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 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe