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Comment by sweeneyrod | Hacker News Reader

How? What are the categories that OCaml's Hashtbl.Make maps between? What are the morphisms in those categories and how are they mapped?

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It maps between a category with HashedType implementations as objects and a category with Make implementations as objects. Unfortunately it's not a very interesting functor because to satisfy the functor laws I think the categories have to be discrete. It's basically a function! (And functions are indeed a special case of functors.)

https://caml.inria.fr/pub/docs/manual-ocaml/libref/Hashtbl.M...

Right, in other words it is just a function. I don't think the categories have to be discrete just to satisfy the functor laws; even ignoring that, what would the morphisms be in principle? I don't think there is an obvious choice, at least.

I think in OCaml, "functor" basically just means "like a function but for modules". They might technically be categorical functors, but it seems quite different to the meaning in Haskell where they are plainly categorical functors (modulo Hask technically not being a category).

Agreed, I'm not sure it's a terribly interesting functor.

Here is one way one might implement a categorical Functor (https://www.reddit.com/r/haskell/comments/eoo16m/base_catego...).

A function S -> T maps a S(ource) type to a T(arget) type, like FunctorOf (-S>) (-T>) does between the source category (-S>) and target category (-T>)

    type  Functor :: forall (s :: Type) (t :: Type). (s -> t) -> Constraint
    class (Category (Src f), Category (Tgt f)) => Functor (f :: s -> t) where
      type Src (f :: s -> t) :: Cat s
      type Tgt (f :: s -> t) :: Cat t
      fmap :: Src f a1 a2 -> Tgt f (f a1) (f a2)

    type FunctorOf :: forall (s :: Type) (t :: Type). Cat s -> Cat t -> (s -> t) -> Constraint
    type FunctorOf src tgt f = (Functor f, Src f ~ src, Tgt f ~ tgt)

The usual endofunctor

    type EndofunctorOf :: forall (ob :: Type). Cat ob -> (ob -> ob) -> Constraint
    type EndofunctorOf @ob cat f = FuntorOf @ob @ob cat cat f

we have in Haskell can be defined as FunctorOf @Type @Type (->) (->), or

    type OldFunctor :: (Type -> Type) -> Constraint
    type OldFunctor f = EndofunctorOf @Type (->) f

For simplicity written with a type synonym, but it cannot be partially applied. So I would use constraint synonym encoding (https://gist.github.com/Icelandjack/5afdaa32f41adf3204ef9025...)

    type FunctorOf :: forall (s :: Type) (t :: Type). Cat s -> Cat t -> Constraint

    class    (Functor f, Src f ~ src, Tgt f ~ tgt) => FunctorOf src tgt f
    instance (Functor f, Src f ~ src, Tgt f ~ tgt) => FunctorOf src tgt f

The other definitions can be eta reduced

    type EndofunctorOf cat = FunctorOf cat cat
    type OldFunctor        = EndofunctorOf (->)

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