[Haskell-cafe] mapTuple

[Grady Lemoine]

I knew there must be a way in GHC to do that second example!

As for the third example, it might be a slick way to do some
super-hyper-refactoring, but I admit I can't think of anything it
would be actually necessary for offhand.

--Grady

On 1/12/07, Cale Gibbard <cgibbard at gmail.com> wrote:
> On 12/01/07, Grady Lemoine <grady.lemoine at gmail.com> wrote:
> > Is there anything in particular you're trying to accomplish?  It seems
> > like this is the type of thing you'd accomplish with typeclasses if
> > you had a less general problem than you've presented.  For example,
> >
> > > mapShowTuple :: (Show a, Show b) => (a, b) -> (String, String)
> > > mapShowTuple (x, y) = (show x, show y)
> >
> > That said, it would be nice do be able to do something a little more
> > general, but still with a specific typeclass, like
> >
> > > mapNumOpTuple :: (Num a, Num b) => (Num c => c -> c) -> (a, b) -> (a, b)
> > > mapNumOpTuple f (x, y) = (f x, f y)
> >
> > (This unfortunately fails to typecheck; GHC chokes with "Illegal
> > polymorphic or qualified type".  On the other hand, I'm still pretty
> > new to Haskell myself, so maybe someone else knows how to write this
> > correctly without doing complex type hackery.)
>
> It's close:
>
> {-# OPTIONS_GHC -fglasgow-exts #-}
>
> mapNumOpPair :: (Num a, Num b) => (forall c. Num c => c -> c) -> (a, b) -> (a,b)
> mapNumOpPair f (x,y) = (f x, f y)
>
> > It would also be nice to be able to generalize over all typeclasses,
> > e.g. (pseudo-code here)
> >
> > mapTypeclassOpTuple :: for all typeclasses C ((C a, C b) => (C c => c
> > -> c) -> (a, b) -> (a, b))
> >
> > but I don't even know how that would fit into Haskell's syntax.  I
> > suspect it's an idea that's been discussed, and I just don't know the
> > term for it.
>
> That's an interesting idea, typeclass variables. It would require a
> bit of a kind system for typeclasses, but that's not so hard. I
> somehow doubt it would get used all *that* much though. Can anyone
> think of a clever way to apply this?
>
>  - Cale
>