Read Sass from stdin (#137)

See #105

lib/src/executable.dart

@@ -16,6 +16,7 @@ import 'util/path.dart';
 main(List<String> args) async {
   var argParser = new ArgParser(allowTrailingOptions: true)
     ..addOption('precision', hide: true)
+    ..addFlag('stdin', help: 'Read the stylesheet from stdin.')
     ..addOption('style',
         abbr: 's',
         help: 'Output style.',
@@ -45,7 +46,8 @@ main(List<String> args) async {
     return;
   }
 
-  if (options['help'] as bool || options.rest.isEmpty) {
+  var stdinFlag = options['stdin'] as bool;
+  if (options['help'] as bool || (options.rest.isEmpty && !stdinFlag)) {
     _printUsage(argParser, "Compile Sass to CSS.");
     exitCode = 64;
     return;
@@ -54,7 +56,16 @@ main(List<String> args) async {
   var color =
       options.wasParsed('color') ? options['color'] as bool : hasTerminal;
   try {
-    var css = compile(options.rest.first, color: color);
+    String css;
+    if (stdinFlag) {
+      css = compileString(await readStdin(), color: color);
+    } else {
+      var input = options.rest.first;
+      css = input == '-'
+          ? compileString(await readStdin(), color: color)
+          : compile(input, color: color);
+    }
+
     if (css.isNotEmpty) print(css);
   } on SassException catch (error, stackTrace) {
     stderr.writeln(error.toString(color: color));

lib/src/io/interface.dart

@@ -2,6 +2,8 @@
 // MIT-style license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
+import 'dart:async';
+
 /// An output sink that writes to this process's standard error.
 class Stderr {
   /// Writes the string representation of [object] to standard error.
@@ -41,6 +43,10 @@ String get currentPath => null;
 /// the file isn't valid UTF-8.
 String readFile(String path) => null;
 
+/// Reads from the standard input for the current process until it closes,
+/// returning the contents.
+Future<String> readStdin() async => null;
+
 /// Returns whether a file at [path] exists.
 bool fileExists(String path) => null;
 

lib/src/io/node.dart

@@ -2,6 +2,9 @@
 // MIT-style license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
+import 'dart:async';
+import 'dart:convert';
+
 import 'package:js/js.dart';
 import 'package:source_span/source_span.dart';
 
@@ -20,6 +23,13 @@ class _Stderr {
   external void write(String text);
 }
 
+@JS()
+class _Stdin {
+  external String read();
+
+  external void on(String event, void callback([object]));
+}
+
 @JS()
 class _SystemError {
   external String get message;
@@ -92,6 +102,33 @@ _readFile(String path, [String encoding]) {
   }
 }
 
+Future<String> readStdin() async {
+  var completer = new Completer<String>();
+  String contents;
+  var innerSink = new StringConversionSink.withCallback((String result) {
+    contents = result;
+    completer.complete(contents);
+  });
+  // Node defaults all buffers to 'utf8'.
+  var sink = UTF8.decoder.startChunkedConversion(innerSink);
+  _stdin.on('data', allowInterop(([chunk]) {
+    assert(chunk != null);
+    sink.add(chunk as List<int>);
+  }));
+  _stdin.on('end', allowInterop(([_]) {
+    // Callback for 'end' receives no args.
+    assert(_ == null);
+    sink.close();
+  }));
+  _stdin.on('error', allowInterop(([e]) {
+    assert(e != null);
+    stderr.writeln('Failed to read from stdin');
+    stderr.writeln(e);
+    completer.completeError(e);
+  }));
+  return completer.future;
+}
+
 /// Cleans up a Node system error's message.
 String _cleanErrorMessage(_SystemError error) {
   // The error message is of the form "$code: $text, $syscall '$path'". We just
@@ -109,6 +146,9 @@ external _Stderr get _stderr;
 
 final stderr = new Stderr(_stderr);
 
+@JS("process.stdin")
+external _Stdin get _stdin;
+
 bool get hasTerminal => _hasTerminal ?? false;
 
 bool get isWindows => _process.platform == 'win32';

lib/src/io/vm.dart

@@ -2,13 +2,14 @@
 // MIT-style license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
+import 'dart:async';
 import 'dart:convert';
 import 'dart:io' as io;
 
+import 'package:path/path.dart' as p;
 import 'package:source_span/source_span.dart';
 
 import '../exception.dart';
-import '../util/path.dart';
 
 export 'dart:io' show exitCode, FileSystemException;
 
@@ -43,6 +44,12 @@ String readFile(String path) {
   }
 }
 
+Future<String> readStdin() async {
+  var completer = new Completer<String>();
+  completer.complete(await io.SYSTEM_ENCODING.decodeStream(io.stdin));
+  return completer.future;
+}
+
 bool fileExists(String path) => new io.File(path).existsSync();
 
 bool dirExists(String path) => new io.Directory(path).existsSync();

pubspec.yaml

@@ -14,6 +14,7 @@ dependencies:
   args: "^0.13.0"
   charcode: "^1.1.0"
   collection: "^1.8.0"
+  convert: "^2.0.1"
   path: "^1.0.0"
   source_span: "^1.4.0"
   string_scanner: ">=0.1.5 <2.0.0"

test/cli_shared.dart

@@ -34,6 +34,49 @@ void sharedTests(Future<TestProcess> runSass(Iterable<String> arguments)) {
     await sass.shouldExit(0);
   });
 
+  test("compiles from stdin with the magic path -", () async {
+    var sass = await runSass(["-"]);
+    sass.stdin.writeln("a {b: 1 + 2}");
+    sass.stdin.close();
+    expect(
+        sass.stdout,
+        emitsInOrder([
+          "a {",
+          "  b: 3;",
+          "}",
+        ]));
+    await sass.shouldExit(0);
+  });
+
+  test("compiles from stdin with --stdin", () async {
+    var sass = await runSass(["--stdin"]);
+    sass.stdin.writeln("a {b: 1 + 2}");
+    sass.stdin.close();
+    expect(
+        sass.stdout,
+        emitsInOrder([
+          "a {",
+          "  b: 3;",
+          "}",
+        ]));
+    await sass.shouldExit(0);
+  });
+
+  test("gracefully reports errors from stdin", () async {
+    var sass = await runSass(["-"]);
+    sass.stdin.writeln("a {b: 1 + }");
+    sass.stdin.close();
+    expect(
+        sass.stderr,
+        emitsInOrder([
+          "Error: Expected expression.",
+          "a {b: 1 + }",
+          "          ^",
+          "  - 1:11  root stylesheet",
+        ]));
+    await sass.shouldExit(65);
+  });
+
   test("supports relative imports", () async {
     await d.file("test.scss", "@import 'dir/test'").create();