A fast immutable list with a functional API.

List

List is a purely functional alternative to arrays. It is an implementation of a fast persistent sequence data structure. Compared to JavaScript's Array List has two major benefits.

• Safety. List is immutable. This makes it safer and better suited for functional programming. It doesn't tempt you with an imperative API and accidental mutations won't be a source of bugs.
• Performance. Since List doesn't allow mutations it can be heavily optimized for pure operations. This makes List much faster for functional programming than arrays. See the benchmarks.

Features

• Familiar functional API. List follows the naming conventions common in functional programming and has arguments ordered for currying/partial application.
• Extensive API. List has all the functions you know from Array and a lot of other functions that'll save the day once you need them.
• Extremely fast. List is a carefully optimized implementation of the highly efficient data-structure relaxed radix balanced trees. Performance has been a central focus from the beginning and we have an extensive benchmark suite to ensure optimal performance.
• Does one thing well. Instead of offering a wealth of data structures List has a tight focus on being the best immutable list possible. It doesn't do everything but is designed to work well with the libraries you're already using.
• Seamless Ramda integration. If you know Ramda you already know how to use List. List includes a Ramda specific module where functions are curried using Ramda's R.curry and where equality comparisons are done using R.equals.
• Type safe. List is written in TypeScript and comes with accurate types that cover the entire library.
• Fully compatible with tree-shaking. List ships with tree-shaking compatible ECMAScript modules. import * as L from "list" in itself adds zero bytes to your bundle when using Webpack. Using a function adds only that function and the very small (<1KB) core of the library. You only pay in size for the functions that you actually use.
• Iterable. Implements the JavaScript iterable protocol. This means that lists can be use in for..of loops, works with destructuring, and can be passed to any function expecting an iterable. See more.
• Fantasy Land support. List implements both the Fantasy Land and the Static Land specification.

Getting started

This section explains how to get started using List. First you'll have to install the library.

npm install list

Then you can import it.

// As an ES module
import * as L from "list";
// Or with require
const L = require("list");

Then you can begin using List instead of arrays and enjoy immutability the performance benefits.

As a replacement for array literals List offers the function list for constructing lists:

// An array literal
const myArray = [0, 1, 2, 3];
// A list "literal"
const myList = L.list(0, 1, 2, 3);

List has all the common functions that you know from native arrays and other libraries.

const myList = L.list(0, 1, 2, 3, 4, 5);
myList.length; //=> 6
L.filter(isEven, myList); //=> list(0, 2, 4)
L.map(n => n * n, myList); //=> list(0, 1, 4, 9, 16, 25)
L.reduce((sum, n) => sum + n, 0, myList); //=> 15
L.slice(2, 5, myList); //=> list(2, 3, 4)
L.concat(myList, L.list(6, 7, 8)); //=> list(0, 1, 2, 3, 4, 5, 6, 7, 8);

You'll probably also end up needing to convert between arrays and List. You can do that with the functions fromArray and toArray.

L.toArray(L.list("foo", "bar")); //=> ["foo", "bar"];
L.fromArray(["foo", "bar"]); //=> L.list("foo", "bar");

List offers a wealth of other useful and high-performing functions. You can see them all in the API documentation

Iterable

List implements the JavaScript iterable protocol. This means that lists can be used with array destructuring just like normal arrays.

const myList = L.list("first", "second", "third", "fourth");
const [first, second] = myList;
first; //=> "first"
second; //=> "second"

Lists can also be used in for..of loops.

for (const element of myList) {
  console.log(element);
}
// logs: first, second, third, fourth

And they can be passed to any function that takes an iterable as its argument. As an example a list can be converted into a native Set.

const mySet = new Set(myList);
mySet.has("third"); //=> true

This works because the Set constructor accepts any iterable as argument.

Lists also work with spread syntax. For instannce, you can call a function like this.

console.log(...list("hello", "there", "i'm", "logging", "elements"));

And each element of the list will be passed as an argument to console.log.

The iterable protocol allows for some very convenient patterns and means that lists can integrate nicely with JavaScript syntax. But, here are two anti-patterns that you should be aware of.

