Experiment with Python Type Annotations.

list-ops.py

# Learned about Python type annotations. 

from typing import Callable
from typing import Any
from typing import TypeVar

T = TypeVar('T')
S = TypeVar('S')
U = TypeVar('U')

def append(list1: [T], list2: [T]) -> [T]:
    for item in list2:
        list1.append(item)
    return list1


def concat(list_of_lists: [[T]]) -> [T]:
    result: [T] = []
    for l in list_of_lists:
        result = append(result, l)
    return result


def _filter(function: Callable[[T], bool], items: [T]) -> [T]:
    result: [T] = []
    for item in items:
        if function(item):
            result.append(item)
    return result


def _length(items: [T]) -> int:
    count = 0
    for item in items:
        count += 1
    return count


def _map(function: Callable[[T], S], items: [T]) -> [S]:
    result: [S] = []
    for item in items:
        result.append(function(item))
    return result


def _foldl(function: Callable[[U, T], U], items: [T], initial: U) -> U:
    result = initial
    for item in items:
        result = function(result, item)
    return result


def _foldr(function: Callable[[T, U], U], items: [T], initial: U) -> U:
    result = initial
    for item in _reverse(items):
        result = function(item, result)
    return result


def _reverse(items: [T]) -> [T]:
    result = []
    for i in range(len(items) - 1, -1, -1):
        result.append(items[i])
    return result