[Haskell] class instance lost during re-export & packaging

[Simon Marlow]

Conal Elliott wrote:
> I have a package P that contains a class instance specified in one 
> module P.M and re-exported in packaging module P (one that gathers & 
> re-exports for the whole P package).  When I import the package module P 
> in a test module, Under ghc-6.6, my instance is not found.  If the test 
> module also says "import P.M()", the instance is found.  Oddly, I only 
> have this problem with classes defined in a package other than P.

That sounds like a bug.  It might be an instance of the last bullet point here:

http://www.haskell.org/ghc/docs/latest/html/users_guide/bugs.html#bugs-ghc

but I suggest you open a ticket for it and we'll check.

> In ghci, after loading the module successfully, if I comment out the 
> "import P.M()" and re-load, the instance is found.

This one is definitely an instance of the known bug above.  GHCi just keeps 
collecting instances from packages and never throws any away.

Cheers,
	Simon