N Queen problem in OCaml

nqueen.ml

let ($) f g = f g

let safe n q qs =
  let (qx, qy) = q in
  if List.exists begin
      fun (x, y) -> qx = x || qy = y
    end qs then false
  else
    let rec row_iter iy =
      if iy >= n then true
      else
        let diff = iy - qy in
        if List.exists begin
          fun (x, y) -> 
            iy = y && (qx + diff = x || qx - diff = x)
        end qs then false
        else row_iter $ iy + 1
    in
    row_iter 0

let find n =
  let rec iter_row y qss =
    if y >= n then qss
    else
      iter_row (y + 1) $
        List.fold_left begin
          fun new_qss qs ->
            let rec iter_col x qss =
              if x >= n then qss
              else
                let q = (x, y) in
                iter_col (x + 1) $ if safe n q qs then (q::qs)::qss else qss
            in
            iter_col 0 new_qss
        end [] qss
  in
  iter_row 0 [[]]

let _ = 
  List.iter begin
    fun qs ->
      print_endline "------------------------------------------------------";
      List.iter begin
        fun (x, y) -> Printf.printf "(%d, %d) " x y
      end qs;
      print_newline ()
  end $ find 8

results

$ ocaml nqueen.ml
------------------------------------------------------
(2, 7) (4, 6) (1, 5) (7, 4) (5, 3) (3, 2) (6, 1) (0, 0)
------------------------------------------------------
(2, 7) (5, 6) (3, 5) (1, 4) (7, 3) (4, 2) (6, 1) (0, 0)
------------------------------------------------------
(4, 7) (1, 6) (3, 5) (6, 4) (2, 3) (7, 2) (5, 1) (0, 0)
------------------------------------------------------
(3, 7) (1, 6) (6, 5) (2, 4) (5, 3) (7, 2) (4, 1) (0, 0)
------------------------------------------------------
            :
            :
------------------------------------------------------
(4, 7) (6, 6) (1, 5) (5, 4) (2, 3) (0, 2) (3, 1) (7, 0)
------------------------------------------------------
(3, 7) (6, 6) (4, 5) (1, 4) (5, 3) (0, 2) (2, 1) (7, 0)
------------------------------------------------------
(5, 7) (2, 6) (4, 5) (6, 4) (0, 3) (3, 2) (1, 1) (7, 0)
------------------------------------------------------
(5, 7) (3, 6) (6, 5) (0, 4) (2, 3) (4, 2) (1, 1) (7, 0)