Add a factory method for creating host callable

lib/src/callable.dart

@@ -3,10 +3,13 @@
 // https://opensource.org/licenses/MIT.
 
 import 'package:meta/meta.dart';
+import 'package:tuple/tuple.dart';
 
+import 'ast/sass.dart';
 import 'callable/async.dart';
 import 'callable/built_in.dart';
 import 'exception.dart';
+import 'util/function_signature.dart';
 import 'value.dart';
 
 export 'callable/async.dart';
@@ -117,4 +120,14 @@ abstract class Callable extends AsyncCallable {
   factory Callable.function(String name, String arguments,
           Value callback(List<Value> arguments)) =>
       BuiltInCallable.function(name, arguments, callback);
+
+  /// Creates a host callable with a single [signature] and a single [callback].
+  ///
+  /// Throws a [SassFormatException] if parsing fails.
+  factory Callable.host(String signature, Value callback(List<Value> arguments),
+      {bool requireParens = true}) {
+    Tuple2<String, ArgumentDeclaration> tuple =
+        parseSignature(signature, requireParens: requireParens);
+    return BuiltInCallable.parsed(tuple.item1, tuple.item2, callback);
+  }
 }

lib/src/callable/async.dart

@@ -5,8 +5,11 @@
 import 'dart:async';
 
 import 'package:meta/meta.dart';
+import 'package:tuple/tuple.dart';
 
+import '../ast/sass.dart';
 import '../exception.dart';
+import '../util/function_signature.dart';
 import '../value.dart';
 import 'async_built_in.dart';
 
@@ -40,4 +43,15 @@ abstract class AsyncCallable {
   factory AsyncCallable.function(String name, String arguments,
           FutureOr<Value> callback(List<Value> arguments)) =>
       AsyncBuiltInCallable.function(name, arguments, callback);
+
+  /// Creates a host callable with a single [signature] and a single [callback].
+  ///
+  /// Throws a [SassFormatException] if parsing fails.
+  factory AsyncCallable.host(
+      String signature, FutureOr<Value> callback(List<Value> arguments),
+      {bool requireParens = true}) {
+    Tuple2<String, ArgumentDeclaration> tuple =
+        parseSignature(signature, requireParens: requireParens);
+    return AsyncBuiltInCallable.parsed(tuple.item1, tuple.item2, callback);
+  }
 }

lib/src/node/compile.dart

@@ -6,22 +6,16 @@ import 'package:js/js.dart';
 import 'package:node_interop/js.dart';
 import 'package:node_interop/util.dart' hide futureToPromise;
 import 'package:term_glyph/term_glyph.dart' as glyph;
-import 'package:tuple/tuple.dart';
 
 import '../../sass.dart';
-import '../ast/sass.dart';
-import '../callable.dart';
-import '../exception.dart';
 import '../importer/no_op.dart';
 import '../importer/node_to_dart/async.dart';
 import '../importer/node_to_dart/async_file.dart';
 import '../importer/node_to_dart/file.dart';
 import '../importer/node_to_dart/sync.dart';
 import '../io.dart';
 import '../logger/node_to_dart.dart';
-import '../parse/scss.dart';
 import '../util/nullable.dart';
-import '../utils.dart';
 import 'compile_options.dart';
 import 'compile_result.dart';
 import 'exception.dart';
@@ -236,42 +230,35 @@ List<AsyncCallable> _parseFunctions(Object? functions, {bool asynch = false}) {
 
   var result = <AsyncCallable>[];
   jsForEach(functions, (signature, callback) {
-    Tuple2<String, ArgumentDeclaration> tuple;
-    try {
-      tuple = ScssParser(signature).parseSignature();
-    } on SassFormatException catch (error, stackTrace) {
-      throwWithTrace(
-          SassFormatException(
-              'Invalid signature "$signature": ${error.message}', error.span),
-          stackTrace);
-    }
-
     if (!asynch) {
-      result.add(BuiltInCallable.parsed(tuple.item1, tuple.item2, (arguments) {
+      Callable? callable;
+      callable = Callable.host(signature, (arguments) {
         var result = (callback as Function)(toJSArray(arguments));
         if (result is Value) return result;
         if (isPromise(result)) {
           throw 'Invalid return value for custom function '
-              '"${tuple.item1}":\n'
+              '"${callable!.name}":\n'
               'Promises may only be returned for sass.compileAsync() and '
               'sass.compileStringAsync().';
         } else {
           throw 'Invalid return value for custom function '
-              '"${tuple.item1}": $result is not a sass.Value.';
+              '"${callable!.name}": $result is not a sass.Value.';
         }
-      }));
+      });
+      result.add(callable);
     } else {
-      result.add(AsyncBuiltInCallable.parsed(tuple.item1, tuple.item2,
-          (arguments) async {
+      AsyncCallable? callable;
+      callable = AsyncCallable.host(signature, (arguments) async {
         var result = (callback as Function)(toJSArray(arguments));
         if (isPromise(result)) {
           result = await promiseToFuture<Object>(result as Promise);
         }
 
