ANNOUNCE: GHC 6.6 Release Candidate

[Tomasz Zielonka]

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