On Mon, Aug 19, 2013 at 1:48 PM, <jabolo...@google.com> wrote: > What is the proper way to implement a non-monadic function that checks > whether a given value is correct and gives a proper error message > otherwise ? What is the recommended option ? > > * Either String a >
Preferred, usually, since Nothing is regarded as an error condition of sorts: the Monad instance for Maybe associates Nothing with `fail`, which is invoked on failed pattern matches; likewise it's mzero for MonadPlus and mempty for Monoid, both of which use it (differently) to reflect certain "failure" scenarios). If nothing else, it would be highly confusing to see Nothing associated with success given its widespread association with failure. Alternatively, have you considered using your own ADT? `data Validity = Success | Failure String` would give you more readable / comprehensible code without needing to worry about assumptions or common usage. -- brandon s allbery kf8nh sine nomine associates allber...@gmail.com ballb...@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net
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