let map = fn (arr, f) {
    let iter = fn (arr, accumulated) {
        if (len(arr) == 0) {
            accumulated
        } else {
            iter(rest(arr), push(accumulated, f(first(arr))))
        }
    };
    iter(arr, [])
};
let a = [1, 2, 3, 4];
let double = fn (x) {
    x * 2
};
map(a, double);