What is an idiomatic way to add lists in Haskell?

Question

Suppose I want to add two lists in Haskell. What is the most usual way to do this?

Here's what I did:

addLists :: (Integral a) => [a] -> [a] -> [a]
addLists xs ys = map add $ zip xs ys
    where add (x, y) = x+y

Answer 1

Applicative Functor style:

import Control.Applicative

addLists xs ys = getZipList $ (+) <$> ZipList xs <*> ZipList ys

Note that this is so ugly because there are two ways to make List an Applicative Functor. The first (and IMHO less useful) way is to take all combination, and that way became the "standard", so (+) <$> [1,2] <*> [30,40] is [31,41,32,42]. The other way is to zip the lists as we need here, but as you can have only one type class instance per type, we have to wrap the lists in ZipLists, and to unwrap the result using getZipList.

Answer 2

There is a zipWith library function that combines two lists by using a supplied function. It does exactly what you want here and you get:

addLists = zipWith (+)

This uses (+) to combine the elements of lists given as further arguments.

Answer 3

addLists xs ys = zipWith (+) xs ys