Support more forms of async returned values.

GraphQL.js already had support for `Promise<T>` and `Promise<Array<T>>`, but not `Array<Promise<T>>`. We had assumed that resolve functions would call Promise.all when necessary based on how GraphQL had been used in the context of PHP/Hack at Facebook where `Array<Promise<T>>` is actually pretty hard to construct. In JavaScript, however, it's easy to forget a call to Promise.all and provide this kind of value.

This change makes GraphQL.js aware of `Array<Promise<T>>` and importantly enables a new expression of error handling for nullable list item types (`[T]` vs `[T!]`): If the second of an array of three values fails, we can log an error accordingly and place a null value for that index.

The primary structural change here is moving the error detection logic to after calling a field's `resolve` rather than before it. This means direct field `resolve` errors have to be dealt with directly, which is a little less elegant, but it means this error detection logic can be easily reused when completing the values of items in a list.

Finally, this adds a more comprehensive test suite for list types in permuation with nullability and Array and Promise return types.

Fixes #17

src/__tests__/starWarsData.js

@@ -88,13 +88,16 @@ var droidData = {
  * Helper function to get a character by ID.
  */
 function getCharacter(id) {
-  if (humanData[id] !== undefined) {
-    return humanData[id];
-  }
-  if (droidData[id] !== undefined) {
-    return droidData[id];
-  }
-  return null;
+  // Returning a promise just to illustrate GraphQL.js's support.
+  return new Promise(resolve => {
+    if (humanData[id] !== undefined) {
+      return resolve(humanData[id]);
+    }
+    if (droidData[id] !== undefined) {
+      return resolve(droidData[id]);
+    }
+    return resolve(null);
+  });
 }
 
 /**
@@ -108,4 +111,3 @@ export var starWarsData = {
   Humans: humanData,
   Droids: droidData,
 };
-

src/error/index.js

@@ -13,7 +13,7 @@ import { getLocation } from '../language';
 import type { Node } from '../language/ast';
 
 
-export class GraphQLError {
+export class GraphQLError extends Error {
   message: string;
   stack: string;
   nodes: ?Array<Node>;
@@ -27,6 +27,7 @@ export class GraphQLError {
     nodes?: Array<any/*Node*/>,
     stack?: any
   ) {
+    super(message);
     this.message = message;
     this.stack = stack || message;
     if (nodes) {
@@ -45,7 +46,16 @@ export class GraphQLError {
   }
 }
 
-(GraphQLError: any).prototype = Error.prototype;
+export function locatedError(error: any, nodes: Array<any>): GraphQLError {
+  if (error instanceof GraphQLError) {
+    return error;
+  }
+  return new GraphQLError(
+    error && error.message,
+    nodes,
+    error ? error.stack : null
+  );
+}
 
 export type GraphQLFormattedError = {
   message: string,