1. Don't overuse for..of loops. Functions like map and foldl are often a better choice. If you want to perform a side-effect for each element in a list you should probably use forEach.
2. Don't use the spread syntax in destructuring

   const [a, b, ...cs] = myList; // Don't do this

   The syntax converts the rest of the iterable (in this case a list) into an array by iterating through the entire iterable. This is slow and it turns our list into an array. This alternative avoids both problems.

   const [[a, b], cs] = splitAt(2, myList); // Do this

   This uses the splitAt function which splits and creates the list cs very efficiently in O(log(n)) time.

Seamless Ramda integration

List is designed to work seamlessly together with Ramda. Ramda offers a large number of useful functions for working with arrays. List implements the same methods on its immutable data structure. This means that Ramda users can keep using the API they're familiar with.

Additionally, List offers an entry point where all functions are curried using Ramda's R.curry and where all equality comparisons are done using R.equals.

import * as L from "list/ramda";
const indexOfFoo1 = indexOf({ foo: 1 });
indexOfFoo1(list({ foo: 0 }, { foo: 1 }, { foo: 2 })); //=> 1

In the example above indexOf is curried and it uses R.equals to find an element equivalent to { foo: 1 }.

Since List implements Ramda's array API it is very easy to convert code from using arrays to using immutable lists. As an example, consider the code below.

import * as R from "ramda";

R.pipe(R.filter(n => n % 2 === 0), R.map(R.multiply(3)), R.reduce(R.add, 0))(
  array
);

It can be converted to code using List as follows.

import * as R from "ramda";
import * as L from "list";

R.pipe(L.filter(n => n % 2 === 0), L.map(R.multiply(3)), L.reduce(R.add, 0))(
  list
);

For each function operating on arrays, the R is simply changed to an L. This works because List exports functions that have the same name and behavior as Ramdas functions.

Implemented Ramda functions

The goal is to implement the entirety of Ramda's array functions for List. The list below keeps track of how many of Ramda functions that are missing and of how many that are already implemented. Currently 45 out of 75 functions have been implemented.

Implemented: adjust, all, any, append, chain, concat, contains, drop, dropLast, dropWhile, filter, find, findIndex, head, flatten, indexOf, init, insert, insertAll, last, length, join, map, none, nth, pair, partition, pluck, prepend, range, reduce, reduceRight, reject, remove, reverse, repeat, slice, splitAt, take, takeWhile, tail, takeLast, times, update, zip, zipWith.

Not implemented: aperture, dropLastWhile, dropRepeats, dropRepeatsWith, endsWith, findLast, findLastIndex, groupWith, indexBy, intersperse, lastIndexOf, mapAccum, mapAccumRight, reduceWhile, scan, sequence, sort, splitEvery, splitWhen, startsWith, takeLastWhile, transpose, traverse, unfold, uniq, uniqBy, uniqWith, unnest without, xprod.

Fantasy Land & Static Land

List currently implements the following Fantasy Land and Static Land specifications: Setoid, semigroup, monoid, foldable, functor, apply, applicative, chain, monad.

The following specifications have not been implemented yet: Traversable.

Since methods hinder tree-shaking the Fantasy Land methods are not included by default. In order to get them you must import it likes this:

import "list/fantasy-land";

API documentation

The API is organized into three parts.

1. Creating lists — Functions that create lists.
2. Updating lists — Functions that transform lists. That is, functions that take one or more lists as arguments and returns a new list.
3. Folds — Functions that extracts values based on lists. They take one or more lists as arguments and returns something that is not a list.

Creating lists

list

Creates a list based on the arguments given.

Complexity: O(n)

Example

const l = list(1, 2, 3, 4); // creates a list of four elements
const l2 = list("foo"); // creates a singleton

empty

Returns an empty list.

Complexity: O(1)

Example

const emptyList = empty(); //=> list()

of

Takes a single arguments and returns a singleton list that contains it.

Complexity: O(1)

Example

of("foo"); //=> list("foo")

pair

Takes two arguments and returns a list that contains them.

Complexity: O(1)

Example

pair("foo", "bar"); //=> list("foo", "bar")

fromArray

Converts an array into a list.

Complexity: O(n)

Example

fromArray([0, 1, 2, 3, 4]); //=> list(0, 1, 2, 3, 4)

range

Returns a list of numbers between an inclusive lower bound and an exclusive upper bound.

