Add boolean short-circuiting.

CHANGELOG.md

@@ -13,6 +13,8 @@
 
 * Properly handle placeholder selectors in selector pseudos.
 
+* Properly short-circuit the `or` and `and` operators.
+
 * Support `--$variable`.
 
 * Don't consider unitless numbers equal to numbers with units.

lib/src/value.dart

@@ -235,12 +235,6 @@ abstract class Value {
   Value singleEquals(Value other) =>
       new SassString("${toCssString()}=${other.toCssString()}");
 
-  /// The SassScript `or` operation.
-  Value or(Value other) => this;
-
-  /// The SassScript `and` operation.
-  Value and(Value other) => other;
-
   /// The SassScript `>` operation.
   SassBoolean greaterThan(Value other) =>
       throw new SassScriptException('Undefined operation "$this > $other".');

lib/src/value/boolean.dart

@@ -31,9 +31,5 @@ class SassBoolean extends Value {
 
   SassBoolean assertBoolean([String name]) => this;
 
-  Value or(Value other) => value ? this : other;
-
-  Value and(Value other) => value ? other : this;
-
   Value unaryNot() => value ? sassFalse : sassTrue;
 }

lib/src/value/null.dart

@@ -20,9 +20,5 @@ class SassNull extends Value {
 
   /*=T*/ accept/*<T>*/(ValueVisitor/*<T>*/ visitor) => visitor.visitNull(this);
 
-  Value or(Value other) => other;
-
-  Value and(Value other) => this;
-
   Value unaryNot() => sassTrue;
 }

lib/src/visitor/perform.dart

@@ -964,33 +964,55 @@ class _PerformVisitor
   Value visitBinaryOperationExpression(BinaryOperationExpression node) {
     return _addExceptionSpan(node.span, () {
       var left = node.left.accept(this);
-      var right = node.right.accept(this);
       switch (node.operator) {
         case BinaryOperator.singleEquals:
+          var right = node.right.accept(this);
           return left.singleEquals(right);
+
         case BinaryOperator.or:
-          return left.or(right);
+          return left.isTruthy ? left : node.right.accept(this);
+
         case BinaryOperator.and:
-          return left.and(right);
+          return left.isTruthy ? node.right.accept(this) : left;
+
         case BinaryOperator.equals:
+          var right = node.right.accept(this);
           return new SassBoolean(left == right);
+
         case BinaryOperator.notEquals:
+          var right = node.right.accept(this);
           return new SassBoolean(left != right);
+
         case BinaryOperator.greaterThan:
+          var right = node.right.accept(this);
           return left.greaterThan(right);
+
         case BinaryOperator.greaterThanOrEquals:
+          var right = node.right.accept(this);
           return left.greaterThanOrEquals(right);
+
         case BinaryOperator.lessThan:
+          var right = node.right.accept(this);
           return left.lessThan(right);
+
         case BinaryOperator.lessThanOrEquals:
+          var right = node.right.accept(this);
           return left.lessThanOrEquals(right);
+
         case BinaryOperator.plus:
+          var right = node.right.accept(this);
           return left.plus(right);
+
         case BinaryOperator.minus:
+          var right = node.right.accept(this);
           return left.minus(right);
+
         case BinaryOperator.times:
+          var right = node.right.accept(this);
           return left.times(right);
+
         case BinaryOperator.dividedBy:
+          var right = node.right.accept(this);
           var result = left.dividedBy(right);
           if (node.allowsSlash && left is SassNumber && right is SassNumber) {
             var leftSlash = left.asSlash ?? _toCss(left, node.left.span);
@@ -1000,7 +1022,9 @@ class _PerformVisitor
             return result;
           }
           break;
+
         case BinaryOperator.modulo:
+          var right = node.right.accept(this);
           return left.modulo(right);
         default:
           return null;