Play with the Collatz conjecture in Formality

Collatz.fm

Collatz.run: IO(Unit)
  do IO {
    var val_str = IO.prompt("start value:");
    let start = Nat.parse_decimal(val_str);
    Collatz.step(start);
  }

Collatz.step(start: Nat): IO(Unit)
  let next = Collatz.next(start);
  do IO {
    IO.print(Nat.show(next));
    case Nat.eql(next, 1):
    | Bool.true => IO.print("Loop found!");
    | Bool.false => Collatz.step(next);;
  }

Collatz.next(start: Nat): Nat
  if Nat.odd(start) then
    Nat.add(Nat.mul(3, start), 1)
  else
    Nat.div(start, 2)