The factorial function using hylomorphism in OCaml.

fact.ml

let rec unfold p g b =
  if p b then [] else
    (match g b with (a, bprime) ->
       a :: unfold p g bprime)

let fact n =
  List.fold_right 
    (* The fold's function input is the times function.*) 
    (fun x y -> x * y)  
    (* The fold's list input is the result of this unfold. *)
    (unfold 
      (* The unfold function's predicate to stop is n=0. *)
      (fun n -> (n = 0)) 
      (* The unfold's g n = (n, n-1) *)
      (fun n -> (n, (n - 1)))
      (* The unfold's input is n, the input to the fact function. *)
       n)   
     (* The fold's base case is one. *)
     1
     ;;