Make visitor interfaces wholly abstract.

lib/src/ast/selector/placeholder.dart

@@ -22,6 +22,4 @@ class PlaceholderSelector extends SimpleSelector {
   bool operator ==(other) => other is PlaceholderSelector && other.name == name;
 
   int get hashCode => name.hashCode;
-
-  String toString() => "%$name";
 }

lib/src/visitor/interface/css.dart

@@ -3,41 +3,13 @@
 // https://opensource.org/licenses/MIT.
 
 import '../../ast/css.dart';
-import 'selector.dart';
-import 'value.dart';
 
-abstract class CssVisitor<T> extends SelectorVisitor<T>
-    implements ValueVisitor<T> {
-  T visitComment(CssComment node) => null;
-  T visitDeclaration(CssDeclaration node) => null;
-  T visitImport(CssImport node) => null;
-
-  T visitAtRule(CssAtRule node) {
-    if (node.children == null) return null;
-    for (var child in node.children) {
-      child.accept(this);
-    }
-    return null;
-  }
-
-  T visitMediaRule(CssMediaRule node) {
-    for (var child in node.children) {
-      child.accept(this);
-    }
-    return null;
-  }
-
-  T visitStyleRule(CssStyleRule node) {
-    for (var child in node.children) {
-      child.accept(this);
-    }
-    return null;
-  }
-
-  T visitStylesheet(CssStylesheet node) {
-    for (var child in node.children) {
-      child.accept(this);
-    }
-    return null;
-  }
+abstract class CssVisitor<T> {
+  T visitAtRule(CssAtRule node);
+  T visitComment(CssComment node);
+  T visitDeclaration(CssDeclaration node);
+  T visitImport(CssImport node);
+  T visitMediaRule(CssMediaRule node);
+  T visitStyleRule(CssStyleRule node);
+  T visitStylesheet(CssStylesheet node);
 }

lib/src/visitor/interface/expression.dart

@@ -5,56 +5,14 @@
 import '../../ast/sass.dart';
 
 abstract class ExpressionVisitor<T> {
-  T visitVariableExpression(VariableExpression node) => null;
-  T visitBooleanExpression(BooleanExpression node) => null;
-  T visitNumberExpression(NumberExpression node) => null;
-  T visitColorExpression(ColorExpression node) => null;
-
-  T visitUnaryOperatorExpression(UnaryOperatorExpression node) {
-    node.operand.accept(this);
-    return null;
-  }
-
-  T visitIdentifierExpression(IdentifierExpression node) {
-    _visitInterpolation(node.text);
-    return null;
-  }
-
-  T visitListExpression(ListExpression node) {
-    for (var expression in node.contents) {
-      expression.accept(this);
-    }
-    return null;
-  }
-
-  T visitMapExpression(MapExpression node) {
-    for (var pair in node.pairs) {
-      pair.first.accept(this);
-      pair.last.accept(this);
-    }
-    return null;
-  }
-
-  T visitFunctionExpression(FunctionExpression node) {
-    for (var expression in node.arguments.positional) {
-      expression.accept(this);
-    }
-    for (var expression in node.arguments.named.values) {
-      expression.accept(this);
-    }
-    node.arguments.rest?.accept(this);
-    node.arguments.keywordRest?.accept(this);
-    return null;
-  }
-
-  T visitStringExpression(StringExpression node) {
-    _visitInterpolation(node.text);
-    return null;
-  }
-
-  void _visitInterpolation(Interpolation node) {
-    for (var value in node.contents) {
-      if (value is Expression) value.accept(this);
-    }
-  }
+  T visitBooleanExpression(BooleanExpression node);
+  T visitColorExpression(ColorExpression node);
+  T visitFunctionExpression(FunctionExpression node);
+  T visitIdentifierExpression(IdentifierExpression node);
+  T visitListExpression(ListExpression node);
+  T visitMapExpression(MapExpression node);
+  T visitNumberExpression(NumberExpression node);
+  T visitStringExpression(StringExpression node);
+  T visitUnaryOperatorExpression(UnaryOperatorExpression node);
+  T visitVariableExpression(VariableExpression node);
 }

lib/src/visitor/interface/selector.dart

@@ -5,37 +5,15 @@
 import '../../ast/selector.dart';
 
