Field accessor type inference woes
Twan van Laarhoven
twanvl at gmail.com
Tue Jul 2 16:26:07 CEST 2013
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
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