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 three 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.
• API: List has a large API of useful functions and offers both chainable methods and curried functions to suit every taste.

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 known from Array and a lot of additional 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. We have an extensive benchmark suite to ensure optimal performance. Here is an explanation how
• Several API styles. In addition to the base API List offers additional API styles. Import list/methods to get chainable methods or alterntively import list/curried to get a version of the API where every function is curried. Both variants are 100% TypeScript compatible.
• 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 was designed to integrate seamlessly with Ramda.
• Type safe. List is implemented in TypeScript. It makes full use of TypeScript features to provide accurate types that covers 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.

Package	Version	Downloads	Dependencies	Dev Deps	Install size	GZIP size
list

Getting started

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

npm i 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. Instead of using [...] to construct an array with the content ... one can use list(...) to construct a list with the same content. Here is an example.

// An array literal
const myArray = [0, 1, 2, 3];
// The List equivalent
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 from and toArray.

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

API styles

List offers several API styles. By default the library exports "plain" functions. Additionally curried functions can be imported from list/curried and an API with chainable methods can be imported from list/methods. The differences are illustrated below.

The default export offers normal plain function.

import * as L from "list";

const l = L.take(5, L.sortBy(p => p.name, L.filter(p => p.age > 22, people)));

In list/methods all functions are available as chainable methods.

import * as L from "list/methods";

const l = people
  .filter(p => p.age > 22)
  .sortBy(p => p.name)
  .take(5);

In list/curried all functions are curried. In the example below the partially applied functions are composed together using Ramda's pipe. Alternatively one could have used Lodash's flowRight.

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

const l = R.pipe(L.filter(p => p.age > 22), L.sortBy(p => p.name), L.take(5))(
  people
);

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 instance, you can call a function like this.

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

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

List also suports iterating backwards over lists through the backwards function.

Iterator anti-patterns

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 functions 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.

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
);

The example can be converted to code using List as follows.

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

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 names 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 61 out of 76 functions have been implemented.

Implemented: adjust, all, any, append, chain, concat, contains, drop, dropLast, dropRepeats, dropRepeatsWith, dropWhile, filter, find, findIndex, findLast, group, groupWith, head, flatten, indexOf, intersperse, init, insert, insertAll, last, lastIndexOf, length, join, map, none, nth, pair, partition, pluck, prepend, range, reduce, reduceRight, reduceWhile, reject, remove, reverse, repeat, scan, sequence, slice, sort, splitAt, splitEvery, splitWhen, take, takeWhile, tail, takeLast,takeLastWhile, traverse, times, update, zip, zipWith.

Not implemented: aperture, dropLastWhile, endsWith, findLastIndex, indexBy, mapAccum, mapAccumRight, startsWith, transpose, unfold, uniq, uniqBy, uniqWith, unnest without, xprod.

Differences compared to Ramda

While List tries to stay as close to Ramda's API as possible there are a few deviations to be aware of.

• List's curried functions do not support the R.__ placeholder. Instead of R.reduce(R.__, 0, l) one alternative is to use an arrow function _ => L.reduce(_, 0, l) instead.
• sort and sortWith are different compared to what they do in Ramda. L.sortWith is equivalent to R.sort and L.sort sorts a list without taking a comparison function. This makes the common case of sorting a list of numbers or strings easier

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, Ord.

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")

from

Converts an array, an array-like or an itearble into a list.

Complexity: O(n)

Example

from([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)

intersperse

Inserts a separator between each element in a list.

Example

intersperse("n", list("ba", "a", "a")); //=> list("ba", "n", "a", "n", "a")

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.

If the index is out of bounds the given list is returned unchanged.

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.

If the index is out of bounds the given list is returned unchanged.

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)
takeWhile(_ => false, list(0, 1, 2, 3, 4, 5)); //=> list()

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)

takeLastWhile

Takes the last 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

takeLastWhile(n => n > 2, list(0, 1, 2, 3, 4, 5)); //=> list(3, 4, 5)
takeLastWhile(_ => false, list(0, 1, 2, 3, 4, 5)); //=> list()

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)]

splitWhen

Splits a list at the first element in the list for which the given predicate returns true.

Complexity: O(n)

Example

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

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)

dropRepeats

Returns a new list without repeated elements.

Complexity: O(n)

Example

dropRepeats(L.list(0, 0, 1, 1, 1, 2, 3, 3, 4, 4)); //=> list(0, 1, 2, 3, 4)

dropRepeatsWith

Returns a new list without repeated elements by using the given function to determine when elements are equal.

Complexity: O(n)

Example

dropRepeatsWith(
  (n, m) => Math.floor(n) === Math.floor(m),
  list(0, 0.4, 1.2, 1.1, 1.8, 2.2, 3.8, 3.4, 4.7, 4.2)
); //=> list(0, 1, 2, 3, 4)

tail

Returns a new list with the first element removed. If the list is empty the empty list is returne.

Complexity: O(1)

Example

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

pop

Returns a new list with the last element removed. If the list is empty the empty list is returned.

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]);

sort

Sorts the given list. The list should contain values that can be compared using the < operator or values that implement the Fantasy Land Ord specification.

Performs a stable sort.

