Address review

lib/src/node/compile.dart

@@ -225,15 +225,17 @@ Importer _parseImporter(Object? importer) {
   }
 }
 
-/// Implements the simplification algorithm for custom function return values.
+/// Implements the simplification algorithm for custom function return Values.
 /// {@link https://github.com/sass/sass/blob/main/spec/types/calculation.md#simplifying-a-calculationvalue}
-Object simplify(Object value) => switch (value) {
+Value _simplifyValue(Value value) => switch (value) {
       SassCalculation() => switch ((
           // Match against...
           value.name, // ...the calculation name
-          value.arguments.map(simplify).toList() // ...and simplified arguments
+          value.arguments // ...and simplified arguments
+              .map(_simplifyCalcArg)
+              .toList()
         )) {
-          ('calc', [var first]) => first,
+          ('calc', [var first]) => first as Value,
           ('calc', _) =>
             throw ArgumentError('calc() requires exactly one argument.'),
           ('clamp', [var min, var value, var max]) =>
@@ -245,8 +247,15 @@ Object simplify(Object value) => switch (value) {
           (var name, _) => throw ArgumentError(
               '"$name" is not a recognized calculation type.'),
         },
-      CalculationOperation() => SassCalculation.operate(
-          value.operator, simplify(value.left), simplify(value.right)),
+      _ => value,
+    };
+
+/// Handles simplifying calculation arguments, which are not guaranteed to be
+/// Value instances.
+Object _simplifyCalcArg(Object value) => switch (value) {
+      SassCalculation() => _simplifyValue(value),
+      CalculationOperation() => SassCalculation.operate(value.operator,
+          _simplifyCalcArg(value.left), _simplifyCalcArg(value.right)),
       _ => value,
     };
 
@@ -264,23 +273,16 @@ List<AsyncCallable> _parseFunctions(Object? functions, {bool asynch = false}) {
       late Callable callable;
       callable = Callable.fromSignature(signature, (arguments) {
         var result = (callback as Function)(toJSArray(arguments));
+        if (result is Value) return _simplifyValue(result);
         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}": $result is not a sass.Value.';
       });
       result.add(callable);
     } else {
@@ -290,15 +292,7 @@ List<AsyncCallable> _parseFunctions(Object? functions, {bool asynch = false}) {
         if (isPromise(result)) {
           result = await promiseToFuture<Object>(result as Promise);
         }
-        if (result is Value) {
-          var simplified = simplify(result);
-          if (simplified is! Value) {
-            throw 'Custom function "${callable.name}" '
-                'returned an object that cannot be simplified to a '
-                'sass.Value.';
-          }
-          return simplified;
-        }
+        if (result is Value) return _simplifyValue(result);
         throw 'Invalid return value for custom function '
             '"${callable.name}": $result is not a sass.Value.';
       });

lib/src/node/value/calculation.dart

@@ -11,20 +11,20 @@ import '../../value.dart';
 import '../reflection.dart';
 
 /// Check that [arg] is a valid argument to a calculation function.
-void assertCalculationValue(Object arg) => switch (arg) {
-      (SassNumber() ||
-            SassString(hasQuotes: false) ||
-            SassCalculation() ||
-            CalculationOperation() ||
-            CalculationInterpolation()) =>
+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) {
+bool _isValidClampArg(Object? arg) => switch (arg) {
       (CalculationInterpolation() || SassString(hasQuotes: false)) => true,
       _ => false,
     };
@@ -38,30 +38,30 @@ final JSClass calculationClass = () {
 
   jsClass.defineStaticMethods({
     'calc': (Object argument) {
-      assertCalculationValue(argument);
+      _assertCalculationValue(argument);
       return SassCalculation.unsimplified('calc', [argument]);
     },
     'min': (Object arguments) {
       var argList = jsToDartList(arguments).cast<Object>();
-      argList.forEach(assertCalculationValue);
+      argList.forEach(_assertCalculationValue);
       return SassCalculation.unsimplified('min', argList);
     },
     'max': (Object arguments) {
       var argList = jsToDartList(arguments).cast<Object>();
-      argList.forEach(assertCalculationValue);
+      argList.forEach(_assertCalculationValue);
       return SassCalculation.unsimplified('max', argList);
     },
     'clamp': (Object min, [Object? value, Object? max]) {
-      if ((value == null && !isValidClampArg(min)) ||
-          (max == null) && !([min, value].any(isValidClampArg))) {
+      if ((value == null && !_isValidClampArg(min)) ||
+          (max == null && ![min, value].any(_isValidClampArg))) {
         jsThrow(JsError('Expected at least one SassString or '
             'CalculationInterpolation in `${[
           min,
           value,
           max
         ].whereNotNull()}`'));
       }
-      [min, value, max].whereNotNull().forEach(assertCalculationValue);
+      [min, value, max].whereNotNull().forEach(_assertCalculationValue);
       return SassCalculation.unsimplified(
           'clamp', [min, value, max].whereNotNull());
     }
@@ -90,8 +90,8 @@ final JSClass calculationOperationClass = () {
     if (operator == null) {
       jsThrow(JsError('Invalid operator: $strOperator'));
     }
-    assertCalculationValue(left);
-    assertCalculationValue(right);
+    _assertCalculationValue(left);
+    _assertCalculationValue(right);
     return SassCalculation.operateInternal(operator, left, right,
         inMinMax: false, simplify: false);
   });

lib/src/value/calculation.dart

@@ -332,30 +332,21 @@ class CalculationOperation {
   /// implementation.
 
   /// The operator.
-  CalculationOperator get operator {
-    return _operator;
-  }
-
+  CalculationOperator get operator => _operator;
   final CalculationOperator _operator;
 
   /// The left-hand operand.
   ///
   /// This is either a [SassNumber], a [SassCalculation], an unquoted
   /// [SassString], a [CalculationOperation], or a [CalculationInterpolation].
-  Object get left {
-    return _left;
-  }
-
+  Object get left => _left;
   final Object _left;
 
   /// The right-hand operand.
   ///
   /// This is either a [SassNumber], a [SassCalculation], an unquoted
   /// [SassString], a [CalculationOperation], or a [CalculationInterpolation].
-  Object get right {
-    return _right;
-  }
-
+  Object get right => _right;
   final Object _right;
 
   CalculationOperation._(this._operator, this._left, this._right);
@@ -421,10 +412,7 @@ class CalculationInterpolation {
   /// We use a getters to allow overriding the logic in the JS API
   /// implementation.
 
-  String get value {
-    return _value;
-  }
-
+  String get value => _value;
   final String _value;
 
   CalculationInterpolation(this._value);