Docs / Language Manual / ArrayAndList

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Array and List

Array

Arrays are our primary ordered data structure. They work the same way as JavaScript arrays.

Reason (Old Syntax)ML (Older Syntax)JS Output
 
let myArray = [|"hello", "world", "how are you"|];

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

Usage

See the Js.Array API.

Access & update an array item like so:

Reason (Old Syntax)ML (Older Syntax)JS Output
 
let myArray = [|"hello", "world", "how are you"|];

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

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

List

ReScript provides a singly linked list too. Lists are:

  • immutable

  • fast at prepending items

  • fast at getting the tail

  • slow at everything else

Reason (Old Syntax)ML (Older Syntax)JS Output
 
let myList = [1, 2, 3];

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

Usage

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:

Reason (Old Syntax)ML (Older Syntax)JS Output
 
let myList = [1, 2, 3];
let anotherList = [0, ...myList];

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

Note that [a, ...b, ...c] is a syntax error. We don't support multiple spread for a list. That'd be an accidental linear operation (O(b)), since each item of b would be one-by-one added to the head of c. You can use List.concat for this, but we highly discourage it.

Updating an arbitrary item in the middle of a list is also discouraged, since its performance and allocation overhead would be linear (O(n)).

Access

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

Reason (Old Syntax)ML (Older Syntax)JS Output
 
let message =
  switch myList {
  | [] => "This list is empty"
  | [a, ...rest] => "The head of the list is the string " ++ Js.Int.toString(a)
  };