[Haskell-cafe] Fundeps and overlapping instances

[Gábor Lehel]

On Tue, Jun 5, 2012 at 2:25 PM, Gábor Lehel <illissius at gmail.com> wrote:
> The encoded version would be:
>
> instance (f a b) => FMap f (HJust a) (HJust b)
>  where type f :<$>: (HJust a) = HJust b
>
> and I think this actually demonstrates a *different* limitation, namely that
>
>> The RHS of an associated type declaration mentions type variable `b'
>>   All such variables must be bound on the LHS
>
> which means that the standard encoding doesn't work for this case.