Re: Field accessor type inference woes

On 02/07/13 10:57, Simon Peyton-Jones wrote:

Here is Plan A: use fundep (or type function)

class Has r f t | r f -> t where getFld :: r -> t

instance Has (R a) “foo” (a -> a) where .. instance Has S “bar” (forall b. b -> b) where ...

Lacking (as we still do) impredicative polymorphism, the S instance declaration is rejected.

How common are such polymorphic fields in practice? You sometimes see them in newtype wrappers and the like. But I think those are not cases where you want overlapping names anyway. So: why not use a Plan A style class, except for polymorphic fields? Perhaps you could still have a (less polymorphic) class for bar above, class HasBar r where getBar :: r -> b -> b Twan