Tuples

Tuples gather a given number of values in a specific order and those values, called components, can be retrieved by their index (position). Probably the most common tuple is the pair. For example, if we were storing coordinates on a two dimensional grid we might use a pair [x, y] to store the coordinates x and y. There is a specific order, so [y, x] is not equal to [x, y] in general. The number of components is part of the type of a tuple, so, for example, we cannot add an extra component to a pair and obtain a triple of the same type: [x, y] has always a different type from [x, y, z], whereas [y, x] might have the same type as [x, y].

Tuple components can be of arbitrary types. A pair is a 2-tuple. If it contains a first component of type t_1 and a second component of type t_2, its type is written [t_1, t_2]. If more components: [t1, t2, ..., t_n]. (We can think of tuple types as products of types.) Tuple types do not have to be defined before they can be used:

const friends = ["Alice", "Bob"];

but it is sometimes more informative to define a type. Type definitions are introduced with the keyword type, like value definitions are with const. Instead of a value expression as a right-hand side, we have a type expression:

type couple = [string, string];

const friends : couple = ["Alice", "Bob"];

Accessing

If we want to get the first and second names of a pair, we can use destructuring. Destructuring a tuple allows us to give names to the elements inside the tuple:

const friends = ["Alice", "Bob"];

const [alice, bob] = friends;

That single definition actually introduces in the current scope two constants, alice and bob. Alternatively, if we still want to give a meaningful name to a useless component, we can use a silent variable for it, by prefixing it with _:

const [alice2, _bob] = friends;

Note how we renamed alice as alice2 in order to avoid a collision (redefinition) with previous one in the same top-level scope.

We can destructure nested tuples in the same manner:

const deep = [1, [2n, "Hello"]];

const [_x, [_y, greeting]] = deep; // greeting == "Hello"

This works well if we want to give a name to a component (like greeting above), but we might simply want the value of a component without naming it. In that case, we use the binary operator []:

const film = deep[1][1] + ", Dolly!" // film == "Hello, Dolly!"

The first component has index 0, the second 1 etc.