Add @if.

lib/src/ast/sass.dart

@@ -28,6 +28,7 @@ export 'sass/statement/content.dart';
 export 'sass/statement/declaration.dart';
 export 'sass/statement/extend_rule.dart';
 export 'sass/statement/function_declaration.dart';
+export 'sass/statement/if.dart';
 export 'sass/statement/include.dart';
 export 'sass/statement/media_rule.dart';
 export 'sass/statement/mixin_declaration.dart';

lib/src/ast/sass/statement/if.dart

@@ -0,0 +1,25 @@
+// Copyright 2016 Google Inc. Use of this source code is governed by an
+// MIT-style license that can be found in the LICENSE file or at
+// https://opensource.org/licenses/MIT.
+
+import 'package:source_span/source_span.dart';
+
+import '../../../visitor/interface/statement.dart';
+import '../expression.dart';
+import '../statement.dart';
+
+class If implements Statement {
+  final Expression expression;
+
+  final List<Statement> children;
+
+  final FileSpan span;
+
+  If(this.expression, Iterable<Statement> children, {this.span})
+      : children = new List.unmodifiable(children);
+
+  /*=T*/ accept/*<T>*/(StatementVisitor/*<T>*/ visitor) =>
+      visitor.visitIf(this);
+
+  String toString() => "@if $expression {${children.join(" ")}}";
+}

lib/src/environment.dart

@@ -33,6 +33,8 @@ class Environment {
   Environment get contentEnvironment => _contentEnvironment;
   Environment _contentEnvironment;
 
+  var _inSemiGlobalScope = false;
+
   Environment()
       : _variables = [normalizedMap()],
         _variableIndices = normalizedMap(),
@@ -63,7 +65,8 @@ class Environment {
   void setVariable(String name, Value value, {bool global: false}) {
     var index = global || _variables.length == 1
         ? 0
-        : _variableIndices.putIfAbsent(name, () => _variables.length - 1);
+        : _variableIndices.putIfAbsent(name,
+              () => _inSemiGlobalScope ? 0 : _variables.length - 1);
     _variables[index][name] = value;
   }
 
@@ -122,14 +125,19 @@ class Environment {
     _contentEnvironment = oldEnvironment;
   }
 
-  /*=T*/ scope/*<T>*/(/*=T*/ callback()) {
+  /*=T*/ scope/*<T>*/(/*=T*/ callback(), {bool semiGlobal: false}) {
+    assert(!semiGlobal || _inSemiGlobalScope || _variables.length == 1);
+
     // TODO: avoid creating a new scope if no variables are declared.
+    var wasInSemiGlobalScope = _inSemiGlobalScope;
+    _inSemiGlobalScope = semiGlobal;
     _variables.add(normalizedMap());
     _functions.add(normalizedMap());
     _mixins.add(normalizedMap());
     try {
       return callback();
     } finally {
+      _inSemiGlobalScope = wasInSemiGlobalScope;
       for (var name in _variables.removeLast().keys) {
         _variableIndices.remove(name);
       }

lib/src/parser.dart

@@ -41,31 +41,14 @@ class Parser {
 
   Stylesheet parse() {
     var start = _scanner.state;
-    var children = <Statement>[];
-    while (true) {
-      children.addAll(_comments());
-      if (_scanner.isDone) break;
-      switch (_scanner.peekChar()) {
-        case $dollar:
-          children.add(_variableDeclaration());
-          break;
-
-        case $at:
-          children.add(_atRule());
-          break;
-
-        case $semicolon:
-          _scanner.readChar();
-          break;
-
-        default:
-          children.add(_styleRule());
-          break;
-      }
-    }
-
+    var statements = _statements(_topLevelStatement);
     _scanner.expectDone();
-    return new Stylesheet(children, span: _scanner.spanFrom(start));
+    return new Stylesheet(statements, span: _scanner.spanFrom(start));
+  }
+
+  Statement _topLevelStatement() {
+    if (_scanner.peekChar() == $at) return _atRule(_topLevelStatement);
+    return _styleRule();
   }
 