Complexity: O(n)

Example

range(3, 8); //=> list(3, 4, 5, 6, 7)

repeat

Returns a list of a given length that contains the specified value in all positions.

Complexity: O(n)

Example

repeat(1, 7); //=> list(1, 1, 1, 1, 1, 1, 1)
repeat("foo", 3); //=> list("foo", "foo", "foo")

times

Returns a list of given length that contains the value of the given function called with current index.

Complexity: O(n)

Example

const twoFirsOdds = times(i => i * 2 + 1, 2);
const dots = times(() => {
  const x = Math.random() * width;
  const y = Math.random() * height;
  return { x, y };
}, 50);

Updating lists

concat

Concatenates two lists.

Complexity: O(log(n))

Example

concat(list(0, 1, 2), list(3, 4)); //=> list(0, 1, 2, 3, 4)

flatten

Flattens a list of lists into a list. Note that this function does not flatten recursively. It removes one level of nesting only.

Complexity: O(n * log(m)) where n is the length of the outer list and m the length of the inner lists.

Example

const nested = list(list(0, 1, 2, 3), list(4), empty(), list(5, 6));
flatten(nested); //=> list(0, 1, 2, 3, 4, 5, 6)

prepend

Prepends an element to the front of a list and returns the new list.

Complexity: O(log(n)), practically constant

Example

const newList = prepend(0, list(1, 2, 3)); //=> list(0, 1, 2, 3)

append

Appends an element to the end of a list and returns the new list.

Complexity: O(log(n)), practically constant

Example

const newList = append(3, list(0, 1, 2)); //=> list(0, 1, 2, 3)

map

Applies a function to each element in the given list and returns a new list of the values that the function return.

Complexity: O(n)

Example

map(n => n * n, list(0, 1, 2, 3, 4)); //=> list(0, 1, 4, 9, 16)

pluck

Extracts the specified property from each object in the list.

Example

const l = list(
  { foo: 0, bar: "a" },
  { foo: 1, bar: "b" },
  { foo: 2, bar: "c" }
);
pluck("foo", l); //=> list(0, 1, 2)

update

Returns a list that has the entry specified by the index replaced with the given value.

Complexity: O(log(n))

Example

update(2, "X", list("a", "b", "c", "d", "e")); //=> list("a", "b", "X", "d", "e")

adjust

Returns a list that has the entry specified by the index replaced with the value returned by applying the function to the value.

Complexity: O(log(n))

Example

adjust(2, inc, list(0, 1, 2, 3, 4, 5)); //=> list(0, 1, 3, 3, 4, 5)

slice

Returns a slice of a list. Elements are removed from the beginning and end. Both the indices can be negative in which case they will count from the right end of the list.

Complexity: O(log(n))

Example

const l = list(0, 1, 2, 3, 4, 5);
slice(1, 4, l); //=> list(1, 2, 3)
slice(2, -2, l); //=> list(2, 3)

take

Takes the first n elements from a list and returns them in a new list.

Complexity: O(log(n))

Example

take(3, list(0, 1, 2, 3, 4, 5)); //=> list(0, 1, 2)

takeWhile

Takes the first elements in the list for which the predicate returns true.

Complexity: O(k + log(n)) where k is the number of elements satisfying the predicate.

Example

takeWhile(n => n < 4, list(0, 1, 2, 3, 4, 5, 6)); //=> list(0, 1, 2, 3)

takeLast

Takes the last n elements from a list and returns them in a new list.

Complexity: O(log(n))

Example

takeLast(3, list(0, 1, 2, 3, 4, 5)); //=> list(3, 4, 5)

splitAt

Splits a list at the given index and return the two sides in a pair. The left side will contain all elements before but not including the element at the given index. The right side contains the element at the index and all elements after it.

Complexity: O(log(n))

Example

const l = list(0, 1, 2, 3, 4, 5, 6, 7, 8);
splitAt(4, l); //=> [list(0, 1, 2, 3), list(4, 5, 6, 7, 8)]

remove

Takes an index, a number of elements to remove and a list. Returns a new list with the given amount of elements removed from the specified index.

Complexity: O(log(n))

Example

const l = list(0, 1, 2, 3, 4, 5, 6, 7, 8);
remove(4, 3, l); //=> list(0, 1, 2, 3, 7, 8)
remove(2, 5, l); //=> list(0, 1, 7, 8)

drop

Returns a new list without the first n elements.

