Roll Observatory packages and add a roll script

packages/async/lib/result.dart

+// 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.
+///
+/// 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.
+  factory 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 value 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 returned 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());
+  }
+
+  /// Converts a result of a result to a single result.
+  ///
+  /// If the result is an error, or it is a `Result` value
+  /// which is then an error, then a result with that error is returned.
+  /// Otherwise both levels of results are value results, and a single
+  /// result with the value is returned.
+  static Result flatten(Result<Result> result) {
+    if (result.isError) return result;
+    return result.asValue.value;
+  }
+
+  /// 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 an [EventSink].
+  ///
+  /// Calls the sink's `add` or `addError` method as appropriate.
+  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> {
+  final T 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 {
+  final error;
+  final StackTrace 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 completer) {
+    completer.completeError(error, stackTrace);
+  }
+  void addTo(EventSink sink) {
+    sink.addError(error, stackTrace);
+  }
+  Future get asFuture => new Future.error(error, stackTrace);
+
+  /// Calls an error handler with the error and stacktrace.
+  ///
+  /// An async error handler function is either a function expecting two
+  /// arguments, which will be called with the error and the stack trace,
+  /// or it has to be a function expecting only one argument,
+  /// which will be called with only the error.
+  void handle(Function errorHandler) {
+    if (errorHandler is ZoneBinaryCallback) {
+      errorHandler(error, stackTrace);
+    } else {
+      errorHandler(error);
+    }
+  }
+}
+
+/// 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<Result> _createSink(EventSink 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(); }
+}