On 09/02/06, Brian Hulley
Brian Hulley wrote:
Brian Hulley wrote:
Brian Hulley wrote:
f :: (forall a m. a -> m a) -> c -> d -> (m c, m d)
The above is wrong - there is no way to quantify m properly. This must be why intersection types need to be written with "&" after all....
What am I saying! It's right after all, and might be better than the & syntax because it makes the dependency clearer (assuming you can't write a function that is both Int->String and Float->Int)
Last correction!
f :: forall m. (forall a. a->m a) -> c -> d -> (m c, m d)
Sorry for all this confusion.
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") --- - Cale