Haskell-style applicative Fold in OCaml

fold.ml

module Fold = struct
  module T = struct
    type ('a, 'e) t = Fold : ('x -> 'e -> 'x) * 'x * ('x -> 'a) -> ('a, 'e) t

    let make ~step ~init ~finish = Fold (step, init, finish)

    let map =
      `Custom
        (fun (Fold (step, init, finish)) ~f ->
          Fold (step, init, fun x -> finish x |> f))

    let return a = Fold ((fun () _ -> ()), (), fun () -> a)

    let apply (Fold (step1, init1, finish1)) (Fold (step2, init2, finish2)) =
      let step (x1, x2) e = (step1 x1 e, step2 x2 e) in
      let init = (init1, init2) in
      let finish (x1, x2) = finish1 x1 (finish2 x2) in
      Fold (step, init, finish)
  end

  include T
  include Applicative.Make2 (T)
end