Prototype of validate step

src/execution/execute.js

@@ -55,6 +55,7 @@ import type {
   InlineFragmentNode,
   FragmentDefinitionNode,
 } from '../language/ast';
+import { assertValidSchema } from '../validation/validateSchema';
 
 
 /**
@@ -179,6 +180,8 @@ function executeImpl(
   operationName,
   fieldResolver
 ) {
+  assertValidSchema(schema);
+
   // If arguments are missing or incorrect, throw an error.
   assertValidExecutionArguments(
     schema,

src/type/schema.js

@@ -64,6 +64,7 @@ export class GraphQLSchema {
   _typeMap: TypeMap;
   _implementations: ObjMap<Array<GraphQLObjectType>>;
   _possibleTypeMap: ?ObjMap<ObjMap<boolean>>;
+  __valid: ?boolean;
 
   constructor(config: GraphQLSchemaConfig): void {
     invariant(

src/validation/validateSchema.js

@@ -0,0 +1,42 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
+
+import invariant from '../jsutils/invariant';
+
+import type { GraphQLSchema } from '../type/schema';
+import type { GraphQLError } from '../error';
+
+/**
+ * Validation runs synchronously, returning an array of encountered errors, or
+ * an empty array if no errors were encountered and the document is valid.
+ */
+export function validateSchema(
+  schema: GraphQLSchema,
+): Array<GraphQLError> {
+  if (schema.__valid === true) {
+    return [];
+  }
+  const errors = [];
+
+  // TODO actually validate the schema
+
+  if (errors.length === 0) {
+    schema.__valid = true;
+  }
+  return errors;
+}
+
+export function assertValidSchema(
+  schema: GraphQLSchema,
+): void {
+  console.error(
+    'The schema has to be validated by calling `validateSchema()` before ' +
+      'usage.',
+  );
+}