map and fmap

[Jeffrey Yasskin]

On 8/20/06, John Hughes <rjmh at cs.chalmers.se> wrote:
>
> From: "Jon Fairbairn" <jon.fairbairn at cl.cam.ac.uk>
>
> > To
> > reinforce what Aaron said, if a programme works now, it'll
> > still work if map suddenly means fmap.
>
> Well, this isn't quite true, is it? Here's an example:
>
> class Foldable f where
>   fold :: (a -> a -> a) -> a -> f a -> a
>
> instance Foldable [] where
>   fold = foldr
>
> example = fold (+) 0 (map (+1) (return 2))
>
> example has the value 3 (of course), but if you replace map by fmap then the
> code no longer compiles.

There's a proposal
http://hackage.haskell.org/trac/haskell-prime/wiki/Defaulting that
mentions extending defaulting to other typeclasses. That seems to fix
this particular problem, but above you mentioned that this was "a
whole new can of worms." Could you elaborate or point me to a
discussion of the worms?

Thanks,
Jeffrey