On Thu, Aug 19, 2004 at 05:42:10PM +0100, Sam Mason wrote:
Hi,
I've been getting into Haskell over the past few months and have just come up against a bit of a brick wall when trying to encapsulate some of the data structures in my code nicely. Basically what I want to have, is a type class where one of the intermediate values is opaque with respect to code using its implementations. This is a simplified version of what I'm trying to accomplish:
class Foo t where encode :: String -> t decode :: t -> String
instance Foo Int where encode = read decode = show
test = decode . encode
This currently fails, because the type checker insists on trying to figure out what its type should be - even though it shouldn't be needed.
You could "fix" it by type-annotating encode: test = decode . (encode :: String->Int) It makes sense that it doesn't work unannotated, since in the general case, you'd have more than one instance of Foo. Certainly you wouldn't want functions to stop working when you add one more instance. I think this sort of abstract manipulation is only possible with existential types. For example: data T = forall t.MkT (String -> t) (t -> String) x = MkT (read :: String->Int) show y = MkT (read :: String->Float) show test (MkT encode decode) = decode . encode --- *Main> test x "0" "0" *Main> test y "6.7" "6.7" -marius