On 12/01/07, Grady Lemoine <[EMAIL PROTECTED]> 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
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