Immutable list implementation 🥝

22 July 2017

Today I want to share with you an implementation of immutable list in JavaScript in one line!

Long story short:

const list = (head, tail) => next => next ? tail : head

That’s it! This is a full implementation of immutable list structure in JavaScript.

This technique called church encoding. Let’s me show how to create a list on three elements.

const three = list(1, list(2, list(3)))

As you may notice in the last list call, we omitted tail argument. That’s because an empty list is presented as undefined.

const empty = void 0

But the list itself is not very useful, we need a way of getting elements from it. For this we’ll create two functions head and tail.

const head = l => l && l()
const tail = l => l && l(true)

Here is how we can retrieve head and tail from our three list.

const el = head(three) // 1
const xs = tail(three) // list(2, list(3))

And there is no way to mutate the three list. We can create only new list from elements of old one.

Lets write an function to retrieve the last element from a list. If tail isn’t an empty list (i.e. not undefined) get the last element of it’s tail. If tail is empty get list’s head. Yep, a recursion. A special one — with a tail call.

const last = l => tail(l) ? last(tail(l)) : head(l)

But to deal with a really big list in JavaScript we need tail call optimization. And luckily ES6 has it.

Next, lets implement a very useful and powerful function foldl.

const foldl = (l, acc, fn) => l ? foldl(tail(l), fn(head(l), acc), fn) : acc

This function folds (reduces) the given list from the left with a function.

foldl(three, 1, (x, acc) => x * acc) // gives 6

Next, lets implement reverse function which returns new list in reverse order.

const reverse = l => foldl(l, empty, list)

Pretty neat. Right? I created a npm library @medv/list with these and other helpful functions. You can play with it to understand why it’s working.

const {list, print, foldl, reverse} = require('@medv/list')
const a = list(1, list(2, list(3)))
const b = reverse(a)
foldl(a, 1, (x, acc) => x * acc)

With foldl and reverse we can implement simple foldr function.

const foldr = (l, acc, fn) => foldl(reverse(l), acc, fn)

And with foldr we can implement function for concatenating two lists.

const concat = (l, r) => foldr(l, r, list)

The complexity of concat is proportional to length(l), so avoid repeatedly concatenating lists of arbitrary length, e.g. concat(l, list(el)). Instead, consider prepending via list(el, l) and then reversing.

But is this recursion fast enough? We can rewrite foldl to loop and run benchmarks (for it I use test of @medv/list).

const foldl = (l, acc, fn) => {
  while (l) {
    acc = fn(head(l), acc)
    l = tail(l)
  return acc

Results are as the same as recursive implementation with --harmony-tailcalls:

$ time npm test

> node --harmony-tailcalls node_modules/.bin/ava

  15 passed

real	0m32,747s
user	0m31,353s
sys	0m2,032s

And without recursion:

$ time npm test

> node node_modules/.bin/ava

  15 passed

real	0m32,861s
user	0m31,556s
sys	0m2,137s

Btw, Are there more performance implementations of immutable list? Definitely.

For example:

const list = (head, tail) => Object.freeze({head, tail})

This implementation, on same benchmarks, 6 time faster. But it’s not so awesome as stack for storing head and tail.

  • @medv/list — immutable list library
  • tto — tic-tak-toe build with this library
  • cons — more about lisp functional programing
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