Yes, I knew that's where you were going :-)

The trouble is, you'd still have to deal with

fail :: Monad m => String -> m a

which would require your failure type to somehow accept a string. Plumbing that in would be a little more awkward than your initial exporations suggest :-\

You have two problems. The first is how to construct a value of your failure type that accepts a String parameter so that you can support users of "fail". The second is that you might need to pass extra information to construct your failure value when naïve code uses fail or mzero, otherwise you will only get useful error values out quite infrequently.

 

Even though those changes represent a major modification to the internals of attoparsec, they shouldn't really affect what you want to do, or anything interesting about how to do it.