From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Jules Bean
Krasimir Angelov wrote:
The monads design is used in Data.Map i.e.
lookup :: (Monad m, Ord k) => k -> Map k a -> m a
which is widely considered a poor design decision and a wart on Data.Map.
Seriously, if you don't return a useful error message, then Maybe is as good as it gets, why not use it?
(And there really is only one kind of error possible here, in each case)
'fail' only adds information in the case it has a useful error message (in which case I'd encourage MonadError m =>)
Is there a reason not to recommend Either, if you want to return error information? It is an instance of MonadError. Alistair ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************