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.
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:
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:
Array spreads
Since 11.1
You can spread arrays of the the same type into new arrays, just like in JavaScript:
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
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:
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: