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());
}
