Consistent error handling is needed if you are returning 0 for success and -1 for failure. Or was it 1 for success and 0 for failure? But there's nothing wrong with a type signature that precisely describes what you can get back. Only argument I'd have with Either is it's not obvious which is the error (although left is the convention and baked into the monad), although in practice you'll be returning "Either ParseError ParseTree" which makes it rather obvious. Also, do-notation over the Maybe monad is just perfect for simple error handling, but sometimes you need to return more than None. Happily, you can switch to Either without changing a great deal. But you can't mix them easily, and that is a suckful thing about monads indeed.