make it easy to get result value

lib/async.dart

@@ -27,6 +27,7 @@ export 'src/restartable_timer.dart';
 export 'src/result/error.dart';
 export 'src/result/future.dart';
 export 'src/result/result.dart';
+export 'src/result/result_extensions.dart';
 export 'src/result/value.dart';
 export 'src/single_subscription_transformer.dart';
 export 'src/sink_base.dart';

lib/src/result/result_extensions.dart

@@ -0,0 +1,16 @@
+import 'result.dart';
+
+extension ResultExtensions<T> on Result<T> {
+  /// Returns the value if the result [isValue].
+  ///
+  /// If the result [isError], it throws the corresponding error and stacktrace.
+  T get requireValue {
+    if (isValue) return asValue!.value;
+    Error.throwWithStackTrace(asError!.error, asError!.stackTrace);
+  }
+
+  /// Returns the value if the result [isValue].
+  ///
+  /// Returns null otherwise
+  T? get valueOrNull => asValue?.value;
+}

test/result/result_extensions_test.dart

@@ -0,0 +1,67 @@
+import 'package:async/async.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('test requiredValue', () {
+    test(
+      'return value when result is value',
+      () {
+        final result = Result.value(10);
+        expect(result.requireValue, 10);
+      },
+    );
+
+    test(
+      'return nullable value when result is nullable value',
+      () {
+        final result = Result<int?>.value(null);
+        expect(result.requireValue, null);
+      },
+    );
+
+    test(
+      'throw exception when result is error',
+      () async {
+        final result = Result.error('error');
+        expect(() => result.requireValue, throwsA(anything));
+      },
+    );
+
+    test(
+      'throw the corresponding and stacktrace exception when result is error',
+      () async {
+        final error = Exception();
+        final stacktrace = StackTrace.current;
+        final result = Result.error(error,stacktrace);
+
+        expect(
+          () => result.requireValue,
+          throwsA(
+            predicate((err) {
+              return err == error;
+            }),
+          ),
+        );
+      },
+    );
+  });
+
+  group('test valueOrNull', () {
+    test('return null when result is error', (){
+      final result = Result.error('error');
+      expect(result.valueOrNull, null);
+    });
+
+    test('return value when result is value ', (){
+      final result = Result.value(10);
+      expect(result.valueOrNull, 10);
+    });
+
+    test('return nullable value when result is nullable value ', (){
+      final result = Result<int?>.value(null);
+      expect(result.valueOrNull, null);
+    });
+
+  });
+
+}