Store numbers' original values.

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

@@ -15,9 +15,13 @@ class NumberExpression implements Expression {
   /// The number's unit, or `null`.
   final String unit;
 
+  /// Whether the number produced should retain its original representation.
+  final bool hasOriginal;
+
   final FileSpan span;
 
-  NumberExpression(this.value, this.span, {this.unit});
+  NumberExpression(this.value, this.span, {this.unit, bool original: false})
+      : hasOriginal = original;
 
   /*=T*/ accept/*<T>*/(ExpressionVisitor/*<T>*/ visitor) =>
       visitor.visitNumberExpression(this);

lib/src/parse/stylesheet.dart

@@ -45,6 +45,9 @@ abstract class StylesheetParser extends Parser {
   /// or `@each`.
   var _inControlDirective = false;
 
+  /// Whether the parser is currently within a parenthesized expression.
+  var _inParentheses = false;
+
   StylesheetParser(String contents, {url}) : super(contents, url: url);
 
   // ## Statements
@@ -1229,38 +1232,44 @@ abstract class StylesheetParser extends Parser {
 
   /// Consumes a parenthesized expression.
   Expression _parentheses() {
-    var start = scanner.state;
-    scanner.expectChar($lparen);
-    whitespace();
-    if (!_lookingAtExpression()) {
+    try {
+      _inParentheses = true;
+      var start = scanner.state;
+      scanner.expectChar($lparen);
+      whitespace();
+      if (!_lookingAtExpression()) {
+        scanner.expectChar($rparen);
+        return new ListExpression([], ListSeparator.undecided,
+            span: scanner.spanFrom(start));
+      }
+
+      var first = _expressionUntilComma();
+      if (scanner.scanChar($colon)) {
+        whitespace();
+        _inParentheses = false;
+        return _map(first, start);
+      }
+
+      if (!scanner.scanChar($comma)) {
+        scanner.expectChar($rparen);
+        return first;
+      }
+      whitespace();
+
+      var expressions = [first];
+      while (true) {
+        if (!_lookingAtExpression()) break;
+        expressions.add(_expressionUntilComma());
+        if (!scanner.scanChar($comma)) break;
+        whitespace();
+      }
+
       scanner.expectChar($rparen);
-      return new ListExpression([], ListSeparator.undecided,
+      return new ListExpression(expressions, ListSeparator.comma,
           span: scanner.spanFrom(start));
+    } finally {
+      _inParentheses = false;
     }
-
-    var first = _expressionUntilComma();
-    if (scanner.scanChar($colon)) {
-      whitespace();
-      return _map(first, start);
-    }
-
-    if (!scanner.scanChar($comma)) {
-      scanner.expectChar($rparen);
-      return first;
-    }
-    whitespace();
-
-    var expressions = [first];
-    while (true) {
-      if (!_lookingAtExpression()) break;
-      expressions.add(_expressionUntilComma());
-      if (!scanner.scanChar($comma)) break;
-      whitespace();
-    }
-
-    scanner.expectChar($rparen);
-    return new ListExpression(expressions, ListSeparator.comma,
-        span: scanner.spanFrom(start));
   }
 
   /// Consumes a map expression.
@@ -1444,7 +1453,7 @@ abstract class StylesheetParser extends Parser {
     }
 
     return new NumberExpression(sign * number, scanner.spanFrom(start),
-        unit: unit);
+        unit: unit, original: !_inParentheses);
   }
 
   /// Consumes a variable expression.

lib/src/value.dart

@@ -281,6 +281,11 @@ abstract class Value {
   /// The SassScript unary `not` operation.
   Value unaryNot() => sassFalse;
 
+  /// Returns a copy of [this] without [SassNumber.original] set.
+  ///
+  /// If this isn't a [SassNumber], returns it as-is.
+  Value withoutOriginal() => this;
+
   /// Returns a valid CSS representation of [this].
   ///
   /// Throws a [SassScriptException] if [this] can't be represented in plain

lib/src/value/number.dart

@@ -161,6 +161,28 @@ class SassNumber extends Value {
   /// This number's denominator units.
   final List<String> denominatorUnits;
 
+  /// Whether this should be represented as a slash-separated series of numbers.
+  ///
+  /// This is `true` if and only if [original] contains a slash.
+  bool get isSlashSeparated => _hasOriginal && _original != null;
+
+  /// The original representation of the number.
+  ///
+  /// This is used to preserve slash-separated numbers in some contexts.
+  String get original {
+    if (_original != null) return _original;
+    if (_hasOriginal) return toCssString();
+    return _original;
+  }
+
+  final String _original;
+
+  /// Whether this number still has its original representation.
+  ///
+  /// This is separate from [_original] so that we can avoid eagerly converting
+  /// all number literals to strings.
+  final bool _hasOriginal;
+
   /// Whether [this] has any units.
   bool get hasUnits => numeratorUnits.isNotEmpty || denominatorUnits.isNotEmpty;
 
@@ -181,14 +203,27 @@ class SassNumber extends Value {
   String get unitString =>
       hasUnits ? _unitString(numeratorUnits, denominatorUnits) : '';
 
