Docs / Language Manual / Array & List

Array and List


Arrays are our main ordered data structure. They work the same way as JavaScript arrays: they can be randomly accessed, dynamically resized, updated, etc.

ReScriptJS Output
let myArray = ["hello", "world", "how are you"]

ReScript arrays' items must have the same type, i.e. homogeneous.


Access & update an array item like so:

ReScriptJS Output
let myArray = ["hello", "world", "how are you"]

let firstItem = myArray[0] // "hello"

myArray[0] = "hey" // now ["hey", "world", "how are you"]


Array spreads

Since 11.1

You can spread arrays of the the same type into new arrays, just like in JavaScript:

ReScriptJS Output
let y = [1, 2]
let x = [4, 5, ...y]
let x2 = [4, 5, ...y, 7, ...y]
let x3 = [...y]

Note that array spreads compiles to Belt.Array.concatMany right now. This is likely to change to native ES6 array spreads in the future.


ReScript provides a singly linked list too. Lists are:

  • immutable

  • fast at prepending items

  • fast at getting the head

  • slow at everything else

ReScriptJS Output
let myList = list{1, 2, 3}

Like arrays, lists' items need to be of the same type.


You'd use list for its resizability, its fast prepend (adding at the head), and its fast split, all of which are immutable and relatively efficient.

Do not use list if you need to randomly access an item or insert at non-head position. Your code would end up obtuse and/or slow.

The standard lib provides a List module.

Immutable Prepend

Use the spread syntax:

ReScriptJS Output
let myList = list{1, 2, 3}
let anotherList = list{0, ...myList}

myList didn't mutate. anotherList is now list{0, 1, 2, 3}. This is efficient (constant time, not linear). anotherList's last 3 elements are shared with myList!

Note that list{a, ...b, ...c} was a syntax error before compiler v10.1. In general, the pattern should be used with care as its performance and allocation overhead are linear (O(n)).


switch (described in the pattern matching section) is usually used to access list items:

ReScriptJS Output
let message =
  switch myList {
  | list{} => "This list is empty"
  | list{a,} => "The head of the list is the string " ++ Int.toString(a)