bug with type families

[Stefan Holdermans]

Peter,

> I've run into a bug that looks to be the same as the one described  
> here:
>
> http://hackage.haskell.org/trac/ghc/ticket/1897

It does not seem like a bug, although the type-error message may be a  
bit confusing as is the fact that GHC happily infers a type for the  
signature-less function: these are I guess the real issues here. But,  
apart from that, the contents of a previous discussion seems to apply  
here:

   http://www.haskell.org/pipermail/haskell-cafe/2008-April/041385.html

Short version: the type of isValid involves |Depend s| and not |s|.  
Since |Depend s| does not uniquely determine |s|, it is not possible  
to resolve which dictionary for Bug to supply for a particular call to  
isValid.

Or---am I overlooking something?

Cheers,

   Stefan