Booleans and unary not.

lib/src/ast/sass/expression/unary_operator.dart

@@ -20,13 +20,19 @@ class UnaryOperatorExpression implements Expression {
   /*=T*/ visit/*<T>*/(ExpressionVisitor/*<T>*/ visitor) =>
       visitor.visitUnaryOperatorExpression(this);
 
-  String toString() => "${operator.operator}${operand}";
+  String toString() {
+    var buffer = new StringBuffer(operator.operator);
+    if (operator == UnaryOperator.not) buffer.writeCharCode($space);
+    buffer.write(operand);
+    return buffer.toString();
+  }
 }
 
 class UnaryOperator {
   static const plus = const UnaryOperator._("plus", "+");
   static const minus = const UnaryOperator._("minus", "-");
   static const divide = const UnaryOperator._("divide", "/");
+  static const not = const UnaryOperator._("not", "not");
 
   final String name;
 

lib/src/parser.dart

@@ -392,7 +392,14 @@ class Parser {
 
   Expression _identifierLike() {
     // TODO: url(), functions
-    return new IdentifierExpression(_interpolatedIdentifier());
+    var identifier = _interpolatedIdentifier();
+    if (identifier.asPlain == "not") {
+      _ignoreComments();
+      return new UnaryOperatorExpression(
+          UnaryOperator.not, _singleExpression());
+    } else {
+      return new IdentifierExpression(identifier);
+    }
   }
 
   /// Consumes tokens up to "{", "}", ";", or "!".

lib/src/value.dart

@@ -2,17 +2,23 @@
 // MIT-style license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
+import 'value/boolean.dart';
 import 'value/string.dart';
 
+export 'value/boolean.dart';
 export 'value/identifier.dart';
 export 'value/list.dart';
 export 'value/string.dart';
 
-class Value {
+abstract class Value {
+  const Value();
+
   // TODO: call the proper stringifying method
   Value unaryPlus() => new SassString("+$this");
 
   Value unaryMinus() => new SassString("-$this");
 
   Value unaryDivide() => new SassString("/$this");
+
+  Value unaryNot() => Boolean.sassFalse;
 }

lib/src/value/boolean.dart

@@ -0,0 +1,20 @@
+// 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 '../value.dart';
+
+class Boolean extends Value {
+  static const sassTrue = const Boolean._(true);
+  static const sassFalse = const Boolean._(false);
+
+  final bool value;
+
+  factory Boolean(bool value) => value ? Boolean.sassTrue : Boolean.sassFalse;
+
+  const Boolean._(this.value);
+
+  Value unaryNot() => value ? sassFalse : sassTrue;
+
+  String toString() => value.toString();
+}

lib/src/visitor/expression/perform.dart

@@ -23,10 +23,12 @@ class PerformExpressionVisitor extends ExpressionVisitor<Value> {
   }
 
   Value visitUnaryOperatorExpression(UnaryOperatorExpression node) {
+    var operand = node.operand.visit(this);
     switch (node.operator) {
-      case UnaryOperator.plus: return node.unaryPlus();
-      case UnaryOperator.minus: return node.unaryMinus();
-      case UnaryOperator.divide: return node.unaryDivide();
+      case UnaryOperator.plus: return operand.unaryPlus();
+      case UnaryOperator.minus: return operand.unaryMinus();
+      case UnaryOperator.divide: return operand.unaryDivide();
+      case UnaryOperator.not: return operand.unaryNot();
       default: throw new StateError("Unknown unary operator ${node.operator}.");
     }
   }