Complexity: O(log(n))

Example

drop(2, list(0, 1, 2, 3, 4, 5)); //=> list(2, 3, 4, 5)

dropWhile

Removes the first elements in the list for which the predicate returns true.

Complexity: O(k + log(n)) where k is the number of elements satisfying the predicate.

Example

dropWhile(n => n < 4, list(0, 1, 2, 3, 4, 5, 6)); //=> list(4, 5, 6)

dropLast

Returns a new list without the last n elements.

Complexity: O(log(n))

Example

dropLast(2, list(0, 1, 2, 3, 4, 5)); //=> list(0, 1, 2, 3)

tail

Returns a new list with the first element removed.

Complexity: O(1)

Example

tail(list(0, 1, 2, 3)); //=> list(1, 2, 3)

pop

Returns a new list with the last element removed.

Aliases: init

Complexity: O(1)

Example

pop(list(0, 1, 2, 3)); //=> list(0, 1, 2)

filter

Returns a new list that only contains the elements of the original list for which the predicate returns true.

Complexity: O(n)

Example

filter(isEven, list(0, 1, 2, 3, 4, 5, 6)); //=> list(0, 2, 4, 6)

reject

Returns a new list that only contains the elements of the original list for which the predicate returns false.

Complexity: O(n)

Example

reject(isEven, list(0, 1, 2, 3, 4, 5, 6)); //=> list(1, 3, 5)

reverse

Reverses a list.

Complexity: O(n)

Example

reverse(list(0, 1, 2, 3, 4, 5)); //=> list(5, 4, 3, 2, 1, 0)

ap

Applies a list of functions to a list of values.

Example

ap(list((n: number) => n + 2, n => 2 * n, n => n * n), list(1, 2, 3)); //=> list(3, 4, 5, 2, 4, 6, 1, 4, 9)

chain

Maps a function over a list and concatenates all the resulting lists together.

Also known as flatMap.

Example

chain(n => list(n, 2 * n, n * n), list(1, 2, 3)); //=> list(1, 2, 1, 2, 4, 4, 3, 6, 9)

partition

Splits the list into two lists. One list that contains all the values for which the predicate returns true and one containing the values for which it returns false.

Complexity: O(n)

Example

partition(isEven, list(0, 1, 2, 3, 4, 5)); //=> list(list(0, 2, 4), list(1, 3, 5))

insert

Inserts the given element at the given index in the list.

Complexity: O(log(n))

Example

insert(2, "c", list("a", "b", "d", "e")); //=> list("a", "b", "c", "d", "e")

insertAll

Inserts the given list of elements at the given index in the list.

Complexity: O(log(n))

Example

insertAll(2, list("c", "d"), list("a", "b", "e", "f")); //=> list("a", "b", "c", "d", "e", "f")

zipWith

This is like mapping over two lists at the same time. The two lists are iterated over in parallel and each pair of elements is passed to the function. The returned values are assembled into a new list.

The shortest list determine the size of the result.

Complexity: O(log(n)) where n is the length of the smallest list.

Example

const names = list("Turing", "Curry");
const years = list(1912, 1900);
zipWith((name, year) => ({ name, year }), names, years);
//=> list({ name: "Turing", year: 1912 }, { name: "Curry", year: 1900 });

zip

Iterate over two lists in parallel and collect the pairs.

Complexity: O(log(n)) where n is the length of the smallest list.

Example

const names = list("a", "b", "c", "d", "e");
const years = list(0, 1, 2, 3, 4, 5, 6);
//=> list(["a", 0], ["b", 1], ["c", 2], ["d", 3], ["e", 4]);

Folds

isList

Returns true if the given argument is a list.

Complexity: O(1)

Example

isList([0, 1, 2]); //=> false
isList("string"); //=> false
isList({ foo: 0, bar: 1 }); //=> false
isList(list(0, 1, 2)); //=> true

equals

Returns true if the two lists are equivalent.

Complexity: O(n)

Example

equals(list(0, 1, 2, 3), list(0, 1, 2, 3)); //=> true
equals(list("a", "b", "c"), list("a", "z", "c")); //=> false

toArray

Converts a list into an array.

