IsoHKD for composite's field type.

[locallycompact]

Hi. I'm working downstream at https://github.com/ConferOpenSource/composite and trying to use the :-> operator in XRec expressions. I was able to get currying working by defining.

instance KnownSymbol s => IsoHKD Identity (s :-> a) where
  type HKD Identity (s :-> a) = a
  unHKD = Identity . Val
  toHKD (Identity (Val x)) = x

However this isn't particularly good, because this is against a different Identity declaration so it coincidentally doesn't conflict. However if I try to do for example this HKD which is what I'm using.

instance Functor m => IsoHKD (ReaderT r m) a where
  type HKD (ReaderT r m) a = r -> m a
  unHKD f = ReaderT $ f
  toHKD (ReaderT f) = f

instance Functor m => IsoHKD (ReaderT r m) (s :-> a) where
  type HKD (ReaderT r m) (s :-> a) = r -> m a
  unHKD f = ReaderT $ f
  toHKD (ReaderT f) = f

Then I get an overlapping instance error.

/home/lc/Source/gitlab.com/shakebook-site/shakebook-lib/test/Spec.hs:90:8: error:
    Conflicting family instance declarations:
      HKD (ReaderT r m) a = r -> m a -- Defined at test/Spec.hs:90:8
      HKD (ReaderT r m) (s :-> a) = r -> m a
        -- Defined at test/Spec.hs:95:8
   |        
90 |   type HKD (ReaderT r m) a = r -> m a

I can't simply copy the ELField example because the constructor for :-> is not a pair, and is defined like

newtype (:->) (s :: Symbol) a = Val { getVal :: a }

So defining

instance KnownSymbol s => IsoHKD ((:->) s) a where
  type HKD ((:->) s) a = a        
  unHKD x = Val x
  toHKD (Val x) = x

isn't actually the right behaviour because the type is not actually part of the functor, it's part of the record field. Is there a right way to do this?

[acowley]

This looks similar to cases where we use something like a type family PayloadType. Iirc, the idea there is specifically to deal with this kind of problem by special casing ElField because of how it involves complicating the record field type.