ErrorT is similar to EitherT, so I just used the latter to make the names less confusing. Either doesn't have to be "stringly typed" per se thanks to sum types

ErrorT is similar to EitherT, so I just used the latter to make the names less confusing. Either doesn't have to be "stringly typed" per se thanks to sum types. You can write a type like: data ErrorType = SomeError | OtherError String | ... The compiler can now check that you handle all possible errors and will give you a warning if you don't, so it's strictly better than using strings. Coincidentally, this is one place where some sort of sub-typing would be nice, I think. This sort of type would be perfect using OCaml's polymorphic variants because then you could share different error cases around while specifying a very specific type for each computation.