bug with type families

[Peter Gavin]

Stefan Holdermans wrote:
> 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?
> 

No, that makes perfect sense to me now. Thanks.

I put up a note on #1897 pointing to that thread, but left it open 
because the error message needs to be fixed :)


Pete