 abstract class SelectorVisitor<T> {
-  T visitAttributeSelector(AttributeSelector attribute) => null;
-  T visitClassSelector(ClassSelector klass) => null;
-  T visitIDSelector(IDSelector id) => null;
-  T visitParentSelector(ParentSelector placeholder) => null;
-  T visitPlaceholderSelector(PlaceholderSelector placeholder) => null;
-  T visitTypeSelector(TypeSelector type) => null;
-  T visitUniversalSelector(UniversalSelector universal) => null;
-
-  T visitComplexSelector(ComplexSelector complex) {
-    for (var component in complex.components) {
-      if (component is CompoundSelector) component.accept(this);
-    }
-    return null;
-  }
-
-  T visitCompoundSelector(CompoundSelector compound) {
-    for (var simple in compound.components) {
-      simple.accept(this);
-    }
-    return null;
-  }
-
-  T visitSelectorList(SelectorList list) {
-    for (var complex in list.components) {
-      complex.accept(this);
-    }
-    return null;
-  }
-
-  T visitPseudoSelector(PseudoSelector pseudo) {
-    if (pseudo.selector != null) visitSelectorList(pseudo.selector);
-    return null;
-  }
+  T visitAttributeSelector(AttributeSelector attribute);
+  T visitClassSelector(ClassSelector klass);
+  T visitComplexSelector(ComplexSelector complex);
+  T visitCompoundSelector(CompoundSelector compound);
+  T visitIDSelector(IDSelector id);
+  T visitParentSelector(ParentSelector placeholder);
+  T visitPlaceholderSelector(PlaceholderSelector placeholder);
+  T visitPseudoSelector(PseudoSelector pseudo);
+  T visitSelectorList(SelectorList list);
+  T visitTypeSelector(TypeSelector type);
+  T visitUniversalSelector(UniversalSelector universal);
 }

lib/src/visitor/interface/statement.dart

@@ -5,77 +5,20 @@
 import '../../ast/sass.dart';
 
 abstract class StatementVisitor<T> {
-  T visitComment(Comment node) => null;
-  T visitContent(Content node) => null;
-  T visitExtendRule(ExtendRule node) => null;
-  T visitImport(Import node) => null;
-  T visitPlainImport(PlainImport node) => null;
-  T visitReturn(Return node) => null;
-  T visitVariableDeclaration(VariableDeclaration node) => null;
-
-  T visitDeclaration(Declaration node) {
-    if (node.children == null) return null;
-    for (var child in node.children) {
-      child.accept(this);
-    }
-    return null;
-  }
-
-  T visitAtRule(AtRule node) {
-    if (node.children == null) return null;
-    for (var child in node.children) {
-      child.accept(this);
-    }
-    return null;
-  }
-
-  T visitFunctionDeclaration(FunctionDeclaration node) {
-    for (var child in node.children) {
-      child.accept(this);
-    }
-    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) {
-      child.accept(this);
-    }
-    return null;
-  }
-
-  T visitMixinDeclaration(MixinDeclaration node) {
-    for (var child in node.children) {
-      child.accept(this);
-    }
-    return null;
-  }
-
-  T visitMediaRule(MediaRule node) {
-    for (var child in node.children) {
-      child.accept(this);
-    }
-    return null;
-  }
-
-  T visitStyleRule(StyleRule node) {
-    for (var child in node.children) {
-      child.accept(this);
-    }
-    return null;
-  }
-
-  T visitStylesheet(Stylesheet node) {
-    for (var child in node.children) {
-      child.accept(this);
-    }
-    return null;
-  }
+  T visitAtRule(AtRule node);
+  T visitComment(Comment node);
+  T visitContent(Content node);
+  T visitDeclaration(Declaration node);
+  T visitExtendRule(ExtendRule node);
+  T visitFunctionDeclaration(FunctionDeclaration node);
+  T visitIf(If node);
+  T visitImport(Import node);
+  T visitInclude(Include node);
+  T visitMediaRule(MediaRule node);
+  T visitMixinDeclaration(MixinDeclaration node);
+  T visitPlainImport(PlainImport node);
+  T visitReturn(Return node);
+  T visitStyleRule(StyleRule node);
+  T visitStylesheet(Stylesheet node);
+  T visitVariableDeclaration(VariableDeclaration node);
 }

lib/src/visitor/interface/value.dart

@@ -5,24 +5,11 @@
 import '../../value.dart';
 
 abstract class ValueVisitor<T> {
-  T visitBoolean(SassBoolean value) => null;
-  T visitIdentifier(SassIdentifier value) => null;
-  T visitNumber(SassNumber value) => null;
-  T visitString(SassString value) => null;
-  T visitColor(SassColor value) => null;
-
-  T visitList(SassList value) {
-    for (var element in value.contents) {
-      element.accept(this);
-    }
-    return null;
-  }
-
-  T visitMap(SassMap value) {
-    value.contents.forEach((key, value) {
-      key.accept(this);
-      value.accept(this);
-    });
-    return null;
-  }
+  T visitBoolean(SassBoolean value);
+  T visitColor(SassColor value);
+  T visitIdentifier(SassIdentifier value);
+  T visitList(SassList value);
+  T visitMap(SassMap value);
+  T visitNumber(SassNumber value);
+  T visitString(SassString value);
 }

lib/src/visitor/perform.dart

@@ -20,8 +20,7 @@ import '../value.dart';
 import 'interface/statement.dart';
 import 'interface/expression.dart';
 
-class PerformVisitor extends StatementVisitor
-    implements ExpressionVisitor<Value> {
+class PerformVisitor implements StatementVisitor, ExpressionVisitor<Value> {
   final List<String> _loadPaths;
 
   Environment _environment;
@@ -64,7 +63,9 @@ class PerformVisitor extends StatementVisitor
   CssStylesheet visitStylesheet(Stylesheet node) {
     _root = new CssStylesheet(node.span);
     _parent = _root;
-    super.visitStylesheet(node);
+    for (var child in node.children) {
+      child.accept(this);
+    }
     return _root;
   }
 
