Aha. That's a bug. The forall'd type variables of an expression type signature should scope over the expression. Thank you -- I'll fix it, though I don't know if it'll get into 6.6 now. Simon | -----Original Message----- | From: Mirko Rahn [mailto:rahn@ira.uka.de] | Sent: 02 October 2006 16:05 | To: Simon Peyton-Jones | Cc: glasgow-haskell-users@haskell.org | Subject: Re: Problem with lexically scoped type variables. | | | > Oh I see. Well, you'd need to put the type annotation for s back in: | > | > t1 = runST ( (trav f [1..10] (1,52) >>= \ (s::STRef s (Set Int)) -> seen | > s) | > :: forall s. ST s [Int] ) | | ...and reach the starting point again. This version gives the same error | message as the original one. | | But what works is to give some more detailed hints to the type system: | | t1S = trav f [1..10] (1,52) :: ST s (STRef s (Set Int)) | t2S = trav f [1..10] (1,52) :: ST s (STUArray s Int Bool) | | t1 = runST ( t1S >>= seen ) | t2 = runST ( t2S >>= seen ) | | This compiles (and works). | | Thank's for advice, MR | | -- | -- Mirko Rahn -- Tel +49-721 608 7504 -- | --- http://liinwww.ira.uka.de/~rahn/ ---