Johannes Waldmann <[EMAIL PROTECTED]> wrote:
> > In Haskell, this is called 'fmap'. :-)
>
> OK, then show me an "instance Functor Set" so that I can use it :-)
instance Function Set where
fmap = Data.Set.mapMonotonic
Ok, so this introduces a precondition on the function being mapped, so
there is a proof obligation on the programmer.
But if contexts-on-datatypes worked correctly,
data Set a = Ord a => ....
then even the "real" map from Data.Set:
map :: (Ord a, Ord b) => (a -> b) -> Set a -> Set b
could be an instance method of Functor. (Because the Ord constraints
would be packaged inside the Set type, rather than needing to be
explicit.)
Regards,
Malcolm
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