double := fun (n: int) -> int {
  return n * 2;
};

assert double(2) == 4;
assert double(18 + 3) == 42;
assert double(double(-3)) == -12;

fac := fun (n: int) -> int {
  if n == 0 { return 1; }
  return n * this(n - 1);
};

assert fac(5) == 120;

inner_val := fun () -> int {
  n := 28;
  return n;
};

assert inner_val() == 28;

inner_fun := fun (n: int) -> int {
  f := fun (n: int) -> int {
    return n * 2;
  };

  return f(f(n));
};

assert inner_fun(4) == 16;

twice := fun (n: int, f: (int -> int)) -> int {
  return f(f(n));
};

incr := fun (n: int) -> int {
  return n + 1;
};

dble := fun (n: int) -> int {
  return n * 2;
};

assert twice(4, incr) == 6;
assert twice(4, dble) == 16;
assert twice(3, fun (x: int) -> int {
  return x + 1;
} ) == 5;

fun sugar(n: int) -> int {
  return n * 2;
}

assert sugar(21) == 42;

fun add3(x, y, z: int) -> int {
  return x + y + z;
}

assert add3(2, 4, 6) == 12;

fun empty(n: string) {
  return;
}

empty('useless');