547,11 @@ class SchemaValidatorTest extends \PHPUnit_Framework_TestCase
     {
         $someIncorrectInputType = new InputObjectType([
             'name' => 'SomeIncorrectInputType',
-            'fields' => [
-                'val' => ['type' => function() use ($argType) {return $argType;} ]
-            ]
+            'fields' => function() use ($argType) {
+                return [
+                    'val' => ['type' => $argType ]
+                ];
+            }
         ]);
 
         $queryType = new ObjectType([
diff --git a/tests/Validator/QuerySecuritySchema.php b/tests/Validator/QuerySecuritySchema.php
index aa95b88..771af77 100644
--- a/tests/Validator/QuerySecuritySchema.php
+++ b/tests/Validator/QuerySecuritySchema.php
@@ -59,24 +59,24 @@ class QuerySecuritySchema
         self::$humanType = new ObjectType(
             [
                 'name' => 'Human',
-                'fields' => [
-                    'firstName' => ['type' => Type::nonNull(Type::string())],
-                    'dogs' => [
-                        'type' => function () {
-                            return Type::nonNull(
+                'fields' => function() {
+                    return [
+                        'firstName' => ['type' => Type::nonNull(Type::string())],
+                        'dogs' => [
+                            'type' => Type::nonNull(
                                 Type::listOf(
                                     Type::nonNull(self::buildDogType())
                                 )
-                            );
-                        },
-                        'complexity' => function ($childrenComplexity, $args) {
-                            $complexity = isset($args['name']) ? 1 : 10;
+                            ),
+                            'complexity' => function ($childrenComplexity, $args) {
+                                $complexity = isset($args['name']) ? 1 : 10;
 
-                            return $childrenComplexity + $complexity;
-                        },
-                        'args' => ['name' => ['type' => Type::string()]],
-                    ],
-                ],
+                                return $childrenComplexity + $complexity;
+                            },
+                            'args' => ['name' => ['type' => Type::string()]],
+                        ],
+                    ];
+                },
             ]
         );
 
diff --git a/tests/Validator/TestCase.php b/tests/Validator/TestCase.php
index f8d2efc..c58d51f 100644
--- a/tests/Validator/TestCase.php
+++ b/tests/Validator/TestCase.php
@@ -94,16 +94,18 @@ abstract class TestCase extends \PHPUnit_Framework_TestCase
         $Cat = new ObjectType([
             'name' => 'Cat',
             'isTypeOf' => function() {return true;},
-            'fields' => [
-                'name' => [
-                    'type' => Type::string(),
-                    'args' => [ 'surname' => [ 'type' => Type::boolean() ] ]
-                ],
-                'nickname' => ['type' => Type::string()],
-                'meows' => ['type' => Type::boolean()],
-                'meowVolume' => ['type' => Type::int()],
-                'furColor' => ['type' => function() use (&$FurColor) {return $FurColor;}]
-            ],
+            'fields' => function() use (&$FurColor) {
+                return [
+                    'name' => [
+                        'type' => Type::string(),
+                        'args' => [ 'surname' => [ 'type' => Type::boolean() ] ]
+                    ],
+                    'nickname' => ['type' => Type::string()],
+                    'meows' => ['type' => Type::boolean()],
+                    'meowVolume' => ['type' => Type::int()],
+                    'furColor' => $FurColor
+                ];
+            },
             'interfaces' => [$Being, $Pet]
         ]);
 
@@ -128,15 +130,17 @@ abstract class TestCase extends \PHPUnit_Framework_TestCase
             'name' => 'Human',
             'isTypeOf' => function() {return true;},
             'interfaces' => [$Being, $Intelligent],
-            'fields' => [
-                'name' => [
-                    'type' => Type::string(),
-                    'args' => ['surname' => ['type' => Type::boolean()]]
-                ],
-                'pets' => ['type' => Type::listOf($Pet)],
-                'relatives' => ['type' => function() use (&$Human) {return Type::listOf($Human); }],
-                'iq' => ['type' => Type::int()]
-            ]
+            'fields' => function() use (&$Human, $Pet) {
+                return [
+                    'name' => [
+                        'type' => Type::string(),
+                        'args' => ['surname' => ['type' => Type::boolean()]]
+                    ],
+                    'pets' => ['type' => Type::listOf($Pet)],
+                    'relatives' => ['type' => Type::listOf($Human)],
+                    'iq' => ['type' => Type::int()]
+                ];
+            }
         ]);
 
         $Alien = new ObjectType([