Lists in PureScript

In PureScript it is important to differentiate Lists and Arrays. While Arrays are compiled into JavaScript Arrays when building, Lists are more like our typical linked-lists, and not translated to Arrays under the hood.

myList = 1 : 2 : 3 : Nil
myArray = [1, 2, 3]

:type myList
--- List Int
:type myArray
--- Array Int

When building the bundle, our Array is translated to the following JS code:

var myArray = [ 1, 2, 3 ];

Let’s go over the most important functions to work with Lists: head, tail, cons, snoc, null, singleton, length, last & index.

head & tail

Our linked list is build up like this: (1 (2 (3 (Nil)))) As you can see, next to the 1 is the complete rest of the list. Next to the 2 is the complete rest of the list, and so on. This rest is called the tail - the element “on top” is the head.

So, the head of the tail of myList is 2.

head myList

Gives us “Just 1”, and

tail myList

gives us “(Just (2 : 3 : Nil))“.

cons & snoc

Another pair of function - cons puts takes a list as the first parameter, and an element as the second parameter. This element is added as the head to the list.

Cons 1 (2 : 3 : Nil)
--- (1 : 2 : 3 : Nil)

snoc adds the provided element to the end of the list.

snoc (1 : 2 : 3 : Nil) 4
--- (1 : 2 : 3 : 4 : Nil)

Important: Cons is written with an uppercase “C”, and the to-be-added element is the first parameter. For snoc, the to-be-added element is the second parameter.

null

Returns true, if the provided list is empty, and false, if this is not the case.

null (1 : Nil)
--- false

null (Nil)
--- true

singleton

Returns a list with a single element in it (Nil doesn’t count)

singleton (1 : 2 : Nil)
--- ((1 : 2 : Nil) : Nil)

length

Returns the number of elements in the list provided

length (1 : 2 : 3 : Nil)
--- 3

last

Returns the last element of the list

last (1 : 2 : 3 : Nil)
--- (Just 3)

index

For accessing a value behind the provided index. Counting starts at 0, so index 1 is the second element:

index (1 : 2 : 3 : Nil) 1
--- (Just 2)

concat

Based on a list of lists, this function concatenates the given lists:

concat ((1 : 2 : Nil) : (3 : Nil) : Nil)
--- (1 : 2 : 3 : Nil)