ligo/gitlab-pages/docs/language-basics/sets-lists-tuples.md

id	title
sets-lists-tuples	Sets, Lists, Tuples

Apart from complex data types such as maps and records, ligo also exposes sets, lists and tuples.

⚠️ Make sure to pick the appropriate data type for your use case; it carries not only semantic but also gas related costs.

Sets

Sets are similar to lists. The main difference is that elements of a set must be unique.

Defining a set

type int_set is set(int);
const my_set: int_set = set 
    1; 
    2; 
    3; 
end

type int_set = int set
let my_set: int_set =
  Set.add 3 (Set.add 2 (Set.add 1 (Set.empty: int set)))

type int_set = set(int);
let my_set: int_set =
  Set.add(3, Set.add(2, Set.add(1, Set.empty: set(int))));

Empty sets

const my_set: int_set = set end;
const my_set_2: int_set = set_empty;

let my_set: int_set = (Set.empty: int set)

let my_set: int_set = (Set.empty: set(int));

Checking if set contains an element

const contains_three: bool = my_set contains 3;
// or alternatively
const contains_three_fn: bool = set_mem(3, my_set);

let contains_three: bool = Set.mem 3 my_set

let contains_three: bool = Set.mem(3, my_set);

Obtaining the size of a set

const set_size: nat = size(my_set);

let set_size: nat = Set.size my_set

let set_size: nat = Set.size(my_set);

Modifying a set

const larger_set: int_set = set_add(4, my_set);
const smaller_set: int_set = set_remove(3, my_set);

let larger_set: int_set = Set.add 4 my_set
let smaller_set: int_set = Set.remove 3 my_set

let larger_set: int_set = Set.add(4, my_set);
let smaller_set: int_set = Set.remove(3, my_set);

Folding a set

function sum(const result: int; const i: int): int is result + i;
// Outputs 6
const sum_of_a_set: int = set_fold(sum, my_set, 0);

let sum (result, i: int * int) : int = result + i
let sum_of_a_set: int = Set.fold sum my_set 0

let sum = (result_i: (int, int)): int => result_i[0] + result_i[1];
let sum_of_a_set: int = Set.fold(sum, my_set, 0);

Lists

Lists are similar to sets, but their elements don't need to be unique and they don't offer the same range of built-in functions.

💡 Lists are useful when returning operations from a smart contract's entrypoint.

Defining a list

type int_list is list(int);
const my_list: int_list = list
    1;
    2;
    3;
end

type int_list = int list
let my_list: int_list = [1; 2; 3]

type int_list = list(int);
let my_list: int_list = [1, 2, 3];

Appending an element to a list

const larger_list: int_list = cons(4, my_list);
const even_larger_list: int_list = 5 # larger_list;

let larger_list: int_list = 4 :: my_list
(* CameLIGO doesn't have a List.cons *)

let larger_list: int_list = [4, ...my_list];
(* ReasonLIGO doesn't have a List.cons *)

> 💡 Lists can be iterated, folded or mapped to different values. You can find additional examples [here](https://gitlab.com/ligolang/ligo/tree/dev/src/test/contracts) and other built-in operators [here](https://gitlab.com/ligolang/ligo/blob/dev/src/passes/operators/operators.ml#L59)

Mapping of a list

function increment(const i: int): int is block { skip } with i + 1;
// Creates a new list with elements incremented by 1
const incremented_list: int_list = list_map(increment, even_larger_list);

let increment (i: int) : int = i + 1
(* Creates a new list with elements incremented by 1 *)
let incremented_list: int_list = List.map increment larger_list

let increment = (i: int): int => i + 1;
(* Creates a new list with elements incremented by 1 *)
let incremented_list: int_list = List.map(increment, larger_list);

Folding of a list:

function sum(const result: int; const i: int): int is block { skip } with result + i;
// Outputs 6
const sum_of_a_list: int = list_fold(sum, my_list, 0);

let sum (result, i: int * int) : int = result + i
// Outputs 6
let sum_of_a_list: int = List.fold sum my_list 0

let sum = ((result, i): (int, int)): int => result + i;
(* Outputs 6 *)
let sum_of_a_list: int = List.fold(sum, my_list, 0);

Tuples

Tuples are used to store related data that has a specific order and defined length without the need for named fields or a dedicated type identity. Probably the most common tuple is a pair of type (a, b). For example, if we were storing coordinates on a two dimensional grid we might use a pair tuple of type int * int to store the coordinates x and y. There is a specific order because x and y must always stay in the same location within the tuple for the data to make sense. There is also a defined length because the tuple pair can only ever have two elements, if we added a third dimension z its type would be incompatible with that of the pair tuple.

Like records, tuples can have members of arbitrary types in the same structure.

Defining a tuple

Unlike a record, tuple types do not have to be defined before they can be used. However below we will give them names for the sake of illustration.

type full_name is string * string;
const full_name: full_name = ("Alice", "Johnson");

type full_name = string * string
(* The parenthesis here are optional *)
let full_name: full_name = ("Alice", "Johnson")

type full_name = (string, string);
(* The parenthesis here are optional *)
let full_name: full_name = ("Alice", "Johnson");

Accessing an element in a tuple

The traditional way to access the elements of a tuple in OCaml is through a pattern match. LIGO does not currently support tuple patterns in its syntaxes.

However, it is possible to access LIGO tuples by their position.

Tuple elements are one-indexed and accessed like so:

const first_name: string = full_name.1;

Tuple elements are zero-indexed and accessed like so:

let first_name: string = full_name.0

let first_name: string = full_name[1];