Queues in

About Queues

The Gleam standard library implements a Queue type in the gleam/queue module. It is similar to the List type, but with a few key differences:

• The Queue type doesn't have a literal syntax that can be used to construct queues or pattern match on them.
• Elements can be efficiently added to and removed from both the front and back of the Queue type, while the List type can only efficiently add and remove elements from the front.

Most of the time you will want to use the List type, but when you need to be able to add and remove from the end of a collection, the Queue type is a good choice.

Queues can be created using the queue.new and queue.from_list functions:

let empty = queue.new()
let one_to_four = queue.from_list([1, 2, 3, 4])

Elements can be added to the queue using the queue.push_front and queue.push_back functions:

let one_to_four = queue.from_list([1, 2, 3, 4])
let one_to_five = queue.push_back(one_to_four, 5)
let zero_to_five = queue.push_front(one_to_five, 0)

Elements can be removed from the queue using the queue.pop_front and queue.pop_back functions:

let one_to_four = queue.from_list([1, 2, 3, 4])

queue.pop_back(one_to_four)
// -> Ok(#(4, queue.from_list([1, 2, 3])))

queue.pop_front(one_to_four)
// -> Ok(#(1, queue.from_list([2, 3, 4])))

queue.pop_front(queue.new())
// -> Error(Nil)

A queue can be converted into a list using the queue.to_list function:

let one_to_four = queue.from_list([1, 2, 3, 4])
queue.to_list(one_to_four)
// -> [1, 2, 3, 4]

Queue equality

Due to how queues are implemented, two queues with the same elements in the same order may not be equal according to the == operator, which compares values structurally. For example:

let empty = queue.new()
let a = queue.push_front(empty, 1)
let b = queue.push_back(empty, 1)
a == b
// -> False

If you need to compare two queues for equality use can use the queue.is_logically_equal function.

queue.is_logically_equal(a, b, fn(x, y) { x == y })))
// -> True