How can you use composition to create complex functions?

1 What is composition?

Composition is the process of applying one function to the result of another function. For example, if you have two functions f and g, you can compose them by writing f(g(x)), which means apply g to x and then apply f to the result. The order of the functions matters, as f(g(x)) is not the same as g(f(x)) in general. You can think of composition as a way of chaining functions together to create a new function.

3 How to use composition in JavaScript?

JavaScript is a multi-paradigm language that supports functional programming features, such as first-class functions, higher-order functions, and closures. You can use these features to create and compose functions in JavaScript. For example, suppose you want to create a function that takes a string and returns its length in uppercase letters. You can use the built-in functions String.prototype.toUpperCase and String.prototype.length to create two simple functions, and then compose them using the arrow syntax:

const toUpper = str => str.toUpperCase();

const getLength = str => str.length;

const upperLength = str => toUpper(getLength(str));

Now you can use the upperLength function to get the desired result:

console.log(upperLength("Hello")); // "5"

You can also use the compose function from libraries like Lodash or Ramda to compose functions from right to left. For example, using Lodash, you can write:

const upperLength = _.compose(toUpper, getLength);

This is equivalent to writing:

const upperLength = str => toUpper(getLength(str));

4 How to use composition in Python?

Python is another multi-paradigm language that supports functional programming features, such as lambda functions, map, filter, and reduce. You can use these features to create and compose functions in Python. For example, suppose you want to create a function that takes a list of numbers and returns the sum of their squares. You can use the built-in functions map and sum to create two simple functions, and then compose them using the lambda syntax:

square = lambda x: x ** 2

sum_of_squares = lambda lst: sum(map(square, lst))

Now you can use the sum_of_squares function to get the desired result:

print(sum_of_squares([1, 2, 3])) # 14

You can also use the functools module to compose functions from left to right. For example, using functools, you can write:

import functools

sum_of_squares = functools.reduce(lambda x, y: x + y ** 2, [1, 2, 3], 0)

This is equivalent to writing:

sum_of_squares = 0

for x in [1, 2, 3]:

sum_of_squares = sum_of_squares + x ** 2

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?