On Thursday 10 March 2011 14:18:24, Anakim Border wrote:
Dear list,
I have the following (simplified) piece of code:
find :: Int -> [Int] find i = runST . (`runContT` return) $ callCC $ \escape -> do return []
which used to compile correctly under GHC 6.12.3.
Now that I've switched to 7.0.2 it gets rejected with the following error:
Couldn't match expected type `forall s. ST s c0' with actual type `m0 r0' Expected type: ContT r0 m0 a0 -> forall s. ST s c0 Actual type: ContT r0 m0 a0 -> m0 r0 In the second argument of `(.)', namely `(`runContT` return)' In the expression: runST . (`runContT` return)
I'm a little bit lost at what exactly is the problem.
If memory serves correctly, it's impredicative polymorphism. The type of (.), (b -> c) -> (a -> b) -> a -> c, can't handle (x -> forall s. ST s [Int]) Previously there was an implementation of impredicative polymorphism which allowed GHC to handle that construct, but that has been removed (because it was unsatisfactory), so GHC 7 doesn't compile that anymore.
Anyone can suggest a solution?
Parentheses. find i = runST ((`runContT` return) $ callCC $ \escape -> do return [])
Thanks!
AB