Feature Request: Make underlying representations accessible #171
Description
In the binary-serialise-cbor repo, there's been some discussion about the difficulty of being able to implement Serialise for Vectors which use more efficient representations of vectors than providing an array of structures in the encoded form. In well-typed/cborg#62 I had proposed making the underlying representation of Vectors accessible via an associated type (this isn't precise when compared to the current implementation but I hope gets the idea across):
class Vector v a where
type VRepr v a :: *
toVRepr :: v a -> VRepr v a
fromVRepr :: VRepr v a -> Maybe (v a)
... -- The rest of the current Vector class
instance Vector v Int where
type VRepr v Int = v Int -- this is certainly not correct
instance (Vector v a, Vector v b) => Vector v (a,b) where
type VRepr v (a,b) = (Vector v a, Vector v b)
-- Or perhaps?
-- type VRepr v (a,b) = (VRepr v a, VRepr v b)
toVRepr (V_M2 _ va vb) = (va,vb) -- AKA unzip
...Which allows for defining instances for the primitive types and instances for things like (Serialise a, Serialise b) => Serialise (a,b) and we get the instances for Serialise (a,b), Serialise (Complex a) etc. essentially for free.
This is something I've run into a few times, where I wanted to take advantage of the underlying array of structures representation of Vectors without needing to know exactly which constructor was being used.
This is (obviously) not a very thorough proposal, I'm making it to spark discussion, and I'll be happy if the result is "no, because ... impossible ...", or "yes, we'll do this and change the major version".