[haskell-llvm] (StructFields s) vs. IsType (Struct s)

[Henning Thielemann]

On Wed, 28 Sep 2011, Henning Thielemann wrote:

> Maybe this is a logical problem:
>
> GHC says
>
>    Could not deduce (llvm-0.10.0.0:LLVM.Core.Type.StructFields s)
>      from the context (IsType (Struct s), ...)
>    ...
>    Possible fix:
>      add (llvm-0.10.0.0:LLVM.Core.Type.StructFields s) to the context ...
>
>
> I could just add the (StructFields s) constraint, but StructFields is not 
> exported. I thought that adding (IsType (Struct s)) is also ok, since the 
> llvm package exports the instance:
>   StructFields s => IsType (Struct s)

Ok, the problem was, that IsFirstClass (Struct s) was needed and there is 
an instance:
   StructFields s => IsFirstClass (Struct s)
  Thus GHC suggested (StructFields s).

I think that GHC does not deduce (StructFields s) from IsType (Struct s), 
because there might be overlapping instances. If it would, it could derive
   IsType (Struct s) -> StructFields s -> IsFirstClass (Struct s)