Map, Filter, and Reduce Review

Higher-order functions like map, filter, and reduce allow us to avoid stateful iteration and mutation of variables.

Take a look at this imperative code that calculates the factorial of a number:

def factorial(n: int) -> int:
    # a procedure that continuously multiplies
    # the current result by the next number
    result: int = 1
    for i in range(1, n + 1):
        result *= i
    return result

Here's the same factorial function using reduce:

import functools


def factorial(n: int) -> int:
    return functools.reduce(lambda x, y: x * y, range(1, n + 1))

In the functional example, we're just combining functions to get the result we want. There's no need to reassign variables or keep track of the program's state in a loop.

A loop is inherently stateful! Depending on which iteration you're on, the i variable has a different value.