ligo/gitlab-pages/docs/reference/list.md

id	title
list-reference	Lists — Linear Collections

import Syntax from '@theme/Syntax';

Lists are linear collections of elements of the same type. Linear means that, in order to reach an element in a list, we must visit all the elements before (sequential access). Elements can be repeated, as only their order in the collection matters. The first element is called the head, and the sub-list after the head is called the tail. For those familiar with algorithmic data structure, you can think of a list a stack, where the top is written on the left.

Defining Lists

const empty_list : list (int) = nil // Or list []
const my_list : list (int) = list [1; 2; 2] // The head is 1

let empty_list : int list = []
let my_list : int list = [1; 2; 2] // The head is 1

let empty_list : list (int) = [];
let my_list : list (int) = [1, 2, 2]; // The head is 1

Adding to Lists

Lists can be augmented by adding an element before the head (or, in terms of stack, by pushing an element on top).

const larger_list : list (int) = 5 # my_list // [5;1;2;2]

let larger_list : int list = 5 :: my_list // [5;1;2;2]

let larger_list : list (int) = [5, ...my_list]; // [5,1,2,2]

Functional Iteration over Lists

A functional iterator is a function that traverses a data structure and calls in turn a given function over the elements of that structure to compute some value. Another approach is possible in PascaLIGO: loops (see the relevant section).

There are three kinds of functional iterations over LIGO lists: the iterated operation, the map operation (not to be confused with the map data structure) and the fold operation.

Iterated Operation over Lists

The first, the iterated operation, is an iteration over the list with a unit return value. It is useful to enforce certain invariants on the element of a list, or fail.

function iter_op (const l : list (int)) : unit is
  block {
    function iterated (const i : int) : unit is
      if i > 3 then Unit else (failwith ("Below range.") : unit)
  } with List.iter (iterated, l)

Note that list_iter is deprecated.

let iter_op (l : int list) : unit =
  let predicate = fun (i : int) -> assert (i > 3)
  in List.iter predicate l

let iter_op = (l : list (int)) : unit => {
  let predicate = (i : int) => assert (i > 3);
  List.iter (predicate, l);
};

Mapped Operation over Lists

We may want to change all the elements of a given list by applying to them a function. This is called a map operation, not to be confused with the map data structure.

function increment (const i : int): int is i + 1

// Creates a new list with all elements incremented by 1
const plus_one : list (int) = List.map (increment, larger_list)

Note that list_map is deprecated.

let increment (i : int) : int = i + 1

// Creates a new list with all elements incremented by 1
let plus_one : int list = List.map increment larger_list

let increment = (i : int) : int => i + 1;

// Creates a new list with all elements incremented by 1
let plus_one : list (int) = List.map (increment, larger_list);

Folded Operation over Lists

A folded operation is the most general of iterations. The folded function takes two arguments: an accumulator and the structure element at hand, with which it then produces a new accumulator. This enables having a partial result that becomes complete when the traversal of the data structure is over.

function sum (const acc : int; const i : int): int is acc + i
const sum_of_elements : int = List.fold (sum, my_list, 0)

Note that list_fold is deprecated.

let sum (acc, i: int * int) : int = acc + i
let sum_of_elements : int = List.fold sum my_list 0

let sum = ((result, i): (int, int)): int => result + i;
let sum_of_elements : int = List.fold (sum, my_list, 0);

List Length

Get the number of elements in a list.

function size_of (const l : list (int)) : nat is List.length (l)

Note that size is deprecated.

let size_of (l : int list) : nat = List.length l

let size_of = (l : list (int)) : nat => List.length (l);