Add capture/release for results in package:async

pkg/async/lib/result.dart

Index: pkg/async/lib/result.dart
diff --git a/pkg/async/lib/result.dart b/pkg/async/lib/result.dart
new file mode 100644
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+++ b/pkg/async/lib/result.dart
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+// Copyright (c) 2013, 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.
+
+/**
+ * Capture asynchronous results into synchronous values, and release them again.
+ *
+ * Capturing a result (either a returned value or a thrown error)
+ * means converting it into a [Result] -
+ * either a [ValueResult] or an [ErrorResult].
+ *
+ * This value can release itself by writing itself either to a
+ * [EventSink] or a [Completer], or by becoming a [Future].
+ */
+library dart.pkg.async.results;
+
+import "dart:async";
+
+/**
+ * The result of a computation.
+ */
+abstract class Result<T> {
+  /**
+   * Create a `Result` with the result of calling [computation].
+   *
+   * This generates either a [ValueResult] with the value returned by
+   * calling `computation`, or an [ErrorResult] with an error thrown by
+   * the call.
+   */
+  Result(T computation()) {
+    try {
+      return new ValueResult(computation());
+    } catch (e, s) {
+      return new ErrorResult(e, s);
+    }
+  }
+
+  /**
+   * Create a `Result` holding a value.
+   *
+   * Alias for [ValueResult.ValueResult].
+   */
+  factory Result.value(T value) = ValueResult<T>;
+
+  /**
+   * Create a `Result` holding an error.
+   *
+   * Alias for [ErrorResult.ErrorResult].
+   */
+  factory Result.error(Object error, [StackTrace stackTrace]) =>
+      new ErrorResult(error, stackTrace);
+
+  // Helper functions.
+  static _captureValue(value) => new ValueResult(value);
+  static _captureError(error, stack) => new ErrorResult(error, stack);
+  static _release(Result v) {
+    if (v.isValue) return v.asValue.value;  // Avoid wrapping in future.
+    return v.asFuture;
+  }
+
+  /**
+   * Capture the result of a future into a `Result` future.
+   *
+   * The resulting future will never have an error.
+   * Errors have been converted to an [ErrorResult] value.
+   */
+  static Future<Result> capture(Future future) {
+    return future.then(_captureValue, onError: _captureError);
+  }
+
+  /**
+   * Release the result of a captured future.
+   *
+   * Converts the [Result] value of the given [future] to a result or error
+   * completion of the returned future.
+   *
+   * If [future] completes with an error, the returned future completes with
+   * the same error.
+   */
+  static Future release(Future<Result> future) {
+    return future.then(_release);
+  }
+
+  /**
+   * Capture the results of a stream into a stream of [Result] values.
+   *
+   * The returned stream will not have any error events.
+   * Errors from the source stream have been converted to [ErrorResult]s.
+   *
+   * Shorthand for transforming the stream using [CaptureStreamTransformer].
+   */
+  static Stream<Result> captureStream(Stream source) {
+    return source.transform(const CaptureStreamTransformer());
+  }
+
+  /**
+   * Release a stream of [result] values into a stream of the results.
+   *
+   * `Result` values of the source stream become value or error events in
+   * the retuned stream as appropriate.
+   * Errors from the source stream become errors in the returned stream.
+   *
+   * Shorthand for transforming the stream using [ReleaseStreamTransformer].
+   */
+  static Stream releaseStream(Stream<Result> source) {
+    return source.transform(const ReleaseStreamTransformer());
+  }
+
+  /**
+   * Whether this result is a value result.
+   *
+   * Always the opposite of [isError].
+   */
+  bool get isValue;
+
+  /**
+   * Whether this result is an error result.
+   *
+   * Always the opposite of [isValue].
+   */
+  bool get isError;
+
+  /**
+   * If this is a value result, return itself.
+   *
+   * Otherwise return `null`.
+   */
+  ValueResult<T> get asValue;
+
+  /**
+   * If this is an error result, return itself.
+   *
+   * Otherwise return `null`.
+   */
+  ErrorResult get asError;
+
+  /**
+   * Complete a completer with this result.
+   */
+  void complete(Completer<T> completer);
+
+  /**
+   * Add this result to a [StreamSink].