   SelectorList parseSelector() {
@@ -116,15 +99,15 @@ class Parser {
   StyleRule _styleRule() {
     var start = _scanner.state;
     var selector = _almostAnyValue();
-    var children = _ruleChildren();
+    var children = _children(_ruleChild);
     return new StyleRule(selector, children,
         span: _scanner.spanFrom(start));
   }
 
-  List<Statement> _ruleChildren() => _children(() {
-    if (_scanner.peekChar() == $at) return _atRule();
+  Statement _ruleChild() {
+    if (_scanner.peekChar() == $at) return _atRule(_ruleChild);
     return _declarationOrStyleRule();
-  });
+  }
 
   Expression _declarationExpression() {
     if (_scanner.peekChar() == $lbrace) {
@@ -169,7 +152,7 @@ class Parser {
     var buffer = declarationOrBuffer as InterpolationBuffer;
     buffer.addInterpolation(_almostAnyValue());
 
-    var children = _ruleChildren();
+    var children = _children(_ruleChild);
     return new StyleRule(
         buffer.interpolation(_scanner.spanFrom(start)), children,
         span: _scanner.spanFrom(start));
@@ -222,7 +205,7 @@ class Parser {
     var postColonWhitespace = _commentText();
     if (_scanner.peekChar() == $lbrace) {
       return new Declaration(name,
-          children: _declarationChildren(),
+          children: _children(_declarationChild),
           span: _scanner.spanFrom(start));
     }
 
@@ -262,7 +245,7 @@ class Parser {
         name,
         value: value,
         children: _scanner.peekChar() == $lbrace
-             ? _declarationChildren()
+             ? _children(_declarationChild)
              : null,
         span: _scanner.spanFrom(start));
   }
@@ -276,7 +259,7 @@ class Parser {
 
     if (_scanner.peekChar() == $lbrace) {
       return new Declaration(name,
-          children: _declarationChildren(),
+          children: _children(_declarationChild),
           span: _scanner.spanFrom(start));
     }
 
@@ -285,15 +268,15 @@ class Parser {
         name,
         value: value,
         children: _scanner.peekChar() == $lbrace
-             ? _declarationChildren()
+             ? _children(_declarationChild)
              : null,
         span: _scanner.spanFrom(start));
   }
 
-  List<Statement> _declarationChildren() => _children(() {
+  Statement _declarationChild() {
     if (_scanner.peekChar() == $at) return _declarationAtRule();
     return _declaration();
-  });
+  }
 
   /// Consumes whitespace if available and returns any comments it contained.
   List<Comment> _comments() {
@@ -310,7 +293,7 @@ class Parser {
 
   // # At Rules
 
-  Statement _atRule() {
+  Statement _atRule(Statement child()) {
     var start = _scanner.state;
     var name = _atRuleName();
 
@@ -318,6 +301,7 @@ class Parser {
       case "content": return _content(start);
       case "extend": return _extend(start);
       case "function": return _functionDeclaration(start);
+      case "if": return _if(start, child);
       case "include": return _include(start);
       case "media": return _mediaRule(start);
       case "mixin": return _mixinDeclaration(start);
@@ -331,6 +315,7 @@ class Parser {
 
     switch (name) {
       case "content": return _content(start);
+      case "if": return _if(start, _declarationChild);
       case "include": return _include(start);
       default: return _disallowedAtRule(start);
     }
@@ -339,6 +324,7 @@ class Parser {
   Statement _functionAtRule() {
     var start = _scanner.state;
     switch (_atRuleName()) {
+      case "if": return _if(start, _functionAtRule);
       case "return": return _return(start);
       default: return _disallowedAtRule(start);
     }
@@ -386,6 +372,9 @@ class Parser {
         span: _scanner.spanFrom(start));
   }
 
+  If _if(LineScannerState start, Statement child()) =>
+      new If(_expression(), _children(child), span: _scanner.spanFrom(start));
+
   Include _include(LineScannerState start) {
     var name = _identifier();
     _ignoreComments();
@@ -397,7 +386,7 @@ class Parser {
     List<Statement> children;
     if (_scanner.peekChar() == $lbrace) {
       _inContentBlock = true;
-      children = _ruleChildren();
+      children = _children(_ruleChild);
       _inContentBlock = false;
     }
 
