What is this🔗

This is a library with lots of list functions that are related to indices. It has often-reinvented deleteAt, setAt, etc, as well as indexed variants of functions from Data.List (e.g. imap, ifilter, izipWith). It has no dependencies, builds in about a second, and works on GHC from 7.4 to 8.0; the functions are optimised and benchmarked (for instance, the zip [0..] idiom is usually twice as slow, and sometimes 20× as slow).

So, this library is intended to be the canonical place for index-related functions. You are encouraged to depend on this library instead of reinventing the functions, using zip [0..], or using lens when all you need is a simple imap or ifoldr (not to mention that lens variants are usually 2–10 times slower for lists).

Why should you care🔗

You shouldn’t, actually. This is a small library, it won’t change anyone’s life, and if you care about speed you probably shouldn’t be using lists anyway (unless you keep your fingers crossed and hope that fusion will kick in). So, consider it more of a public service announcement – “hey, just in case you ever need them, index-related functions live here”.

Usage🔗

Unfortunately, Data.List.Indexed was taken by IndexedList, which implements such exciting things as “counted lists” and “conic lists”. Nope, I’m not bitter at all. Okay, maybe a bit, even tho it’s completely unfair to IndexedList. Anyway:

import Data.List.Index

And you can use functions from Data.List by prepending i to them. There’s also indexed :: [a] -> [(Int,a)] and a family of functions for modifying the element at an index (deleteAt, setAt, modifyAt, updateAt, insertAt).

Watch out – ifoldl has the index as the second parameter of the function:

ifoldl :: (b -> Int -> a -> b) -> b -> [a] -> b

That’s the same convention that containers and vector use. Other functions pass the index as the first argument, as expected.