Now fixed in the HEAD, and will be in 5.04.2 Thanks for pointing it out. Simon | -----Original Message----- | From: Jorge Adriano [mailto:jadrian@mat.uc.pt] | Sent: 14 November 2002 21:10 | To: Iavor S. Diatchki | Cc: Haskell Cafe; glasgow-haskell-users@haskell.org | Subject: Bug? [was: Implicit params] | | On Thursday 14 November 2002 18:47, Iavor S. Diatchki wrote: | > hello, | > | > > Well, actually you must be right since the pure field defines a pure | > > (projection) function... Hmmm, ok, can someone explain this to me, | > > | > > data E s = E{ | > > refi :: STRef s Int, | > > refc :: STRef s Char, | > > m :: Int | > > } | > > | > > -- this is fine, obviously... | > > pure :: E s -> Int | > > pure e = m e | > > | > > -- but this is not... | > > pure2 :: (?e :: E s) => Int | > > pure2 = m (?e) | > > | > > Why exactly isn't this allowed? What is the workaround? | > > Error msg: | > > ------------------------------------------------------------------------ | > > Ambiguous constraint `?e :: E s' | > > At least one of the forall'd type variables mentioned by the | > > constraint | > > must be reachable from the type after the '=>' | > > In the type: forall s. (?e :: E s) => Int | > > While checking the type signature for `pure2' | > > Failed, modules loaded: none. | > > ------------------------------------------------------------------------ - | > >- | > | > this seems like a bug in GHC. Hugs is happy with it. The "s" in the | > pure2 signature is not ambiguous because it is determined when you give | > the value of the implicit parameter. in fact the way i think about | > implicit parameters is simply as a nice notation for computations in the | > environment monad, so in my mind the above two definitions are pretty | > much the same. | | Thanks Iavor! | GHC people, can you confirm this, Is it a bug? I'm using Ghc 5.0.4 SuSE rpms. | Is there a workaround? | | Thanks, | J.A. | | | | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users@haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users