@@ -406,7 +395,7 @@ class Parser {
   }
 
   MediaRule _mediaRule(LineScannerState start) =>
-      new MediaRule(_mediaQueryList(), _ruleChildren(),
+      new MediaRule(_mediaQueryList(), _children(_ruleChild),
           span: _scanner.spanFrom(start));
 
   MixinDeclaration _mixinDeclaration(LineScannerState start) {
@@ -425,7 +414,7 @@ class Parser {
     _ignoreComments();
     _inMixin = true;
     _mixinHasContent = false;
-    var children = _ruleChildren();
+    var children = _children(_ruleChild);
     _inMixin = false;
 
     return new MixinDeclaration(name, arguments, children,
@@ -448,7 +437,7 @@ class Parser {
 
     return new AtRule(name,
         value: value,
-        children: _scanner.peekChar() == $lbrace ? _ruleChildren() : null,
+        children: _scanner.peekChar() == $lbrace ? _children(_ruleChild) : null,
         span: _scanner.spanFrom(start));
   }
 
@@ -1668,28 +1657,50 @@ class Parser {
         character == $dollar || isNameStart(character) || isDigit(character);
   }
 
-  List<Statement> _children(Statement consumeChild()) {
+  List<Statement> _children(Statement child()) {
     _scanner.expectChar($lbrace);
     var children = <Statement>[];
-    loop: while (true) {
+    while (true) {
       children.addAll(_comments());
       switch (_scanner.peekChar()) {
         case $dollar:
           children.add(_variableDeclaration());
-          continue loop;
+          break;
 
         case $semicolon:
           _scanner.readChar();
-          continue loop;
+          break;
 
         case $rbrace:
-          break loop;
-      }
+          _scanner.expectChar($rbrace);
+          return children;
 
-      children.add(consumeChild());
+        default:
+          children.add(child());
+          break;
+      }
     }
-    _scanner.expectChar($rbrace);
-    return children;
+  }
+
+  List<Statement> _statements(Statement statement()) {
+    var statements = <Statement>[];
+    while (!_scanner.isDone) {
+      switch (_scanner.peekChar()) {
+        case $dollar:
+          statements.add(_variableDeclaration());
+          break;
+
+        case $semicolon:
+          _scanner.readChar();
+          break;
+
+        default:
+          statements.add(statement());
+          break;
+      }
+      statements.addAll(_comments());
+    }
+    return statements;
   }
 
   String _rawText(void consumer()) {

lib/src/value.dart

@@ -19,6 +19,8 @@ abstract class Value {
   /// Whether the value will be represented in CSS as the empty string.
   bool get isBlank => false;
 
+  bool get isTruthy => true;
+
   const Value();
 
   /*=T*/ accept/*<T>*/(ValueVisitor/*<T>*/ visitor);

lib/src/value/boolean.dart

@@ -11,6 +11,8 @@ const sassFalse = const SassBoolean._(false);
 class SassBoolean extends Value {
   final bool value;
 
+  bool get isTruthy => value;
+
   factory SassBoolean(bool value) => value ? sassTrue : sassFalse;
 
   const SassBoolean._(this.value);

lib/src/visitor/interface/statement.dart

@@ -34,6 +34,13 @@ abstract class StatementVisitor<T> {
     return null;
   }
 
+  T visitIf(If node) {
+    for (var child in node.children) {
+      child.accept(this);
+    }
+    return null;
+  }
+
   T visitInclude(Include node) {
     if (node.children == null) return null;
     for (var child in node.children) {

lib/src/visitor/perform.dart

@@ -148,6 +148,12 @@ class PerformVisitor extends StatementVisitor
         new UserDefinedCallable(node, _environment.closure()));
   }
 
+  void visitIf(If node) {
+    var condition = node.expression.accept(this);
+    if (!condition.isTruthy) return;
+    _environment.scope(() => super.visitIf(node), semiGlobal: true);
+  }
+
   void visitInclude(Include node) {
     var mixin = _environment.getMixin(node.name) as UserDefinedCallable;
     if (mixin == null) throw node.span.message("Undefined mixin.");