         if (result is Value) return result;
         throw 'Invalid return value for custom function '
-            '"${tuple.item1}": $result is not a sass.Value.';
-      }));
+            '"${callable!.name}": $result is not a sass.Value.';
+      });
+      result.add(callable);
     }
   });
   return result;

lib/src/node/legacy.dart

@@ -10,9 +10,7 @@ import 'dart:typed_data';
 import 'package:js/js.dart';
 import 'package:node_interop/js.dart';
 import 'package:path/path.dart' as p;
-import 'package:tuple/tuple.dart';
 
-import '../ast/sass.dart';
 import '../callable.dart';
 import '../compile.dart';
 import '../compile_result.dart';
@@ -21,7 +19,6 @@ import '../importer/legacy_node.dart';
 import '../io.dart';
 import '../logger.dart';
 import '../logger/node_to_dart.dart';
-import '../parse/scss.dart';
 import '../syntax.dart';
 import '../util/nullable.dart';
 import '../utils.dart';
@@ -208,22 +205,12 @@ List<AsyncCallable> _parseFunctions(RenderOptions options, DateTime start,
 
   var result = <AsyncCallable>[];
   jsForEach(functions, (signature, callback) {
-    Tuple2<String, ArgumentDeclaration> tuple;
-    try {
-      tuple = ScssParser(signature).parseSignature(requireParens: false);
-    } on SassFormatException catch (error, stackTrace) {
-      throwWithTrace(
-          SassFormatException(
-              'Invalid signature "$signature": ${error.message}', error.span),
-          stackTrace);
-    }
-
     var context = RenderContext(options: _contextOptions(options, start));
     context.options.context = context;
 
     var fiber = options.fiber;
     if (fiber != null) {
-      result.add(BuiltInCallable.parsed(tuple.item1, tuple.item2, (arguments) {
+      result.add(Callable.host(signature, (arguments) {
         var currentFiber = fiber.current;
         var jsArguments = [
           ...arguments.map(wrapValue),
@@ -240,16 +227,15 @@ List<AsyncCallable> _parseFunctions(RenderOptions options, DateTime start,
             // `Zone.current` in an inconsistent state.
             ? runZoned(() => fiber.yield())
             : result);
-      }));
+      }, requireParens: false));
     } else if (!asynch) {
-      result.add(BuiltInCallable.parsed(
-          tuple.item1,
-          tuple.item2,
+      result.add(Callable.host(
+          signature,
           (arguments) => unwrapValue((callback as JSFunction)
-              .apply(context, arguments.map(wrapValue).toList()))));
+              .apply(context, arguments.map(wrapValue).toList())),
+          requireParens: false));
     } else {
-      result.add(AsyncBuiltInCallable.parsed(tuple.item1, tuple.item2,
-          (arguments) async {
+      result.add(AsyncCallable.host(signature, (arguments) async {
         var completer = Completer<Object?>();
         var jsArguments = [
           ...arguments.map(wrapValue),
@@ -258,7 +244,7 @@ List<AsyncCallable> _parseFunctions(RenderOptions options, DateTime start,
         var result = (callback as JSFunction).apply(context, jsArguments);
         return unwrapValue(
             isUndefined(result) ? await completer.future : result);
-      }));
+      }, requireParens: false));
     }
   });
   return result;

lib/src/util/function_signature.dart

@@ -0,0 +1,28 @@
+// Copyright 2022 Google Inc. Use of this source code is governed by an
+// MIT-style license that can be found in the LICENSE file or at
+// https://opensource.org/licenses/MIT.
+
+import 'package:tuple/tuple.dart';
+
+import '../ast/sass.dart';
+import '../exception.dart';
+import '../parse/scss.dart';
+import '../utils.dart';
+
+/// Parses a function signature of the format allowed by Node Sass's functions
+/// option and returns its name and declaration.
+///
+/// If [requireParens] is `false`, this allows parentheses to be omitted.
+///
+/// Throws a [SassFormatException] if parsing fails.
+Tuple2<String, ArgumentDeclaration> parseSignature(String signature,
+    {bool requireParens = true}) {
+  try {
+    return ScssParser(signature).parseSignature(requireParens: requireParens);
+  } on SassFormatException catch (error, stackTrace) {
+    throwWithTrace(
+        SassFormatException(
+            'Invalid signature "$signature": ${error.message}', error.span),
+        stackTrace);
+  }
+}