let str_join = module{
    sep=" ",
    list=NULL,
} => {
    let joiner = func (acc, item) =>  select (acc.out == ""), NULL, {
        true = acc{
            out="@@" % (acc.out,item),
        },
        false = acc{
            out="@@@" % (acc.out, acc.sep, item),
        },
    };

    let result = reduce(joiner, {sep=mod.sep, out=""}, (mod.list)).out;
};

let len = func(list) => reduce(func(acc, item) => acc + 1, 0, list);

let reverse = func(list) => reduce(func (acc, item) =>  [item] + acc, [], list);

let enumerate = module{
    start = 0,
    step = 1,
    list = NULL,
} => {
    let reducer = func (acc, item) =>  acc{
        count = acc.count + acc.step,
        list=acc.list + [[acc.count, item]],
    };

    let acc = {count=mod.start, list=[], step=mod.step};

    let enumerated = reduce(reducer, acc, (mod.list));
    let result = enumerated.list;
};

let zip = module{
    list1 = NULL,
    list2 = NULL,
} => {
    let len = import "std/lists.ucg".len;


    let len1 = len(mod.list1);
    let len2 = len(mod.list2);

    let rng = select (len1 >= len2), NULL, {
        true = 0:(len1 - 1),
        false = 0:(len2 - 1),
    };

    let reducer = func (acc, item) =>  acc{
        result = acc.result + [[acc.list1.(item), acc.list2.(item)]],
        idxs = acc.idxs + [item]
    };

    let acc = {
        list1 = mod.list1,
        list2 = mod.list2,
        result = [],
        idxs = [],
    };

    let result = reduce(reducer, acc, rng).result;
};