#2309: containers: specialize functions that fail in a Monad to Maybe

[Duncan Coutts]

On Wed, 2008-05-28 at 11:11 +0100, Conor McBride wrote:
> > Ross Paterson wrote:
> >> Several functions on containers used to have types like
> >> 	lookup :: (Ord k) => k -> Map k a -> Maybe a
> >> but these were generalized to
> >> 	lookup :: (Monad m, Ord k) => k -> Map k a -> m a
> >> The proposal is to change these signatures back, specializing them
> >> to Maybe.
> >
> > I'm all for Maybe in the question Monad VS Maybe.
> 
> Me too. (+1) for Ross's proposal.

If we need any more (+1) votes you can count mine.

> > Also, it's not entirely clear whether MonadZero is the most general  
> > abstraction. Is ApplicativeZero better? Or something else?
> 
> Maybe is the most general abstraction. Requiring
> (>>=), or even (<*>) seems excessive. What we need
> is "any f with zero and return", so why not pick
> the canonical, initial, inductively defined such
> thing?

Good. I like that argument.

As others have pointed out, it's easy to lift maybe into something else
when that's needed.

maybe :: a -> (b -> a) -> Maybe b -> a

Duncan