Add an explicit distinction between statements and expressions.

lib/src/ast/comment.dart

@@ -4,10 +4,10 @@
 
 import 'package:source_span/source_span.dart';
 
-import '../visitor.dart';
-import 'node.dart';
+import '../visitor/statement.dart';
+import 'statement.dart';
 
-class CommentNode implements AstNode {
+class CommentNode implements Statement {
   final String text;
 
   final bool isSilent;
@@ -18,7 +18,7 @@ class CommentNode implements AstNode {
   CommentNode(this.text, {bool silent, this.span})
       : isSilent = silent;
 
-  /*=T*/ visit/*<T>*/(AstVisitor/*<T>*/ visitor) =>
+  /*=T*/ visit/*<T>*/(StatementVisitor/*<T>*/ visitor) =>
       visitor.visitComment(this);
 
   String toString() => text;

lib/src/ast/declaration.dart

@@ -4,12 +4,12 @@
 
 import 'package:source_span/source_span.dart';
 
-import '../visitor.dart';
+import '../visitor/statement.dart';
 import 'expression.dart';
 import 'expression/interpolation.dart';
-import 'node.dart';
+import 'statement.dart';
 
-class DeclarationNode implements AstNode {
+class DeclarationNode implements Statement {
   final InterpolationExpression name;
 
   final Expression value;
@@ -18,7 +18,7 @@ class DeclarationNode implements AstNode {
 
   DeclarationNode(this.name, this.value, {this.span});
 
-  /*=T*/ visit/*<T>*/(AstVisitor/*<T>*/ visitor) =>
+  /*=T*/ visit/*<T>*/(StatementVisitor/*<T>*/ visitor) =>
       visitor.visitDeclaration(this);
 
   String toString() => "$name: $value;";

lib/src/ast/expression.dart

@@ -2,6 +2,9 @@
 // MIT-style license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
+import '../visitor/expression.dart';
 import 'node.dart';
 
-abstract class Expression implements AstNode {}
+abstract class Expression implements AstNode {
+  /*=T*/ visit/*<T>*/(ExpressionVisitor/*<T>*/ visitor);
+}

lib/src/ast/expression/identifier.dart

@@ -4,7 +4,7 @@
 
 import 'package:source_span/source_span.dart';
 
-import '../../visitor.dart';
+import '../../visitor/expression.dart';
 import '../expression.dart';
 import 'interpolation.dart';
 
@@ -15,7 +15,7 @@ class IdentifierExpression implements Expression {
 
   IdentifierExpression(this.text);
 
-  /*=T*/ visit/*<T>*/(AstVisitor/*<T>*/ visitor) =>
+  /*=T*/ visit/*<T>*/(ExpressionVisitor/*<T>*/ visitor) =>
       visitor.visitIdentifierExpression(this);
 
   String toString() => text.toString();

lib/src/ast/expression/interpolation.dart

@@ -4,7 +4,7 @@
 
 import 'package:source_span/source_span.dart';
 
-import '../../visitor.dart';
+import '../../visitor/expression.dart';
 import '../expression.dart';
 
 class InterpolationExpression implements Expression {
@@ -40,7 +40,7 @@ class InterpolationExpression implements Expression {
     }
   }
 
-  /*=T*/ visit/*<T>*/(AstVisitor/*<T>*/ visitor) =>
+  /*=T*/ visit/*<T>*/(ExpressionVisitor/*<T>*/ visitor) =>
       visitor.visitInterpolationExpression(this);
 
   String toString() =>

lib/src/ast/expression/list.dart

@@ -5,7 +5,7 @@
 import 'package:source_span/source_span.dart';
 
 import '../../utils.dart';
-import '../../visitor.dart';
+import '../../visitor/expression.dart';
 import '../expression.dart';
 
 class ListExpression implements Expression {
@@ -23,7 +23,7 @@ class ListExpression implements Expression {
       : contents = contents,
         span = span ?? spanForList(contents);
 
-  /*=T*/ visit/*<T>*/(AstVisitor/*<T>*/ visitor) =>
+  /*=T*/ visit/*<T>*/(ExpressionVisitor/*<T>*/ visitor) =>
       visitor.visitListExpression(this);
 
   // TODO: parenthesize nested lists if necessary

lib/src/ast/expression/string.dart

@@ -4,7 +4,7 @@
 
 import 'package:source_span/source_span.dart';
 
-import '../../visitor.dart';
+import '../../visitor/expression.dart';
 import '../expression.dart';
 import 'interpolation.dart';
 
