Add args support.

It doesn't really do anything yet.

bin/sass.dart

@@ -4,14 +4,35 @@
 
 import 'dart:io';
 
+import 'package:args/args.dart';
 import 'package:path/path.dart' as p;
 import 'package:sass/src/parser.dart';
 import 'package:sass/src/visitor/perform.dart';
 import 'package:sass/src/visitor/serialize.dart';
 
 void main(List<String> args) {
-  var parser = new Parser(new File(args.first).readAsStringSync(),
-      url: p.toUri(args.first));
+  var argParser = new ArgParser()
+    ..addOption('precision', hide: true)
+    ..addOption('style',
+        abbr: 's',
+        help: 'Output style.',
+        allowed: ['expanded'],
+        defaultsTo: 'expanded')
+    ..addFlag('help',
+        abbr: 'h', help: 'Print this usage information.', negatable: false);
+  var options = argParser.parse(args);
+
+  if (options['help']) {
+    print("Compile Sass to CSS.\n");
+    print("Usage: sass <input>\n");
+    print(argParser.usage);
+    exit(64);
+  }
+
+  var file = options.rest.first;
+  var parser =
+      new Parser(new File(file).readAsStringSync(), url: p.toUri(file));
   var cssTree = new PerformVisitor().visitStylesheet(parser.parse());
-  print(toCss(cssTree));
+  var css = toCss(cssTree);
+  if (css.isNotEmpty) print(css);
 }

lib/src/visitor/serialize.dart

@@ -15,8 +15,8 @@ import 'interface/css.dart';
 import 'interface/selector.dart';
 import 'interface/value.dart';
 
-String toCss(CssNode node) {
-  var visitor = new _SerializeCssVisitor();
+String toCss(CssNode node, {OutputStyle style}) {
+  var visitor = new _SerializeCssVisitor(style: style);
   node.accept(visitor);
   var result = visitor._buffer.toString();
   if (result.codeUnits.any((codeUnit) => codeUnit > 0x7F)) {
@@ -42,10 +42,15 @@ String selectorToCss(Selector selector) {
 
 class _SerializeCssVisitor
     implements CssVisitor, ValueVisitor, SelectorVisitor {
+  final OutputStyle _style;
+
   final _buffer = new StringBuffer();
 
   var _indentation = 0;
 
+  _SerializeCssVisitor({OutputStyle style})
+      : _style = style ?? OutputStyle.expanded;
+
   void visitStylesheet(CssStylesheet node) {
     for (var child in node.children) {
       child.accept(this);
@@ -408,3 +413,14 @@ class _SerializeCssVisitor
     _indentation--;
   }
 }
+
+class OutputStyle {
+  static const expanded = const OutputStyle._("expanded");
+  static const nested = const OutputStyle._("nested");
+
+  final String _name;
+
+  const OutputStyle._(this._name);
+
+  String toString() => _name;
+}

pubspec.yaml

@@ -8,6 +8,7 @@ environment:
   sdk: '>=1.8.0 <2.0.0'
 
 dependencies:
+  args: "^0.13.0"
   charcode: "^1.1.0"
   collection: "^1.1.0"
   path: "^1.0.0"