hello, well that's because haskell is not as pure as it claims to be :-) if every monad supports errors depends on what you mean by "supporting errors". i would argue that supporting errors and divergence is not the same thing. the difference is that one should be able to handle an error thrown by a computation, while clearly we cannot detect nonterminating computations. bye iavor Jon Cast wrote:

David Roundy wrote:

...

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.

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-- ================================================== | Iavor S. Diatchki, Ph.D. student | | Department of Computer Science and Engineering | | School of OGI at OHSU | | http://www.cse.ogi.edu/~diatchki | ==================================================