So what do I do if I want to write a function that takes another function as a parameter, and some of the possible functions use Maybe and some use Either? What would this code look like?
Maybe and Either are both monads and by convention Left errCode is the fail method of the Either monad. This will work exactly as you think it should. It will also work with, e.g., io actions that might fail.
If I understand you correctly, such a possibility can never arise in Haskell's type system. A possible function would be (a -> Maybe b) -> a -> b, and this won't accept anything that returns an Either.
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So what do I do if I want to write a function that takes another function as a parameter, and some of the possible functions use Maybe and some use Either? What would this code look like?
f: (a -> m b) -> ... -> m Result
Maybe and Either are both monads and by convention Left errCode is the fail method of the Either monad. This will work exactly as you think it should. It will also work with, e.g., io actions that might fail.
If I understand you correctly, such a possibility can never arise in Haskell's type system. A possible function would be (a -> Maybe b) -> a -> b, and this won't accept anything that returns an Either.