Complexity: O(n * log(n))

Example

sort(list(5, 3, 1, 8, 2)); //=> list(1, 2, 3, 5, 8)
sort(list("e", "a", "c", "b", "d"); //=> list("a", "b", "c", "d", "e")

sortBy

Sort the given list by passing each value through the function and comparing the resulting value. The function should either return values comparable using < or values that implement the Fantasy Land Ord specification.

Performs a stable sort.

Complexity: O(n * log(n))

Example

sortBy(
  o => o.n,
  list({ n: 4, m: "foo" }, { n: 3, m: "bar" }, { n: 1, m: "baz" })
);
//=> list({ n: 1, m: "baz" }, { n: 3, m: "bar" }, { n: 4, m: "foo" })

sortBy(s => s.length, list("foo", "bar", "ba", "aa", "list", "z"));
//=> list("z", "ba", "aa", "foo", "bar", "list")

sortWith

Sort the given list by comparing values using the given function. The function receieves two values and should return -1 if the first value is stricty larger than the second, 0 is they are equal and 1 if the first values is strictly smaller than the second.

Note that the comparison function is equivalent to the one required by Array.prototype.sort.

Performs a stable sort.

Complexity: O(n * log(n))

Example

sortWith((a, b) => {
  if (a === b) {
    return 0;
  } else if (a < b) {
    return -1;
  } else {
    return 1;
  }
}, list(5, 3, 1, 8, 2)); //=> list(1, 2, 3, 5, 8)

group

Returns a list of lists where each sublist's elements are all equal.

Complexity: O(n)

Example

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

groupWith

Returns a list of lists where each sublist's elements are pairwise equal based on the given comparison function.

Note that only adjacent elements are compared for equality. If all equal elements should be grouped together the list should be sorted before grouping.

Complexity: O(n)

Example

const floorEqual = (a, b) => Math.round(a) === Math.round(b);
groupWith(floorEqual, list(1.1, 1.3, 1.8, 2, 2.2, 3.3, 3.4));
//=> list(list(1.1, 1.3), list(1.8, 2, 2.2), list(3.3, 3.4))

const sameLength = (a, b) => a.length === b.length;
groupWith(sameLength, list("foo", "bar", "ab", "bc", "baz"));
//=> list(list("foo", "bar"), list("ab", "bc"), list("baz))

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

equalsWith

Returns true if the two lists are equivalent when comparing each pair of elements with the given comparison function.

Complexity: O(n)

Example

equalsWith(
  (n, m) => n.length === m.length,
  list("foo", "hello", "one"),
  list("bar", "world", "two")
); //=> true

toArray

Converts a list into an array.

Complexity: O(n)

Example

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

backwards

Returns an iterable that iterates backwards over the given list.

Complexity: O(1)

Example

const l = list(0, 1, 2, 3, 4)
for (const n of backwards(l)) {
  if (l < 2) {
    break;
  }
  console.log(l);
}
// => logs 4, 3, and then 2

nth

Gets the nth element of the list. If n is out of bounds undefined is returned.

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

foldlWhile

Similar to foldl. But, for each element it calls the predicate function before the folding function and stops folding if it returns false.

Aliases: reduceWhile

Complexity: O(n)

Example

const isOdd = (_acc:, x) => x % 2 === 1;

const xs = L.list(1, 3, 5, 60, 777, 800);
foldlWhile(isOdd, (n, m) => n + m, 0, xs) //=> 9

const ys = L.list(2, 4, 6);
foldlWhile(isOdd, (n, m) => n + m, 111, ys) //=> 111

scan

Folds a function over a list from left to right while collecting all the intermediate steps in a resulting list.

Complexity: O(n)

Example

const l = list(1, 3, 5, 4, 2);
L.scan((n, m) => n + m, 0, l); //=> list(0, 1, 4, 9, 13, 15));
L.scan((s, m) => s + m.toString(), "", l); //=> list("", "1", "13", "135", "1354", "13542")

traverse

Map each element of list to an applicative, evaluate these applicatives from left to right, and collect the results.

This works with Fantasy Land applicatives.

Complexity: O(n)

Example

const safeDiv = n => d => d === 0 ? nothing : just(n / d)

L.traverse(Maybe, safeDiv(10), list(2, 4, 5)); //=> just(list(5, 2.5, 2))
L.traverse(Maybe, safeDiv(10), list(2, 0, 5)); //=> nothing

sequence

Evaluate each applicative in the list from left to right, and collect the results.

Complexity: O(n)

Example

L.sequence(Maybe, list(just(1), just(2), just(3))); //=> just(list(1, 2, 3))
L.sequence(Maybe, list(just(1), just(2), nothing())); //=> nothing

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 -1 is returned.

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

lastIndexOf

Returns the index of the last element in the list that is equal to the given element. If no such element is found -1 is returned.

Complexity: O(n)

Example

const l = L.list(12, 4, 2, 18, 89, 2, 18, 7);
L.lastIndexOf(18, l); //=> 6
L.lastIndexOf(2, l); //=> 5
L.lastIndexOf(12, l); //=> 0

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

findLast

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

Complexity: O(n)

Example

findLast(isEven, list(1, 3, 5, 6, 7, 8, 9)); //=> 8
findLast(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.