struct Doubler {
mut:
    val   int
    until int
}

fn (mut it Doubler) next() ?int {
    v := it.val
    if v > it.until {
        return none
    }
    it.val *= 2
    return v
}

fn doubler(start int, until int) Doubler {
    return Doubler{start, until}
}

fn test_for_in_iterator() {
    mut d := doubler(5, 30)
    mut vals := []int{}
    for val in d {
        vals << val
    }
    assert vals == [5, 10, 20]
}

fn test_for_in_empty_iterator() {
    mut d := doubler(5, 2)
    mut vals := []int{}
    for val in d {
        vals << val
    }
    assert vals == []
}

fn test_for_in_iterator_with_tmp_expr() {
    mut vals := []int{}
    for val in doubler(2, 64) {
        vals << val
    }
    assert vals == [2, 4, 8, 16, 32, 64]
}