go router added

lib/core/utils/either.dart

@@ -0,0 +1,70 @@
+typedef OnError<E> = void Function(E error);
+
+abstract class Either<E, S> {
+  const Either();
+
+  E? getError();
+
+  S? getSuccess();
+
+  bool isError();
+
+  bool isSuccess();
+
+  void when({
+    required OnError<E> error,
+    required void Function(S success) success,
+  });
+}
+
+class Success<E, S> extends Either<E, S> {
+  final S _success;
+
+  const Success(this._success);
+
+  @override
+  E? getError() => null;
+
+  @override
+  S? getSuccess() => _success;
+
+  @override
+  bool isError() => false;
+
+  @override
+  bool isSuccess() => true;
+
+  @override
+  void when({
+    required OnError<E> error,
+    required void Function(S success) success,
+  }) {
+    success(_success);
+  }
+}
+
+class Error<E, S> extends Either<E, S> {
+  final E _error;
+
+  const Error(this._error);
+
+  @override
+  E? getError() => _error;
+
+  @override
+  S? getSuccess() => null;
+
+  @override
+  bool isError() => true;
+
+  @override
+  bool isSuccess() => false;
+
+  @override
+  void when({
+    required OnError<E> error,
+    required void Function(S success) success,
+  }) {
+    error(_error);
+  }
+}