ACM International Collegiate Programming Contest, Asia Regional (Tokyo), 1998, Problem A

gistfile1.ml

open List;;

let generate_isprime x =
  let rec generate_isprime x = 
    match x with
        1 -> []
      | n -> n :: generate_isprime (n-1)
  in
    rev (generate_isprime x);;

let filter_isprime l =
  let rec filter_isprime l =
    match l with
        [] -> []
      | x :: xs ->
          let
              filtered = filter (fun y -> y mod x != 0) xs
          in
            x :: filter_isprime(filtered)
  in
    1 :: filter_isprime(l);;

let rec find_pair n l =
  match l with
      [] -> []
    | x :: xs ->
        let
            ys = filter (fun z -> z == (n - x)) xs
        in
            if length ys != 0
            then (x, hd(ys)) :: find_pair n xs
            else find_pair n xs;;

let count_goldbach n =
  length (find_pair n (filter_isprime (generate_isprime n)));;

let get_text filename =
  let input = open_in filename in
    try
      while true do
        let
            n = int_of_string (input_line input)
        in
          if n != 0 then print_endline (string_of_int (count_goldbach n))
          else ()
      done
    with
        End_of_file -> ();;

Arg.parse [] (fun s -> get_text (s)) ("");;