Cale Gibbard wrote:
On 09/02/06, Brian Hulley <[EMAIL PROTECTED]> wrote:
Brian Hulley wrote:
f :: forall m. (forall a. a->m a) -> c -> d -> (m c, m d)
Of course this type doesn't work on your original example, since (,)
is a type constructor with two parameters, not one, but this type
signature does actually work just fine in GHC.
---
data Pair x = Pair x x deriving Show
data Id x = Id x deriving Show
f :: forall m c d. (forall a. a -> m a) -> c -> d -> (m c, m d)
f g x y = (g x, g y)
---
*Main> f (\x -> Pair x x) 3 "hello"
(Pair 3 3,Pair "hello" "hello")
*Main> f (\x -> Id x) 3 "hello"
(Id 3,Id "hello")
Perhaps in practice this kind of workaround is sufficient, although this
requires you to put the results into a form that can be syntactically
matched against m a.
I wonder though if it would be possible to have a type system that would
work for my original example by automatically creating internal declarations
type T1 a = (a,a) and type T2 a = a so that m would match against T1 and T2
respectively.
It is difficult to know if this would be a worthwhile thing to do or if most
practical applications of polymorphism in any case involve transformations
whose shape can be captured sufficiently with multi-param typeclasses etc.
Regards, Brian
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