[Haskell-beginners] Maybe, Either

[Brent Yorgey]

On Sat, Sep 12, 2009 at 09:13:58PM +0300, Michael Snoyman wrote:
> I often times have to write a lookup function that returns its value into
> any monad instead of just Maybe. For example:
> 
> mLookup :: (Eq k, Monad m) => k -> [(k, v)] -> m v
> mLookup k pairs = case lookup k pairs of
>                                 Nothing -> fail "mLookup: nothing found"
>                                 Just v -> return v
> 

This is actually the type that the lookup function USED to have, but
it was changed since monads actually have nothing to do with failing
(the fail method is just a hack used to handle pattern-match failures
in do-notation).  Probably a better implementation of this would be

  mLookup :: (Eq k, MonadPlus m) => k -> [(k,v)] -> m v
  mLookup k pairs = maybe mzero return (lookup k pairs)

-Brent