# this function returns an iterator when called
fn count_by(delta, limit) {
    let counter = 0;
    return fn() {
        if counter >= limit {
            return nil;
        }
        let prev_value = counter;
        counter += delta;
        return prev_value;
    };
}

# counter is an iterator
# iterators are functions that:
#  - take no arguments
#  - return nil once done
#  - once returned nil once, must do so for all subsequent calls
# the interpreter only checks the first requirement
let counter = count_by(2, 5);
println(counter()); # 0
println(counter()); # 2
println(counter()); # 4
# println(counter()); # nil
# println(counter()); # nil

println("counting by 3s up to 20:");
for n: count_by(3, 20) {
    println(n);
}

println("counting by 7s up to 40:");
for n: count_by(7, 40) {
    println(n);
}