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 <mikkonecny@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 <mikkonecny@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 <mikkonecny@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 <mikkonecny@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

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