fixes #221, initial sync*, async, async* implementation

lib/runtime/_generators.js

Index: lib/runtime/_generators.js
diff --git a/lib/runtime/_generators.js b/lib/runtime/_generators.js
new file mode 100644
index 0000000000000000000000000000000000000000..5dd6e2f023e4c826a9ff9c2de5fea07f15caf7b8
--- /dev/null
+++ b/lib/runtime/_generators.js
@@ -0,0 +1,234 @@
+// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+/**
+ * This library adapts ES6 generators to implement Dart's async/await.
+ *
+ * It's designed to interact with Dart's Future/Stream and follow Dart
+ * async/await semantics.
+ *
+ * See https://github.com/dart-lang/dev_compiler/issues/245 for ideas on
+ * reconciling Dart's Future and ES6 Promise.
+ *
+ * Inspired by `co`: https://github.com/tj/co/blob/master/index.js, which is a
+ * stepping stone for proposed ES7 async/await, and uses ES6 Promises.
+ */
+dart_library.library('dart_runtime/_generators', null, /* Imports */[
+], /* Lazy Imports */[
+  'dart_runtime/_operations',
+  'dart/_js_helper',
+  'dart/core',
+  'dart/collection',
+  'dart/async'
+], function(exports, _operations, _js_helper, core, collection, async) {
+  'use strict';
+
+  const _jsIterator = Symbol('_jsIterator');
+  const _current = Symbol('_current');
+
+  function syncStar(gen, E, ...args) {
+    const SyncIterable_E = _js_helper.SyncIterable$(E);
+    return new SyncIterable_E(gen, args);
+  }
+  exports.syncStar = syncStar;
+
+  function async_(gen, T, ...args) {
+    let iter;
+    function onValue(res) {
+      if (res === void 0) res = null;
+      return next(iter.next(res));
+    }
+    function onError(err) {
+      // If the awaited Future throws, we want to convert this to an exception
+      // thrown from the `yield` point, as if it was thrown there.
+      //
+      // If the exception is not caught inside `gen`, it will emerge here, which
+      // will send it to anyone listening on this async function's Future<T>.
+      //
+      // In essence, we are giving the code inside the generator a chance to
+      // use try-catch-finally.
+      return next(iter.throw(err));
+    }
+    function next(ret) {
+      if (ret.done) return ret.value;
+      // Checks if the awaited value is a Future.
+      let future = ret.value;
+      if (!_operations.instanceOf(future, async.Future$)) {
+        future = async.Future.value(future);
+      }
+      // Chain the Future so `await` receives the Future's value.
+      return future.then(onValue, {onError: onError});
+    }
+    return async.Future$(T).new(function() {
+      iter = gen(...args)[Symbol.iterator]();
+      return onValue();
+    });
+  }
+  exports.async = async_;
+
+  // Implementation inspired by _AsyncStarStreamController in
+  // dart-lang/sdk's runtime/lib/core_patch.dart
+  //
+  // Given input like:
+  //
+  //     foo() async* {
+  //       yield 1;
+  //       yield* bar();
+  //       print(await baz());
+  //     }
+  //
+  // This generates as:
+  //
+  //    function foo() {
+  //      return dart.asyncStar(function*(stream) {
+  //        if (stream.add(1)) return;
+  //        yield;
+  //        if (stream.addStream(bar()) return;
+  //        yield;
+  //        print(yield baz());
+  //      });
+  //    }
+  class _AsyncStarStreamController {
+    constructor(generator, T, args) {
+      this.isAdding = false;
+      this.isWaiting = false;
+      this.isScheduled = false;
+      this.isSuspendedAtYield = false;
+      this.canceler = null;
+      this.iterator = generator(this, ...args)[Symbol.iterator]();
+      this.controller = async.StreamController$(T).new({
+        onListen: () => this.scheduleGenerator(),
+        onResume: () => this.onResume(),
+        onCancel: () => this.onCancel()
+      });
+    }
+
+    onResume() {
+      if (this.isSuspendedAtYield) {
+        this.scheduleGenerator();
+      }
+    }
+
+    onCancel() {
+      if (this.controller.isClosed) {
+        return null;
+      }
+      if (this.canceler == null) {
+        this.canceler = async.Completer.new();
+        this.scheduleGenerator();
+      }
+      return this.canceler.future;
+    }
+
+    close() {
+      if (this.canceler != null && !this.canceler.isCompleted) {
+        // If the stream has been cancelled, complete the cancellation future
+        // with the error.
+        this.canceler.complete();
+      }
+      this.controller.close();
+    }
+
+    scheduleGenerator() {
+      // TODO(jmesserly): is this paused check in the right place? Assuming the
+      // async* Stream yields, then is paused (by other code), the body will
+      // already be scheduled. This will cause at least one more iteration to
+      // run (adding another data item to the Stream) before actually pausing.
+      // It could be fixed by moving the `isPaused` check inside `runBody`.
+      if (this.isScheduled || this.controller.isPaused ||
+          this.isAdding || this.isWaiting) {
+        return;
+      }
+      this.isScheduled = true;
+      async.scheduleMicrotask(() => this.runBody());
+    }
+
+    runBody(opt_awaitValue) {
+      this.isScheduled = false;
+      this.isSuspendedAtYield = false;
+      this.isWaiting = false;
+      let iter;
+      try {
+        iter = this.iterator.next(opt_awaitValue);
+      } catch (e) {
+        this.addError(e, _operations.stackTrace(e));
+        this.close();
+        return;
+      }
+      if (iter.done) {
+        this.close();
+        return;
+      }
+
+      // If we're suspended at a yield/yield*, we're done for now.
+      if (this.isSuspendedAtYield || this.isAdding) return;
+
+      // Handle `await`: if we get a value passed to `yield` it means we are
+      // waiting on this Future. Make sure to prevent scheduling, and pass the
+      // value back as the result of the `yield`.
+      //
+      // TODO(jmesserly): is the timing here correct? The assumption here is
+      // that we should schedule `await` in `async*` the same as in `async`.
+      this.isWaiting = true;
+      let future = iter.value;
+      if (!_operations.instanceOf(future, async.Future$)) {
+        future = async.Future.value(future);
+      }
+      return future.then((x) => this.runBody(x),
+          { onError: (e, s) => this.throwError(e, s) });
+    }
+
+    // Adds element to stream, returns true if the caller should terminate
+    // execution of the generator.
+    add(event) {
+      // If stream is cancelled, tell caller to exit the async generator.
+      if (!this.controller.hasListener) return true;
+      this.controller.add(event);
+      this.scheduleGenerator();
+      this.isSuspendedAtYield = true;
+      return false;
+    }
+
+    // Adds the elements of stream into this controller's stream.
+    // The generator will be scheduled again when all of the
+    // elements of the added stream have been consumed.
+    // Returns true if the caller should terminate
+    // execution of the generator.
+    addStream(stream) {
+      // If stream is cancelled, tell caller to exit the async generator.
+      if (!this.controller.hasListener) return true;
+
+      this.isAdding = true;
+      this.controller.addStream(stream, {cancelOnError: false}).then(() => {
+        this.isAdding = false;
+        this.scheduleGenerator();
+      }, { onError: (e, s) => this.throwError(e, s) });
+    }
+
+    throwError(error, stackTrace) {
+      try {
+        this.iterator.throw(error);
+      } catch (e) {
+        this.addError(e, stackTrace);
+      }
+    }
+
+    addError(error, stackTrace) {
+      if ((this.canceler != null) && !this.canceler.isCompleted) {
+        // If the stream has been cancelled, complete the cancellation future
+        // with the error.
+        this.canceler.completeError(error, stackTrace);
+        return;
+      }
+      if (!this.controller.hasListener) return;
+      this.controller.addError(error, stackTrace);
+    }
+  }
+
+  /** Returns a Stream of T implemented by an async* function. */
+  function asyncStar(gen, T, ...args) {
+    return new _AsyncStarStreamController(gen, T, args).controller.stream;
+  }
+  exports.asyncStar = asyncStar;
+});