Field accessor type inference woes

[AntC]

> Simon Peyton-Jones <simonpj <at> microsoft.com> writes:
> 
>  ...; it’s a tradeoff between polymorphism and overloading.
>  

Hah! my post crossed with Simon's. This time I'll be succinct.
There's **three** alternatives. ...

>
>           data R a = MkR { foo :: a -> a }
>           data S    = MkS { bar :: forall b. b -> b }
>  

Try Plan C: use a cleverer (associated) type function

    class Has r f t    where
       type GetResult r f t :: *               -- ?? default to t
       getFld :: r -> GetResult r f t

    instance (t ~ a->a) => Has (R a) “foo” t where
       type GetResult (R a) "foo" t = a -> a   -- ?? ignore t
       getFld ...
    instance (t ~ b->b) => Has S “bar” t where 
       type GetResult S "bar" t = t      -- 'improved' t
       getFld ...

In the 'chained' accessors that Edward raises,
I think the presence of the type function 'fools' type inference into 
thinking there's a dependency.

So (foo . bar) has type (and abusing notation):

    ( Has r "bar" t_bar, Has (GetResult r "bar" t_bar) "foo" t_foo )
     => r -> (GetResult (GetResult r "bar" t_bar) "foo" t_foo)