#include "share/atspre_staload.hats"
#include "share/HATS/atslib_staload_libats_libc.hats"

fnx fact_boring(n : int) : int =
  case+ n of
    | 0 => 1
    | n => n * fact_boring(n - 1)

(* fnx collatz{n:nat} *)
(* (n: int(n)) : int = let *)
(* fun loop{n:nat}{l:addr} .<n>. *)
(* (pf: !int @ l | n: int n, res: ptr l) : void = *)
(* if n > 1  *)
// TODO rewrite this for collatz?
fnx fact {n : nat} (n : int(n)) : int =
  let
    fun loop {n : nat}{l : addr} .<n>. ( pf : !int @ l | n : int(n)
                                       , res : ptr(l)
                                       ) : void =
      if n > 0 then
        let
          val () = !res := n * !res
        in
          loop(pf | n - 1, res)
        end
    
    // end of [loop]
    var res: int with pf = 1
    val () = loop(pf | n, addr@res)
  in
    res
  end

implement main0 () =
  let
    val x = fact(30)
  in
    println!(tostring_int(x))
  end