Add rgb().

lib/src/functions.dart

@@ -5,11 +5,41 @@
 import 'ast/sass.dart';
 import 'callable.dart';
 import 'environment.dart';
+import 'exception.dart';
 import 'value.dart';
 
 void defineCoreFunctions(Environment environment) {
+  environment.setFunction(new BuiltInCallable(
+      "rgb",
+      new ArgumentDeclaration(
+          [new Argument("red"), new Argument("green"), new Argument("blue")]),
+      (arguments) {
+    var red = arguments[0].assertNumber("red");
+    var green = arguments[1].assertNumber("green");
+    var blue = arguments[2].assertNumber("blue");
+
+    return new SassColor.rgb(
+        _percentageOrUnitless(red, 255, "red"),
+        _percentageOrUnitless(green, 255, "green"),
+        _percentageOrUnitless(blue, 255, "blue"));
+  }));
+
   environment.setFunction(new BuiltInCallable(
       "inspect",
       new ArgumentDeclaration([new Argument("value")]),
       (arguments) => new SassString(arguments.single.toString())));
 }
+
+int _percentageOrUnitless(SassNumber number, int max, String name) {
+  num value;
+  if (!number.hasUnits) {
+    value = number.value;
+  } else if (number.hasUnit("%")) {
+    value = max * number.value / 100;
+  } else {
+    throw new InternalException(
+        '\$$name: Expected $number to have no units or "%".');
+  }
+
+  return value.clamp(0, max).round();
+}

lib/src/value.dart

@@ -4,6 +4,9 @@
 
 import 'exception.dart';
 import 'value/boolean.dart';
+import 'value/color.dart';
+import 'value/map.dart';
+import 'value/number.dart';
 import 'value/string.dart';
 import 'visitor/interface/value.dart';
 import 'visitor/serialize.dart';
@@ -31,6 +34,31 @@ abstract class Value {
 
   /*=T*/ accept/*<T>*/(ValueVisitor/*<T>*/ visitor);
 
+  SassBoolean assertBoolean([String name]) {
+    if (name == null) throw new InternalException("$this is not a boolean.");
+    throw new InternalException("\$$name: $this is not a boolean.");
+  }
+
+  SassColor assertColor([String name]) {
+    if (name == null) throw new InternalException("$this is not a color.");
+    throw new InternalException("\$$name: $this is not a color.");
+  }
+
+  SassMap assertMap([String name]) {
+    if (name == null) throw new InternalException("$this is not a map.");
+    throw new InternalException("\$$name: $this is not a map.");
+  }
+
+  SassNumber assertNumber([String name]) {
+    if (name == null) throw new InternalException("$this is not a number.");
+    throw new InternalException("\$$name: $this is not a number.");
+  }
+
+  SassString assertString([String name]) {
+    if (name == null) throw new InternalException("$this is not a string.");
+    throw new InternalException("\$$name: $this is not a string.");
+  }
+
   Value or(Value other) => this;
 
   Value and(Value other) => other;

lib/src/value/boolean.dart

@@ -20,6 +20,8 @@ class SassBoolean extends Value {
   /*=T*/ accept/*<T>*/(ValueVisitor/*<T>*/ visitor) =>
       visitor.visitBoolean(this);
 
+  SassBoolean assertBoolean([String name]) => this;
+
   Value or(Value other) => value ? this : other;
 
   Value and(Value other) => value ? other : this;

lib/src/value/color.dart

@@ -17,6 +17,8 @@ class SassColor extends Value {
 
   /*=T*/ accept/*<T>*/(ValueVisitor/*<T>*/ visitor) => visitor.visitColor(this);
 
+  SassColor assertColor([String name]) => this;
+
   Value plus(Value other) {
     if (other is! SassNumber && other is! SassColor) return super.plus(other);
     throw new InternalException('Undefined operation "$this + $other".');

lib/src/value/map.dart

@@ -23,6 +23,8 @@ class SassMap extends Value {
 
   /*=T*/ accept/*<T>*/(ValueVisitor/*<T>*/ visitor) => visitor.visitMap(this);
 
+  SassMap assertMap([String name]) => this;
+
   bool operator ==(other) =>
       other is SassMap && const MapEquality().equals(other.contents, contents);
 

lib/src/value/number.dart

@@ -145,7 +145,7 @@ class SassNumber extends Value {
 
   final List<String> denominatorUnits;
 
-  bool get hasUnits => numeratorUnits.isEmpty && denominatorUnits.isEmpty;
+  bool get hasUnits => numeratorUnits.isNotEmpty || denominatorUnits.isNotEmpty;
 
   bool get isInt => value is int || almostEquals(value % 1, 0.0);
 
@@ -171,6 +171,14 @@ class SassNumber extends Value {
   /*=T*/ accept/*<T>*/(ValueVisitor/*<T>*/ visitor) =>
       visitor.visitNumber(this);
 
+  SassNumber assertNumber([String name]) => this;
+
+  /// Returns whether [this] has [unit] as its only unit (and as a numerator).
+  bool hasUnit(String unit) =>
+      numeratorUnits.length == 1 &&
+      denominatorUnits.isEmpty &&
+      numeratorUnits.first == unit;
+
   num valueInUnits(List<String> newNumerators, List<String> newDenominators) {
     if ((newNumerators.isEmpty && newDenominators.isEmpty) ||
         (numeratorUnits.isEmpty && denominatorUnits.isEmpty) ||

lib/src/value/string.dart

@@ -16,6 +16,8 @@ class SassString extends Value {
   /*=T*/ accept/*<T>*/(ValueVisitor/*<T>*/ visitor) =>
       visitor.visitString(this);
 
+  SassString assertString([String name]) => this;
+
   Value plus(Value other) {
     if (other is SassString) {
       return new SassString(text + other.text,

lib/src/visitor/perform.dart

@@ -789,7 +789,8 @@ class PerformVisitor implements StatementVisitor, ExpressionVisitor<Value> {
       positional.add(argumentList);
     }
 
-    var result = callable.callback(positional);
+    var result =
+        _addExceptionSpan(() => callable.callback(positional), invocation.span);
 
     if (argumentList == null) return result;
     if (named.isEmpty) return result;