In article <DE924A789E3C124F8C8A319F273357E7014E30CF@lon-msg-01.europe.corp.microso ft.com>, "Simon Peyton-Jones" <simonpj@microsoft.com> wrote:
Here's a less complicated variant of the same problem:
class C a b | a -> b where {}
instance C Int Int where {}
f :: (C Int b) => Int -> b f x = x
Is the defn of f legal? Both GHC and Hugs reject it because the inferred type of f is more like C Int Int => Int -> Int
If this were allowed, it would effectively allow type-lambda. For instance, I have a type function T that maps Int to Bool and Bool to Char: class C a b | a -> b instance C Int Bool instance C Bool Char newtype T a = MkT (forall b.(C a b) => b) helperIn :: (forall b.(C a b) => b) -> T a helperIn b = MkT b; -- currently won't work helperOut :: T a -> (forall b.(C a b) => b) helperOut (MkT b) = b; Here T is a type-constructor that does that. If I like, I can represent Char as "T (T Int)", though of course I need to use the helper functions to actually use it as a Char. -- Ashley Yakeley, Seattle WA