public void MapErr() { Result <int, Exception> res1 = new Ok <int, Exception>(1); Assert.Equal(1, res1.MapErr(e => e).Unwrap()); Result <int, Exception> res2 = new Err <int, Exception>(new Exception("this is test.")); Assert.Equal(typeof(Err <int, string>), res2.MapErr(e => "mapped error.").GetType()); }