let foldr = fn (arr, initial, f) {
    let iter = fn (arr, result) {
        if (len(arr) == 0) {
            result
        } else {
            iter(rest(arr), f(first(arr), result))
        }
    };
    iter(arr, initial)
};
let a = [1, 2, 3, 4];
let sum = fn (x, y) {
    x + y
};
foldr(a, 0, sum);