Complexity: O(n)

Example

toArray(list(0, 1, 2, 3, 4)); //=> [0, 1, 2, 3, 4]

nth

Gets the nth element of the list.

Complexity: O(log(n)), practically constant

Example

const l = list(0, 1, 2, 3, 4);
nth(2, l); //=> 2

length

Returns the length of a list. I.e. the number of elements that it contains.

Complexity: O(1)

Example

length(list(0, 1, 2, 3)); //=> 4

first

Returns the first element of the list. If the list is empty the function returns undefined.

Aliases: head

Complexity: O(1)

Example

first(list(0, 1, 2, 3)); //=> 0
first(list()); //=> undefined

last

Returns the last element of the list. If the list is empty the function returns undefined.

Complexity: O(1)

Example

last(list(0, 1, 2, 3)); //=> 3
last(list()); //=> undefined

foldl

Folds a function over a list. Left-associative.

Aliases: reduce

Complexity: O(n)

Example

foldl((n, m) => n - m, 1, list(2, 3, 4, 5));
1 - 2 - 3 - 4 - 5; //=> -13

foldr

Folds a function over a list. Right-associative.

Aliases: reduceRight

Complexity: O(n)

Example

foldr((n, m) => n - m, 5, list(1, 2, 3, 4));
1 - (2 - (3 - (4 - 5))); //=> 3

forEach

Invokes a given callback for each element in the list from left to right. Returns undefined.

This function is very similar to map. It should be used instead of map when the mapping function has side-effects. Whereas map constructs a new list forEach merely returns undefined. This makes forEach faster when the new list is unneeded.

Complexity: O(n)

Example

const l = list(0, 1, 2);
forEach(element => console.log(element));
//=> 0
//=> 1
//=> 2

every

Returns true if and only if the predicate function returns true for all elements in the given list.

Aliases: all

Complexity: O(n)

Example

const isEven = n => n % 2 === 0;
every(isEven, empty()); //=> true
every(isEven, list(2, 4, 6, 8)); //=> true
every(isEven, list(2, 3, 4, 6, 7, 8)); //=> false
every(isEven, list(1, 3, 5, 7)); //=> false

some

Returns true if and only if there exists an element in the list for which the predicate returns true.

Aliases: any

Complexity: O(n)

Example

const isEven = n => n % 2 === 0;
some(isEven, empty()); //=> false
some(isEven, list(2, 4, 6, 8)); //=> true
some(isEven, list(2, 3, 4, 6, 7, 8)); //=> true
some(isEven, list(1, 3, 5, 7)); //=> false

indexOf

Returns the index of the first element in the list that is equal to the given element. If no such element is found the function returns -1.

Complexity: O(n)

Example

const l = list(12, 4, 2, 89, 6, 18, 7);
indexOf(12, l); //=> 0
indexOf(89, l); //=> 3
indexOf(10, l); //=> -1

find

Returns the first element for which the predicate returns true. If no such element is found the function returns undefined.

Complexity: O(n)

Example

find(isEven, list(1, 3, 5, 6, 7, 9, 10)); //=> 6
find(isEven, list(1, 3, 5, 7, 9)); //=> undefined

findIndex

Returns the index of the first element for which the predicate returns true. If no such element is found the function returns -1.

Complexity: O(n)

Example

findIndex(isEven, list(1, 3, 5, 6, 7, 9, 10)); //=> 3
findIndex(isEven, list(1, 3, 5, 7, 9)); //=> -1

none

Returns true if and only if the predicate function returns false for all elements in the given list.

Complexity: O(n)

Example

const isEven = n => n % 2 === 0;
none(isEven, empty()); //=> true
none(isEven, list(2, 4, 6, 8)); //=> false
none(isEven, list(2, 3, 4, 6, 7, 8)); //=> false
none(isEven, list(1, 3, 5, 7)); //=> true

includes

Returns true if the list contains the specified element. Otherwise it returns false.

Aliases: contains

Complexity: O(n)

Example

includes(3, list(0, 1, 2, 3, 4, 5)); //=> true
includes(3, list(0, 1, 2, 4, 5)); //=> false

join

Concats the strings in a list separated by a specified separator.

Complexity: O(n)

Example

join(", ", list("one", "two", "three")); //=> "one, two, three"

Benchmarks

The benchmarks are located in the bench directory.