Add also scope operator.

lib/functional.dart

@@ -3,4 +3,5 @@ library functional;
 
 export 'src/functional/either.dart' show Either;
 export 'src/functional/optional.dart' show Optional;
+export 'src/functional/scope.dart' show ScopeFunctionExtension;
 export 'src/functional/types.dart';

lib/src/functional/scope.dart

@@ -0,0 +1,18 @@
+extension ScopeFunctionExtension<T> on T {
+  /// Evaluates the [callback] with the receiver as the argument and returns
+  /// its return value.
+  ///
+  /// In a cascade, this is useful to chain calls to other functions:
+  ///
+  ///     [1, 2, 3]
+  ///       ..also((list) => print('Before: $list'))
+  ///       ..add(4)
+  ///       ..also((list) => print('After: $list'));
+  ///
+  /// On a nullable value, this is useful to only evaluate code on non-null
+  /// objects:
+  ///
+  ///     nullableObject?.also((nonNull) => print('Not null: $nonNull'));
+  ///
+  S also<S>(S Function(T value) callback) => callback(this);
+}

test/functional_test.dart

@@ -401,4 +401,25 @@ void main() {
       });
     });
   });
+  group('scope', () {
+    test('also 1', () {
+      final results = <String>[];
+      [1, 2, 3]
+        ..also((list) => results.add('Before: $list'))
+        ..add(4)
+        ..also((list) => results.add('After: $list'));
+      expect(results, ['Before: [1, 2, 3]', 'After: [1, 2, 3, 4]']);
+    });
+    test('also 2', () {
+      final results = <String>[];
+      for (final value in <int?>[null, 42]) {
+        final result = value?.also((value) {
+          results.add('Value: $value');
+          return value;
+        });
+        expect(result, value);
+      }
+      expect(results, ['Value: 42']);
+    });
+  });
 }