The gather function fails to memoise some actions

[viercc]

Memorizing an action to compute a field of the HKD type t is done only when it's queried using hitchF. Because gather calls actions directly, evaluating gather can perform the action for a field up to 2 times (the field itself and as a dependency via hitchF.)

Unless it's going to be removed, I think one way to get rid of the duplications in gather is requiring a record of lenses to the field itself, like this:

newtype LensB t a = LensB
  { toVLLens :: forall g h. Functor g => (h a -> g (h a)) -> t h -> g (t h) }

class HasSelfLenses t where
    selfLenses :: t (LensB t)

gather :: (TraversableB t, HasSelfLenses t, Monad m) => TangleFT t f m (t f)
gather = btraverse (\l -> hitchF (toVLLens l)) selfLenses

HasSelfLenses is very ad hoc of course. I'm not very confident, but they seem to be related to Representable.

class RepresentableB (t :: (k -> Type) -> Type) where
    data Key t :: k -> Type
    bindex :: Key t a -> t f -> f a
    btabulate :: (forall a. Key t a -> f a) -> t f

[fumieval]

Good point. barbies-th has AccessosB so I guess it's easiest to use it.