Excellent example. It's very hard to give good error messages for impredicative polymorphism. I've tried to improve this one a bit. (Test is tcfail165.hs) Simon | -----Original Message----- | From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users- | bounces@haskell.org] On Behalf Of Tomasz Zielonka | Sent: 01 September 2006 19:55 | To: Simon Marlow | Cc: glasgow-haskell-users@haskell.org | Subject: Re: ANNOUNCE: GHC 6.6 Release Candidate | | 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 | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users@haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users