Add reactive-inspired stream transformers:

test/scan_test.dart

+// Copyright (c) 2016, 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.
+
+// Test stream transformers.
+import "dart:async";
+
+import "package:async/async.dart";
+import "package:test/test.dart";
+
+void main() {
+  test("addition", () {
+    var intStream = new Stream<int>.fromIterable([2, 3, 4, 5, 6]);
+    var scanned = intStream.transform(new Scan<int, int>(0, (a, v) => a + v));
+    expect(scanned.toList(), completion([2, 5, 9, 14, 20]));
+  });
+
+  test("subtraction", () {

[nweiz, 2016/03/01 02:10:04]

Given this, I think testing addition is probably redundant.

[Lasse Reichstein Nielsen, 2016/03/01 16:51:17]

Acknowledged.

+    // Will not work correctly if parameters are swapped.
+    var intStream = new Stream<int>.fromIterable([2, 3, 4, 5, 6]);
+    var scanned = intStream.transform(new Scan<int, int>(20, (a, v) => a - v));
+    expect(scanned.toList(), completion([18, 15, 11, 6, 0]));
+  });
+
+  test("no events", () {
+    var emptyStream = new Stream<int>.empty();
+    var scanned = emptyStream.transform(new Scan<int, int>(
+        42, (a, v) => throw "unreachable"));
+    expect(scanned.toList(), completion(isEmpty));
+  });
+
+  test("types", () {
+    var intStream = new Stream<int>.fromIterable([2, 3, 4, 5, 6]);
+    var scan = new Scan<int, String>("", (String a, int v) => "$a$v");
+    var scanned = intStream.transform(scan);
+    expect(scanned is Stream<String>, isTrue);
+    expect(scanned.toList(), completion(["2", "23", "234", "2345", "23456"]));
+  });
+
+  test("types 2", () {
+    var intStream = new Stream<int>.fromIterable([2, 3, 4, 5, 6]);
+    var scan = new Scan<int, String>("", (String a, int v) => "$a$v");
+    var scanned = scan.bind(intStream);
+    expect(scanned is Stream<String>, isTrue);
+    expect(scanned.toList(), completion(["2", "23", "234", "2345", "23456"]));
+  });
+
+  test("types 3", () {

[nweiz, 2016/03/01 02:10:04]

Explain what these tests are actually asserting.

[Lasse Reichstein Nielsen, 2016/03/01 16:51:17]

Acknowledged.

+    var stream = new Stream<String>.fromIterable(["2", "3", "4", "5", "6"]);
+    var scan = new Scan<String, String>("", (String a, String v) => "$a$v");
+    var scanned = stream.transform(scan);
+    expect(scanned is Stream<String>, isTrue);
+    expect(scanned.toList(), completion(["2", "23", "234", "2345", "23456"]));
+  });
+
+  test("error", () async {
+    errorStream(int start, int end, int errorAfter) async* {
+      for (int i = start; i < end; i++) {
+        yield i;
+        if (i == errorAfter) {
+          yield* () async* { throw "BAD"; }();  // Stream with error.

[nweiz, 2016/03/01 02:10:04]

I think it's cleaner to yield Future.error.

[Lasse Reichstein Nielsen, 2016/03/01 16:51:17]

Probably true. I got tired of writing "new Future.error(...).asStream()", but it
is more readable.

I plan to add a "new Stream.error(e, s)" constructor at some point.

+        }
+      }
+    }
+    var scanned = errorStream(0, 5, 3)
+        .transform(new Scan<int, String>("", (a, b) => "$a$b"));
+    var list = [];
+    var result = scanned.forEach(list.add);
+    try {
+      await result;
+    } catch (e) {
+      expect(e, "BAD");
+    }
+    expect(list, ["0", "01", "012", "0123"]);
+  });
+}