-  /// Returns a number, optionally with a single numerator unit.
+  /// Creates a number, optionally with a single numerator unit.
   ///
   /// This matches the numbers that can be written as literals.
   /// [SassNumber.withUnits] can be used to construct more complex units.
-  SassNumber(num value, [String unit])
-      : this.withUnits(value, numeratorUnits: unit == null ? null : [unit]);
+  SassNumber(this.value, [String unit])
+      : numeratorUnits =
+            unit == null ? const [] : new List.unmodifiable([unit]),
+        denominatorUnits = const [],
+        _original = null,
+        _hasOriginal = false;
 
-  /// Returns a number with full [numeratorUnits] and [denominatorUnits].
+  /// Like [new SassNumber], but sets [original] based on the string
+  /// representation of the number.
+  SassNumber.withOriginal(this.value, [String unit])
+      : numeratorUnits =
+            unit == null ? const [] : new List.unmodifiable([unit]),
+        denominatorUnits = const [],
+        _original = null,
+        _hasOriginal = true;
+
+  /// Creates a number with full [numeratorUnits] and [denominatorUnits].
   SassNumber.withUnits(this.value,
       {Iterable<String> numeratorUnits, Iterable<String> denominatorUnits})
       : numeratorUnits = numeratorUnits == null
@@ -196,11 +231,28 @@ class SassNumber extends Value {
             : new List.unmodifiable(numeratorUnits),
         denominatorUnits = denominatorUnits == null
             ? const []
-            : new List.unmodifiable(denominatorUnits);
+            : new List.unmodifiable(denominatorUnits),
+        _original = null,
+        _hasOriginal = false;
+
+  SassNumber._(this.value, this.numeratorUnits, this.denominatorUnits,
+      [String original])
+      : _original = original,
+        _hasOriginal = original != null;
 
   /*=T*/ accept/*<T>*/(ValueVisitor/*<T>*/ visitor) =>
       visitor.visitNumber(this);
 
+  /// Returns a copy of [this] without [original] set.
+  SassNumber withoutOriginal() {
+    if (!_hasOriginal) return this;
+    return new SassNumber._(value, numeratorUnits, denominatorUnits);
+  }
+
+  /// Returns a copy of [this] with [this.original] set to [original].
+  SassNumber _withOriginal(String original) =>
+      new SassNumber._(value, numeratorUnits, denominatorUnits, original);
+
   SassNumber assertNumber([String name]) => this;
 
   /// Returns [value] as an [int], if it's an integer value according to
@@ -403,8 +455,10 @@ class SassNumber extends Value {
 
   Value dividedBy(Value other) {
     if (other is SassNumber) {
-      return _multiplyUnits(this.value / other.value, this.numeratorUnits,
+      var result = _multiplyUnits(this.value / other.value, this.numeratorUnits,
           this.denominatorUnits, other.denominatorUnits, other.numeratorUnits);
+      if (!this._hasOriginal || !other._hasOriginal) return result;
+      return result._withOriginal("${this.original}/${other.original}");
     }
     if (other is! SassColor) super.dividedBy(other);
     throw new SassScriptException('Undefined operation "$this / $other".');

lib/src/visitor/perform.dart

@@ -622,7 +622,8 @@ class _PerformVisitor implements StatementVisitor, ExpressionVisitor<Value> {
   }
 
   void visitVariableDeclaration(VariableDeclaration node) {
-    _environment.setVariable(node.name, node.expression.accept(this),
+    _environment.setVariable(
+        node.name, node.expression.accept(this).withoutOriginal(),
         global: node.isGlobal);
   }
 
@@ -732,8 +733,9 @@ class _PerformVisitor implements StatementVisitor, ExpressionVisitor<Value> {
 
   SassNull visitNullExpression(NullExpression node) => sassNull;
 
-  SassNumber visitNumberExpression(NumberExpression node) =>
-      new SassNumber(node.value, node.unit);
+  SassNumber visitNumberExpression(NumberExpression node) => node.hasOriginal
+      ? new SassNumber.withOriginal(node.value, node.unit)
+      : new SassNumber(node.value, node.unit);
 
   SassColor visitColorExpression(ColorExpression node) => node.value;
 
@@ -761,7 +763,7 @@ class _PerformVisitor implements StatementVisitor, ExpressionVisitor<Value> {
       var function = _environment.getFunction(plainName);
       if (function != null) {
         if (function is BuiltInCallable) {
-          return _runBuiltInCallable(node, function);
+          return _runBuiltInCallable(node, function).withoutOriginal();
         } else if (function is UserDefinedCallable) {
           return _runUserDefinedCallable(node, function, () {
             for (var statement in function.declaration.children) {
@@ -771,7 +773,7 @@ class _PerformVisitor implements StatementVisitor, ExpressionVisitor<Value> {
 
             throw _exception("Function finished without @return.",
                 function.declaration.span);
-          });
+          }).withoutOriginal();
         } else {
           return null;
         }

lib/src/visitor/serialize.dart

@@ -353,6 +353,11 @@ class _SerializeCssVisitor
   }
 
   void visitNumber(SassNumber value) {
+    if (value.isSlashSeparated) {
+      _buffer.write(value.original);
+      return;
+    }
+
     _writeNumber(value.value);
 
     if (!_inspect) {