Comment by raddan

You don’t need 2 and 3. You can express them just with with T. For example, you could use an Either type for “good, here’s your value/bad, here’s your error.” If you want to get more sophisticated, you can add a type with extra error cases. In my own combinator libraries, I also include extra failure information to make diagnosing failed parses easy (and to generate pretty output). This does not change the fact that T is sufficient.