@@ -23,7 +23,7 @@ class StringExpression implements Expression {
   /// Unlike [text], his doesn't resolve escapes and does include quotes.
   InterpolationExpression get asInterpolation => throw new UnimplementedError();
 
-  /*=T*/ visit/*<T>*/(AstVisitor/*<T>*/ visitor) =>
+  /*=T*/ visit/*<T>*/(ExpressionVisitor/*<T>*/ visitor) =>
       visitor.visitStringExpression(this);
 
   StringExpression(this.text);

lib/src/ast/node.dart

@@ -4,10 +4,6 @@
 
 import 'package:source_span/source_span.dart';
 
-import '../visitor.dart';
-
 abstract class AstNode {
   SourceSpan get span;
-
-  /*=T*/ visit/*<T>*/(AstVisitor/*<T>*/ visitor);
 }

lib/src/ast/statement.dart

@@ -0,0 +1,10 @@
+// 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 '../visitor/statement.dart';
+import 'node.dart';
+
+abstract class Statement implements AstNode {
+  /*=T*/ visit/*<T>*/(StatementVisitor/*<T>*/ visitor);
+}

lib/src/ast/style_rule.dart

@@ -4,23 +4,23 @@
 
 import 'package:source_span/source_span.dart';
 
-import '../visitor.dart';
+import '../visitor/statement.dart';
 import 'expression/interpolation.dart';
-import 'node.dart';
+import 'statement.dart';
 
-class StyleRuleNode implements AstNode {
+class StyleRuleNode implements Statement {
   final InterpolationExpression selector;
 
-  final List<AstNode> children;
+  final List<Statement> children;
 
   final SourceSpan span;
 
   // TODO: validate that children only contains variable, at-rule, declaration,
   // or style nodes?
-  StyleRuleNode(this.selector, Iterable<AstNode> children, {this.span})
+  StyleRuleNode(this.selector, Iterable<Statement> children, {this.span})
       : children = new List.unmodifiable(children);
 
-  /*=T*/ visit/*<T>*/(AstVisitor/*<T>*/ visitor) =>
+  /*=T*/ visit/*<T>*/(StatementVisitor/*<T>*/ visitor) =>
       visitor.visitStyleRule(this);
 
   String toString() => "$selector {${children.join(" ")}}";

lib/src/ast/stylesheet.dart

@@ -4,18 +4,18 @@
 
 import 'package:source_span/source_span.dart';
 
-import '../visitor.dart';
-import 'node.dart';
+import '../visitor/statement.dart';
+import 'statement.dart';
 
-class StylesheetNode implements AstNode {
-  final List<AstNode> children;
+class StylesheetNode implements Statement {
+  final List<Statement> children;
 
   final SourceSpan span;
 
-  StylesheetNode(Iterable<AstNode> children, {this.span})
+  StylesheetNode(Iterable<Statement> children, {this.span})
       : children = new List.unmodifiable(children);
 
-  /*=T*/ visit/*<T>*/(AstVisitor/*<T>*/ visitor) =>
+  /*=T*/ visit/*<T>*/(StatementVisitor/*<T>*/ visitor) =>
       visitor.visitStylesheet(this);
 
   String toString() => children.map((child) => "$child").join(" ");

lib/src/ast/variable_declaration.dart

@@ -4,11 +4,11 @@
 
 import 'package:source_span/source_span.dart';
 
-import '../visitor.dart';
+import '../visitor/statement.dart';
 import 'expression.dart';
-import 'node.dart';
+import 'statement.dart';
 
-class VariableDeclarationNode implements AstNode {
+class VariableDeclarationNode implements Statement {
   final String name;
 
   final Expression expression;
@@ -24,7 +24,7 @@ class VariableDeclarationNode implements AstNode {
       : isGuarded = guarded,
         isGlobal = global;
 
-  /*=T*/ visit/*<T>*/(AstVisitor/*<T>*/ visitor) =>
+  /*=T*/ visit/*<T>*/(StatementVisitor/*<T>*/ visitor) =>
       visitor.visitVariableDeclaration(this);
 
   String toString() => "\$$name: $expression;";

lib/src/parser.dart

@@ -5,7 +5,6 @@
 import 'package:charcode/charcode.dart';
 import 'package:string_scanner/string_scanner.dart';
 
-import 'ast/node.dart';
 import 'ast/comment.dart';
 import 'ast/declaration.dart';
 import 'ast/expression.dart';
@@ -13,6 +12,8 @@ import 'ast/expression/identifier.dart';
 import 'ast/expression/interpolation.dart';
 import 'ast/expression/list.dart';
 import 'ast/expression/string.dart';
+import 'ast/node.dart';
+import 'ast/statement.dart';
 import 'ast/style_rule.dart';
 import 'ast/stylesheet.dart';
 import 'ast/variable_declaration.dart';
@@ -36,7 +37,7 @@ class Parser {
 
