On Fri, Sep 01, 2006 at 11:03:09AM +0100, Simon Marlow wrote:
Please test as much as possible, bugs are much cheaper if we find them before the release!
I was playing with impredicativity, when I got this strange error message: Prelude> :l Imp [1 of 1] Compiling Imp ( Imp.hs, interpreted ) Imp.hs:15:17: Couldn't match expected type `forall a. (Show a) => a -> String' against inferred type `a -> String' Expected type: forall a1. (Show a1) => a1 -> String Inferred type: forall a1. (Show a1) => a1 -> String In the second argument of `putMVar', namely `(show :: forall a. (Show a) => a -> String)' In the expression: putMVar var (show :: forall a. (Show a) => a -> String) Failed, modules loaded: none. I am still trying to understand this extension, so my code probably makes not much sense, but it's alarming that the compiler cannot unify two types that are even equal. Maybe the bug is in the error message? Here is the code: module Imp where import Control.Concurrent main = do var <- newEmptyMVar :: IO (MVar (forall a. Show a => a -> String)) let thread x = do forkIO $ sequence_ $ repeat $ do f <- takeMVar var putStrLn (f x) threadDelay 100000 thread (1 :: Integer) thread "abcdef" putMVar var (show :: forall a. Show a => a -> String) threadDelay 10000000 I am using ghc-6.5.20060831 with -fglasgow-exts Best regards Tomasz