Someone on the ghc-users list might have an idea. I recommend filing a bug report:
http://hackage.haskell.org/trac/ghc/wiki/ReportABug Even if the behavior isn't wrong, in could use a better error message! Antoine 2011/4/14 Michal Konečný <mikkone...@googlemail.com>: > Hi, > > I have stumbled across some strange behaviour in ghc7. > > The following compiles fine with ghc 6.12.3: > > {-# LANGUAGE ImplicitParams #-} > {-# LANGUAGE TypeFamilies #-} > {-# LANGUAGE NoMonomorphismRestriction #-} > > module Test where > > class C t where > type TF t > ttt :: TF t -> t > > b :: (C t, ?x :: TF t) => t > b = ttt ?x > > but ghc7 says: > > Could not deduce (?x::TF t) > arising from a use of implicit parameter `?x' > from the context (C t, ?x::TF t) > bound by the type signature for b :: (C t, ?x::TF t) => t > at Test.hs:13:1-10 > In the first argument of `ttt', namely `?x' > In the expression: ttt ?x > In an equation for `b': b = ttt ?x > > Moreover, when I comment out the type declaration for b, it compiles and the > inferred type for b is identical to the one in the above program: > > *Test> :t b > b :: (C t, ?x::TF t) => t > > It feels to me like a bug but I am not entirely confident. Any ideas? > > Michal > -- > |o| Michal Konecny <mikkone...@gmail.com> > |o| http://www-users.aston.ac.uk/~konecnym/ > |o| office: (+42) (0)121 204 3462 > |o| PGP key http://www-users.aston.ac.uk/~konecnym/ki.aston > > -- > |o| Michal Konecny <mikkone...@gmail.com> > |o| http://www-users.aston.ac.uk/~konecnym/ > |o| office: (+42) (0)121 204 3462 > |o| PGP key http://www-users.aston.ac.uk/~konecnym/ki.aston > > -- > |o| Michal Konecny <mikkone...@gmail.com> > |o| http://www-users.aston.ac.uk/~konecnym/ > |o| office: (+42) (0)121 204 3462 > |o| PGP key http://www-users.aston.ac.uk/~konecnym/ki.aston > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe