> Types that can actually restrict values Do you mean run-time checks? If yes then all you need is a normal oop class with an `assert` in your constructor. If

> Types that can actually restrict values Do you mean run-time checks? If yes then all you need is a normal oop class with an `assert` in your constructor. If you mean compile-time checks then that's a much much harder problem. > Enumerated errors If you have sum types/enums then enumerated errors are just a normal enum. Like type err = FileDoesntExist | PermissionError | NoDiskSpace type result = Ok file | Err err