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
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