array-and-list.mdx

title	description	canonical
Array & List	Arrays and List data structures	/docs/manual/v11.0.0/array-and-list

Array and List

Array

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

<CodeTab labels={["ReScript", "JS Output"]}>

letmyArray= ["hello", "world", "how are you"]

varmyArray=["hello","world","how are you"];

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

Usage

Access

Accessing items in an array will return an option and can be done like so:

<CodeTab labels={["ReScript", "JS Output"]}>

letmyArray= ["hello", "world", "how are you"] letfirstItem=myArray[0] // Some("hello")lettenthItem=myArray->Array.get(10) // None

varmyArray=["hello","world","how are you"];varfirstItem=myArray[0];vartenthItem=myArray[10];

The behavior of returning an option is new to V11 when you have Core open. It provides a safer way to access array items, which is especially useful when you're not sure if the index is out of bounds. If you would like to not use an option, you can use Array.getUnsafe.

Update

Items in an array can be updated by assigning a value to an index or using a function:

<CodeTab labels={["ReScript", "JS Output"]}>

letmyArray= ["hello", "world", "how are you"] myArray[0] ="hey"// now ["hey", "world", "how are you"]myArray->Array.push("?") // ["hey", "world", "how are you", "?"]myArray->Array.set(0, "bye") // ["bye", "world", "how are you", "?"]

varmyArray=["hello","world","how are you"];myArray[0]="hey";myArray.push("?");myArray[0]="bye";

Array spreads

Since 11.1

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

<CodeTab labels={["ReScript", "JS Output"]}>

lety= [1, 2] letx= [4, 5, ...y] letx2= [4, 5, ...y, 7, ...y] letx3= [...y]

varBelt_Array=require("rescript/lib/js/belt_Array.js");vary=[1,2];varx=Belt_Array.concatMany([[4,5],y]);varx2=Belt_Array.concatMany([[4,5],y,[7],y]);varx3=Belt_Array.concatMany([y]);

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

List

ReScript provides a singly linked list too. Lists are:

• immutable
• fast at prepending items
• fast at getting the head
• slow at everything else

<CodeTab labels={["ReScript", "JS Output"]}>

letmyList=list{1, 2, 3}

varmyList={hd: 1,tl: {hd: 2,tl: {hd: 3,tl: 0}}};

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:

<CodeTab labels={["ReScript", "JS Output"]}>

letmyList=list{1, 2, 3} letanotherList=list{0, ...myList}

varmyList={hd: 1,tl: {hd: 2,tl: {hd: 3,tl: 0}}};varanotherList={hd: 0,tl: 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)).

Access

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

<CodeTab labels={["ReScript", "JS Output"]}>

letmessage=switchmyList { | list{} =>"This list is empty" | list{a, ...rest} =>"The head of the list is the string "++Int.toString(a) }

varmessage=myList ? "The head of the list is the string "+(1).toString() : "This list is empty";