Expose calculations in JS API

lib/src/node/compile.dart

@@ -233,14 +233,17 @@ Object simplify(Object value) => switch (value) {
           value.name, // ...the calculation name
           value.arguments.map(simplify).toList() // ...and simplified arguments
         )) {
-          ('calc', [var first, ...]) => first,
+          ('calc', [var first]) => first,
+          ('calc', _) =>
+            throw ArgumentError('calc() must contain a single argument.'),
           ('clamp', [var min, var value, var max]) =>
             SassCalculation.clamp(min, value, max),
           ('clamp', _) =>
             throw ArgumentError('clamp() requires exactly 3 arguments.'),
           ('min', var args) => SassCalculation.min(args),
           ('max', var args) => SassCalculation.max(args),
-          (var name, var args) => SassCalculation.unsimplified(name, args)
+          (var name, _) =>
+            throw ArgumentError('Unknown calculation function "$name".'),
         },
       CalculationOperation() => SassCalculation.operate(
           value.operator, simplify(value.left), simplify(value.right)),
@@ -260,18 +263,24 @@ List<AsyncCallable> _parseFunctions(Object? functions, {bool asynch = false}) {
     if (!asynch) {
       late Callable callable;
       callable = Callable.fromSignature(signature, (arguments) {
-        var result =
-            simplify((callback as Function)(toJSArray(arguments)) as Object);
-        if (result is Value) return result;
+        var result = (callback as Function)(toJSArray(arguments));
         if (isPromise(result)) {
           throw 'Invalid return value for custom function '
               '"${callable.name}":\n'
               'Promises may only be returned for sass.compileAsync() and '
               'sass.compileStringAsync().';
-        } else {
-          throw 'Invalid return value for custom function '
-              '"${callable.name}": $result is not a sass.Value.';
         }
+        if (result is Value) {
+          var simplified = simplify(result);
+          if (simplified is! Value) {
+            throw 'Custom function "${callable.name}" '
+                'returned an object that cannot be simplified to a '
+                'sass.Value.';
+          }
+          return simplified;
+        }
+        throw 'Invalid return value for custom function '
+            '"${callable.name}": $result is not a sass.Value.';
       });
       result.add(callable);
     } else {
@@ -281,9 +290,15 @@ List<AsyncCallable> _parseFunctions(Object? functions, {bool asynch = false}) {
         if (isPromise(result)) {
           result = await promiseToFuture<Object>(result as Promise);
         }
-
-        var simplified = simplify(result as Object);
-        if (simplified is Value) return simplified;
+        if (result is Value) {
+          var simplified = simplify(result);
+          if (simplified is! Value) {
+            throw 'Custom function "${callable.name}" '
+                'returned an object that cannot be simplified to a '
+                'sass.Value.';
+          }
+          return simplified;
+        }
         throw 'Invalid return value for custom function '
             '"${callable.name}": $result is not a sass.Value.';
       });

lib/src/node/value/calculation.dart

@@ -11,24 +11,23 @@ import '../../value.dart';
 import '../reflection.dart';
 
 /// Check that [arg] is a valid argument to a calculation function.
-void assertCalculationValue(Object arg) {
-  if (arg is! SassNumber &&
-      arg is! SassString &&
-      arg is! SassCalculation &&
-      arg is! CalculationOperation &&
-      arg is! CalculationInterpolation) {
-    jsThrow(JsError('Argument `$arg` must be one of '
-        'SassNumber, SassString, SassCalculation, CalculationOperation, '
-        'CalculationInterpolation'));
-  }
-  if (arg is SassString && arg.hasQuotes) {
-    jsThrow(JsError('Argument `$arg` must be unquoted SassString'));
-  }
-}
-
-/// Check that [arg] is an unquoted string or interpolation
-bool isValidClampArg(Object? arg) => ((arg is CalculationInterpolation) ||
-    (arg is SassString && !arg.hasQuotes));
+void assertCalculationValue(Object arg) => switch (arg) {
+      (SassNumber() ||
+            SassString(hasQuotes: false) ||
+            SassCalculation() ||
+            CalculationOperation() ||
+            CalculationInterpolation()) =>
+        null,
+      _ => jsThrow(JsError(
+          'Argument `$arg` must be one of SassNumber, unquoted SassString, '
+          'SassCalculation, CalculationOperation, CalculationInterpolation')),
+    };
+
+/// Check that [arg] is an unquoted string or interpolation.
+bool isValidClampArg(Object? arg) => switch (arg) {
+      (CalculationInterpolation() || SassString(hasQuotes: false)) => true,
+      _ => false,
+    };
 
 /// The JavaScript `SassCalculation` class.
 final JSClass calculationClass = () {
@@ -54,7 +53,7 @@ final JSClass calculationClass = () {
     },
     'clamp': (Object min, [Object? value, Object? max]) {
       if ((value == null && !isValidClampArg(min)) ||
-          (max == null) && !([min, value]).any(isValidClampArg)) {
+          (max == null) && !([min, value].any(isValidClampArg))) {
         jsThrow(JsError('Expected at least one SassString or '
             'CalculationInterpolation in `${[
           min,
@@ -104,6 +103,8 @@ final JSClass calculationOperationClass = () {
 
   jsClass.defineGetters({
     'operator': (CalculationOperation self) => self.operator.operator,
+    'left': (CalculationOperation self) => self.left,
+    'right': (CalculationOperation self) => self.right,
   });
 
   getJSClass(SassCalculation.operateInternal(
@@ -123,6 +124,10 @@ final JSClass calculationInterpolationClass = () {
     'hashCode': (CalculationInterpolation self) => self.hashCode,
   });
 
+  jsClass.defineGetters({
+    'value': (CalculationInterpolation self) => self.value,
+  });
+
   getJSClass(CalculationInterpolation('')).injectSuperclass(jsClass);
   return jsClass;
 }();

lib/src/value/calculation.dart

@@ -328,10 +328,11 @@ class SassCalculation extends Value {
 /// {@category Value}
 @sealed
 class CalculationOperation {
+  /// We use a getters to allow overriding the logic in the JS API
+  /// implementation.
+
   /// The operator.
   CalculationOperator get operator {
-    // We use a getter to allow overriding the logic in the JS API
-    // implementation
     return _operator;
   }
 
@@ -341,15 +342,23 @@ class CalculationOperation {
   ///
   /// This is either a [SassNumber], a [SassCalculation], an unquoted
   /// [SassString], a [CalculationOperation], or a [CalculationInterpolation].
-  final Object left;
+  Object get left {
+    return _left;
+  }
+
+  final Object _left;
 
   /// The right-hand operand.
   ///
   /// This is either a [SassNumber], a [SassCalculation], an unquoted
   /// [SassString], a [CalculationOperation], or a [CalculationInterpolation].
-  final Object right;
+  Object get right {
+    return _right;
+  }
 
-  CalculationOperation._(this._operator, this.left, this.right);
+  final Object _right;
+
+  CalculationOperation._(this._operator, this._left, this._right);
 
   bool operator ==(Object other) =>
       other is CalculationOperation &&
@@ -409,9 +418,16 @@ enum CalculationOperator {
 /// {@category Value}
 @sealed
 class CalculationInterpolation {
-  final String value;
+  /// We use a getters to allow overriding the logic in the JS API
+  /// implementation.
+
+  String get value {
+    return _value;
+  }
+
+  final String _value;
 
-  CalculationInterpolation(this.value);
+  CalculationInterpolation(this._value);
 
   bool operator ==(Object other) =>
       other is CalculationInterpolation && value == other.value;