   StylesheetNode parse() {
     var start = _scanner.state;
-    var children = <AstNode>[];
+    var children = <Statement>[];
     do {
       children.addAll(_comments());
       switch (_scanner.peekChar()) {
@@ -103,9 +104,9 @@ class Parser {
         span: _scanner.spanFrom(start));
   }
 
-  List<AstNode> _styleRuleChildren() {
+  List<Statement> _styleRuleChildren() {
     _expectChar($lbrace);
-    var children = <AstNode>[];
+    var children = <Statement>[];
     do {
       children.addAll(_comments());
       switch (_scanner.peekChar()) {

lib/src/visitor.dart

@@ -2,20 +2,24 @@
 // MIT-style license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
-import 'ast/node.dart';
 import 'ast/comment.dart';
 import 'ast/declaration.dart';
 import 'ast/expression.dart';
-import 'ast/expression/identifier.dart';
-import 'ast/expression/interpolation.dart';
-import 'ast/expression/list.dart';
-import 'ast/expression/string.dart';
+import 'ast/node.dart';
+import 'ast/statement.dart';
 import 'ast/style_rule.dart';
 import 'ast/stylesheet.dart';
 import 'ast/variable_declaration.dart';
+import 'visitor/expression.dart';
+import 'visitor/statement.dart';
 
-class AstVisitor<T> {
-  T visit(AstNode node) => node.visit(this);
+class AstVisitor<T> extends ExpressionVisitor<T>
+    implements StatementVisitor<T> {
+  T visit(AstNode node) {
+    if (node is Statement) return node.visit(this);
+    if (node is Expression) return node.visit(this);
+    throw new ArgumentError("Unknown node type $node.");
+  }
 
   T visitComment(CommentNode node) => null;
 
@@ -44,28 +48,4 @@ class AstVisitor<T> {
     node.expression.visit(this);
     return null;
   }
-
-  T visitIdentifierExpression(IdentifierExpression node) {
-    visitInterpolationExpression(node.text);
-    return null;
-  }
-
-  T visitInterpolationExpression(InterpolationExpression node) {
-    for (var value in node.contents) {
-      if (value is Expression) value.visit(this);
-    }
-    return null;
-  }
-
-  T visitListExpression(ListExpression node) {
-    for (var expression in node.contents) {
-      expression.visit(this);
-    }
-    return null;
-  }
-
-  T visitStringExpression(StringExpression node) {
-    visitInterpolationExpression(node.text);
-    return null;
-  }
 }

lib/src/visitor/expression.dart

@@ -0,0 +1,37 @@
+// 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 '../ast/expression.dart';
+import '../ast/expression/identifier.dart';
+import '../ast/expression/interpolation.dart';
+import '../ast/expression/list.dart';
+import '../ast/expression/string.dart';
+
+class ExpressionVisitor<T> {
+  T visit(Expression expression) => expression.visit(this);
+
+  T visitIdentifierExpression(IdentifierExpression node) {
+    visitInterpolationExpression(node.text);
+    return null;
+  }
+
+  T visitInterpolationExpression(InterpolationExpression node) {
+    for (var value in node.contents) {
+      if (value is Expression) value.visit(this);
+    }
+    return null;
+  }
+
+  T visitListExpression(ListExpression node) {
+    for (var expression in node.contents) {
+      expression.visit(this);
+    }
+    return null;
+  }
+
+  T visitStringExpression(StringExpression node) {
+    visitInterpolationExpression(node.text);
+    return null;
+  }
+}

lib/src/visitor/statement.dart

@@ -0,0 +1,32 @@
+// 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 '../ast/comment.dart';
+import '../ast/declaration.dart';
+import '../ast/statement.dart';
+import '../ast/style_rule.dart';
+import '../ast/stylesheet.dart';
+import '../ast/variable_declaration.dart';
+
+class StatementVisitor<T> {
+  T visit(Statement node) => node.visit(this);
+
+  T visitComment(CommentNode node) => null;
+  T visitDeclaration(DeclarationNode node) => null;
+  T visitVariableDeclaration(VariableDeclarationNode node) => null;
+
+  T visitStyleRule(StyleRuleNode node) {
+    for (var child in node.children) {
+      child.visit(this);
+    }
+    return null;
+  }
+
+  T visitStylesheet(StylesheetNode node) {
+    for (var child in node.children) {
+      child.visit(this);
+    }
+    return null;
+  }
+}