@@ -101,7 +102,9 @@ class PerformVisitor extends StatementVisitor
     if (node.children != null) {
       var oldDeclarationName = _declarationName;
       _declarationName = name.value;
-      super.visitDeclaration(node);
+      for (var child in node.children) {
+        child.accept(this);
+      }
       _declarationName = oldDeclarationName;
     }
   }
@@ -137,15 +140,19 @@ class PerformVisitor extends StatementVisitor
 
     _withParent(new CssAtRule(node.name, node.span, value: value), () {
       if (_selector == null) {
-        super.visitAtRule(node);
+        for (var child in node.children) {
+          child.accept(this);
+        }
       } else {
         // If we're in a style rule, copy it into the at-rule so that
         // declarations immediately inside it have somewhere to go.
         //
         // For example, "a {@foo {b: c}}" should produce "@foo {a {b: c}}".
-        _withParent(new CssStyleRule(_selector, _selector.span),
-            () => super.visitAtRule(node),
-            removeIfEmpty: true);
+        _withParent(new CssStyleRule(_selector, _selector.span), () {
+          for (var child in node.children) {
+            child.accept(this);
+          }
+        }, removeIfEmpty: true);
       }
     }, through: (node) => node is CssStyleRule);
   }
@@ -158,7 +165,11 @@ class PerformVisitor extends StatementVisitor
   void visitIf(If node) {
     var condition = node.expression.accept(this);
     if (!condition.isTruthy) return;
-    _environment.scope(() => super.visitIf(node), semiGlobal: true);
+    _environment.scope(() {
+      for (var child in node.children) {
+        child.accept(this);
+      }
+    }, semiGlobal: true);
   }
 
   void visitImport(Import node) {
@@ -254,16 +265,20 @@ class PerformVisitor extends StatementVisitor
     _withParent(new CssMediaRule(queries, node.span), () {
       _withMediaQueries(queries, () {
         if (_selector == null) {
-          super.visitMediaRule(node);
+          for (var child in node.children) {
+            child.accept(this);
+          }
         } else {
           // If we're in a style rule, copy it into the media query so that
           // declarations immediately inside @media have somewhere to go.
           //
           // For example, "a {@media screen {b: c}}" should produce
           // "@media screen {a {b: c}}".
-          _withParent(new CssStyleRule(_selector, _selector.span),
-              () => super.visitMediaRule(node),
-              removeIfEmpty: true);
+          _withParent(new CssStyleRule(_selector, _selector.span), () {
+            for (var child in node.children) {
+              child.accept(this);
+            }
+          }, removeIfEmpty: true);
         }
       });
     },
@@ -313,9 +328,13 @@ class PerformVisitor extends StatementVisitor
     var selector =
         new CssValue<SelectorList>(parsedSelector, node.selector.span);
 
-    _withParent(_extender.addSelector(selector, node.span),
-        () => _withSelector(selector, () => super.visitStyleRule(node)),
-        through: (node) => node is CssStyleRule, removeIfEmpty: true);
+    _withParent(_extender.addSelector(selector, node.span), () {
+      _withSelector(selector, () {
+        for (var child in node.children) {
+          child.accept(this);
+        }
+      });
+    }, through: (node) => node is CssStyleRule, removeIfEmpty: true);
   }
 
   void visitVariableDeclaration(VariableDeclaration node) {

lib/src/visitor/serialize.dart

@@ -12,6 +12,8 @@ import '../ast/selector.dart';
 import '../util/character.dart';
 import '../value.dart';
 import 'interface/css.dart';
+import 'interface/selector.dart';
+import 'interface/value.dart';
 
 String toCss(CssNode node) {
   var visitor = new _SerializeCssVisitor();
@@ -38,7 +40,8 @@ String selectorToCss(Selector selector) {
   return visitor._buffer.toString();
 }
 
-class _SerializeCssVisitor extends CssVisitor {
+class _SerializeCssVisitor
+    implements CssVisitor, ValueVisitor, SelectorVisitor {
   final _buffer = new StringBuffer();
 
   var _indentation = 0;
@@ -311,6 +314,12 @@ class _SerializeCssVisitor extends CssVisitor {
     });
   }
 
+  void visitCompoundSelector(CompoundSelector compound) {
+    for (var simple in compound.components) {
+      simple.accept(this);
+    }
+  }
+
   void visitIDSelector(IDSelector id) {
     _buffer.writeCharCode($hash);
     _buffer.write(id.name);
@@ -326,6 +335,11 @@ class _SerializeCssVisitor extends CssVisitor {
     if (parent.suffix != null) _buffer.write(parent.suffix);
   }
 
+  void visitPlaceholderSelector(PlaceholderSelector placeholder) {
+    _buffer.writeCharCode($percent);
+    _buffer.write(placeholder.name);
+  }
+
   void visitPseudoSelector(PseudoSelector pseudo) {
     _buffer.writeCharCode($colon);
     if (pseudo.type == PseudoType.element) _buffer.writeCharCode($colon);