+   */
+  void addTo(EventSink<T> sink);
+
+  /**
+   * Creates a future completed with this result as a value or an error.
+   */
+  Future<T> get asFuture;
+}
+
+/**
+ * A result representing a returned value.
+ */
+class ValueResult<T> implements Result<T> {
+  /** The returned value that this result represents. */
+  final T value;
+  /** Create a value result with the given [value]. */
+  ValueResult(this.value);
+  bool get isValue => true;
+  bool get isError => false;
+  ValueResult<T> get asValue => this;
+  ErrorResult get asError => null;
+  void complete(Completer<T> completer) {
+    completer.complete(value);
+  }
+  void addTo(EventSink<T> sink) {
+    sink.add(value);
+  }
+  Future<T> get asFuture => new Future.value(value);
+}
+
+/**
+ * A result representing a thrown error.
+ */
+class ErrorResult implements Result {
+  /** The thrown object that this result represents. */
+  final error;
+  /** The stack trace, if any, associated with the throw. */
+  final StackTrace stackTrace;
+  /** Create an error result with the given [error] and [stackTrace]. */
+  ErrorResult(this.error, this.stackTrace);
+  bool get isValue => false;
+  bool get isError => true;
+  ValueResult get asValue => null;
+  ErrorResult get asError => this;
+  void complete(Completer<T> completer) {
+    completer.completeError(error, stackTrace);
+  }
+  void addTo(EventSink<T> sink) {
+    sink.addError(error, stackTrace);
+  }
+  Future get asFuture => new Future.error(error, stackTrace);
+}
+
+/**
+ * A stream transformer that captures a stream of events into [Result]s.
+ *
+ * The result of the transformation is a stream of [Result] values and
+ * no error events.
+ */
+class CaptureStreamTransformer<T> implements StreamTransformer<T, Result<T>> {
+  const CaptureStreamTransformer();
+
+  Stream<Result<T>> bind(Stream<T> source) {
+    return new Stream<Result<T>>.eventTransformed(source, _createSink);
+  }
+
+  static EventSink _createSink(EventSink<Result> sink) {
+    return new CaptureSink(sink);
+  }
+}
+
+/**
+ * A stream transformer that releases a stream of result events.
+ *
+ * The result of the transformation is a stream of values and
+ * error events.
+ */
+class ReleaseStreamTransformer<T> implements StreamTransformer<Result<T>, T> {
+  const ReleaseStreamTransformer();
+
+  Stream<T> bind(Stream<Result<T>> source) {
+    return new Stream<T>.eventTransformed(source, _createSink);
+  }
+
+  static EventSink _createSink(EventSink<Result> sink) {
+    return new ReleaseSink(sink);
+  }
+}
+
+/**
+ * An event sink wrapper that captures the incoming events.
+ *
+ * Wraps an [EventSink] that expects [Result] values.
+ * Accepts any value and error result,
+ * and passes them to the wrapped sink as [Result] values.
+ *
+ * The wrapped sink will never receive an error event.
+ */
+class CaptureSink<T> implements EventSink<T> {
+  final EventSink _sink;
+
+  CaptureSink(EventSink<Result<T>> sink) : _sink = sink;
+  void add(T value) { _sink.add(new ValueResult(value)); }
+  void addError(Object error, StackTrace stackTrace) {
+    _sink.add(new ErrorResult(error, stackTrace));
+  }
+  void close() { _sink.close(); }
+}
+
+/**
+ * An event sink wrapper that releases the incoming result events.
+ *
+ * Wraps an output [EventSink] that expects any result.
+ * Accepts [Result] values, and puts the result value or error into the
+ * corresponding output sink add method.
+ */
+class ReleaseSink<T> implements EventSink<Result<T>> {
+  final EventSink _sink;
+  ReleaseSink(EventSink<T> sink) : _sink = sink;
+  void add(Result<T> result) {
+    if (result.isValue) {
+      _sink.add(result.asValue.value);
+    } else {
+      ErrorResult error = result.asError;
+      _sink.addError(error.error, error.stackTrace);
+    }
+  }
+  void addError(Object error, StackTrace stackTrace) {
+    // Errors may be added by intermediate processing, even if it is never
+    // added by CaptureSink.
+    _sink.addError(error, stackTrace);
+  }
+
+  void close() { _sink.close(); }
+}