Use 'case'. Instead of | let MkStateT m2 = k a | m2 s1 ) say case k a of MkStateT m2 -> m2 s1 Simon | -----Original Message----- | From: mathieu [mailto:tenfingers@ifrance.com] | Sent: 10 October 2002 21:42 | To: ghc users | Subject: brain explosion in polymorphic state monad | | Hello, | | I am trying to define a polymorphic state monad using glasgow extensions and I got a | brain explosion of ghc when i try to compile it. | | Here is the code : | | newtype StateT s m a = MkStateT (s -> m (a, s)) | | instance Monad m => Monad (StateT s m) where | return x = MkStateT (\s -> return (x, s)) | MkStateT m1 >>= k = | MkStateT | (\s0 -> do (a, s1) <- m1 s0 | let MkStateT m2 = k a | m2 s1 ) | | data Thread a = forall b . MkThread (StateT (Thread b) [] a) | | instance Monad Thread where | return = MkThread . return | MkThread p >>= k = MkThread ( do x <- p | let MkThread p' = k x | p' ) | | I got this error : | My brain just exploded. | I can't handle pattern bindings for existentially-quantified constructors. | In the binding group | MkThread p' = k x | In the first argument of `MkThread', namely | `(do | x <- p | let MkThread p' = k x | p')' | In the definition of `>>=': | MkThread (do | x <- p | let MkThread p' = k x | p') | | How can i define (>>=) for my thread monad ? | | Thanks in advance for any piece of advice, | Mathieu | | -- | There are only 10 types of people in the world: | Those who understand binary and those who don't. | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users@haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users