let rec fact n =
  if n < 1 then 1
  else n * (fact (n-1))

let x = fact 5;;
(*
The double semicolon is needed above.

This may be related:
http://ocamlverse.net/content/faq_if_semicolon.html
*)
Printf.printf "%d" x
(* 120 *)

(*************************************************)

(* https://github.com/wiredsister/example-ocaml/blob/master/examples/factorial.ml *)
let rec fact2 = function
  | 0          -> 1
  | n when n>1 -> n * (fact2 (n-1))
  | _          -> 0;;

Printf.printf "%d" (fact 6)
(* 720 *)