[Haskell-cafe] ghc7 bug? associated type + implicit parameters

[Michal Konečný]

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 at 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 at 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 at 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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