factorial: integer -> integer
fun factorial(a) {
  if (a == 0) {
    return 1
  } else {
    return a * factorial(a - 1)
  }
}

fun main() {
  n = 0 as integer|void;
  s = "";
  print("Enter integer to obtain factorial of it, or 'q' to quit.");
  while true {
    s = input(">> ");
    if s == "q" { break; }
    n = int(s);
    typecase n is integer { print(str(factorial(n))); }
    typecase n is void { print("Integer needed"); }
  }
}