David Roundy
i would say that if you are wanting to report errors to users, you should not use "fail" or "error". you should instead explicitly report the error. when i program i typically use "error" in situations that i think are imposible, but if there is a bug one gets a better error messeage. i don't use fail (well except sometimes implicitly in list comprahensions). the reason i avoid "fail" is that it seems hackish to me -- it implies that every monad supports errors ^^^^^^^^^^^^^^^^^^^^^^^^^^^
In Haskell, this is indeed the case---every monad is after all a monad transformer applied to Haskell's identity monad, which supports errors (hence the existence of Haskell's `error' function :). So even if a monad designer doesn't explicitly add error support, the monad still supports errors. Jon Cast