from typing import Callable


# return the length of list
def length(list: list) -> int:
    length: int = 0
    for _ in list:
        length += 1
    return length


# append list2 at the end of list1
def append(list1: list, list2: list) -> list:
    new_list: list = []
    for index, item in enumerate(list1, 0):
        new_list.insert(index, item)
    for index, item in enumerate(list2, length(list1)):
        new_list.insert(index, item)
    return new_list


def concat(lists: list[list]):
    new_list: list = []
    for item_list in lists:
        new_list = append(new_list, item_list)
    return new_list


def filter(function: Callable, list_of_items: list):
    filtered_list: list = []
    insert_index: int = 0
    for item in list_of_items:
        if function(item):
            filtered_list.insert(insert_index, item)
            insert_index += 1
    return filtered_list


def map(function, list_of_items):
    mapped_list: list = []
    for index, item in enumerate(list_of_items):
        mapped_list.insert(index, function(item))
    return mapped_list


def foldl(
    function: Callable, list_of_items: list[int | str], initial: int | str
) -> int | str:
    result: int | str = initial
    if len(list_of_items) == 0:
        return result
    for index in range(0, len(list_of_items)):
        result = function(result, list_of_items[index])
    return result


def foldr(function: Callable, list_of_items: list, initial: int | str) -> int | str:
    result: int | str = initial
    if len(list_of_items) == 0:
        return result
    for index in range(len(list_of_items) - 1, -1, -1):
        result = function(result, list_of_items[index])
    return result


# reverse the order of the elements in the list
def reverse(list_of_items: list) -> list:
    # not doing this
    # return list[::-1]
    reversed_list: list = []
    for item in list_of_items:
        reversed_list.insert(0, item)
    return reversed_list