compound-data-types.md

% Compound Data Types

Rust, like many programming languages, has a number of different data types that are built-in. You've already done some simple work with integers and strings, but next, let's talk about some more complicated ways of storing data.

Tuples

The first compound data type we're going to talk about are called tuples. Tuples are an ordered list of a fixed size. Like this:

let x = (1, "hello");

The parentheses and commas form this two-length tuple. Here's the same code, but with the type annotated:

let x: (i32, &str) = (1, "hello");

As you can see, the type of a tuple looks just like the tuple, but with each position having a type name rather than the value. Careful readers will also note that tuples are heterogeneous: we have an i32 and a &str in this tuple. You have briefly seen &str used as a type before, and we'll discuss the details of strings later. In systems programming languages, strings are a bit more complex than in other languages. For now, just read &str as a string slice, and we'll learn more soon.

You can access the fields in a tuple through a destructuring let. Here's an example:

let (x, y, z) = (1, 2, 3);

println!("x is {}", x);

Remember before when I said the left-hand side of a let statement was more powerful than just assigning a binding? Here we are. We can put a pattern on the left-hand side of the let, and if it matches up to the right-hand side, we can assign multiple bindings at once. In this case, let "destructures," or "breaks up," the tuple, and assigns the bits to three bindings.

This pattern is very powerful, and we'll see it repeated more later.

There are also a few things you can do with a tuple as a whole, without destructuring. You can assign one tuple into another, if they have the same contained types and arity. Tuples have the same arity when they have the same length.

let mut x = (1, 2); // x: (i32, i32)
let y = (2, 3); // y: (i32, i32)

x = y;

You can also check for equality with ==. Again, this will only compile if the tuples have the same type.

let x = (1, 2, 3);
let y = (2, 2, 4);

if x == y {
    println!("yes");
} else {
    println!("no");
}

This will print no, because some of the values aren't equal.

Note that the order of the values is considered when checking for equality, so the following example will also print no.

let x = (1, 2, 3);
let y = (2, 1, 3);

if x == y {
    println!("yes");
} else {
    println!("no");
}

One other use of tuples is to return multiple values from a function:

fn next_two(x: i32) -> (i32, i32) { (x + 1, x + 2) }

fn main() {
    let (x, y) = next_two(5);
    println!("x, y = {}, {}", x, y);
}

Even though Rust functions can only return one value, a tuple is one value, that happens to be made up of more than one value. You can also see in this example how you can destructure a pattern returned by a function, as well.

Tuples are a very simple data structure, and so are not often what you want. Let's move on to their bigger sibling, structs.

Structs

A struct is another form of a record type, just like a tuple. There's a difference: structs give each element that they contain a name, called a field or a member. Check it out:

struct Point {
    x: i32,
    y: i32,
}

fn main() {
    let origin = Point { x: 0, y: 0 }; // origin: Point

    println!("The origin is at ({}, {})", origin.x, origin.y);
}

There's a lot going on here, so let's break it down. We declare a struct with the struct keyword, and then with a name. By convention, structs begin with a capital letter and are also camel cased: PointInSpace, not Point_In_Space.

We can create an instance of our struct via let, as usual, but we use a key: value style syntax to set each field. The order doesn't need to be the same as in the original declaration.

Finally, because fields have names, we can access the field through dot notation: origin.x.

The values in structs are immutable by default, like other bindings in Rust. Use mut to make them mutable:

struct Point {
    x: i32,
    y: i32,
}

fn main() {
    let mut point = Point { x: 0, y: 0 };

    point.x = 5;

    println!("The point is at ({}, {})", point.x, point.y);
}

This will print The point is at (5, 0).

Tuple Structs and Newtypes

Rust has another data type that's like a hybrid between a tuple and a struct, called a tuple struct. Tuple structs do have a name, but their fields don't:

struct Color(i32, i32, i32);
struct Point(i32, i32, i32);

These two will not be equal, even if they have the same values:

# struct Color(i32, i32, i32);
# struct Point(i32, i32, i32);
let black = Color(0, 0, 0);
let origin = Point(0, 0, 0);

It is almost always better to use a struct than a tuple struct. We would write Color and Point like this instead:

struct Color {
    red: i32,
    blue: i32,
    green: i32,
}

struct Point {
    x: i32,
    y: i32,
    z: i32,
}

Now, we have actual names, rather than positions. Good names are important, and with a struct, we have actual names.

There is one case when a tuple struct is very useful, though, and that's a tuple struct with only one element. We call this a newtype, because it lets you create a new type that's similar to another one:

struct Inches(i32);

let length = Inches(10);

let Inches(integer_length) = length;
println!("length is {} inches", integer_length);

As you can see here, you can extract the inner integer type through a destructuring let, as we discussed previously in 'tuples.' In this case, the let Inches(integer_length) assigns 10 to integer_length.

Enums

Finally, Rust has a "sum type", an enum. Enums are an incredibly useful feature of Rust, and are used throughout the standard library. An enum is a type which ties a set of alternates to a specific name. For example, below we define Character to be either a Digit or something else. These can be used via their fully scoped names: Character::Other (more about :: below).

enum Character {
    Digit(i32),
    Other,
}

An enum variant can be defined as most normal types. Below are some example types have been listed which also would be allowed in an enum.

struct Empty;
struct Color(i32, i32, i32);
struct Length(i32);
struct Status { Health: i32, Mana: i32, Attack: i32, Defense: i32 }
struct HeightDatabase(Vec<i32>);

So you see that depending on the sub-datastructure, the enum variant, same as a struct, may or may not hold data. That is, in Character, Digit is a name tied to an i32 where Other is just a name. However, the fact that they are distinct makes this very useful.

As with structures, enums don't by default have access to operators such as compare ( == and !=), binary operations (* and +), and order (< and >=). As such, using the previous Character type, the following code is invalid:

// These assignments both succeed
let ten  = Character::Digit(10);
let four = Character::Digit(4);

// Error: `*` is not implemented for type `Character`
let forty = ten * four;

// Error: `<=` is not implemented for type `Character`
let four_is_smaller = four <= ten;

// Error: `==` is not implemented for type `Character`
let four_equals_ten = four == ten;

This may seem rather limiting, but it's a limitation which we can overcome. There are two ways: by implementing equality ourselves, or by using the match keyword. We don't know enough about Rust to implement equality yet, but we can use the Ordering enum from the standard library, which does:

enum Ordering {
    Less,
    Equal,
    Greater,
}

Because we did not define Ordering, we must import it (from the std library) with the use keyword. Here's an example of how Ordering is used:

use std::cmp::Ordering;

fn cmp(a: i32, b: i32) -> Ordering {
    if a < b { Ordering::Less }
    else if a > b { Ordering::Greater }
    else { Ordering::Equal }
}

fn main() {
    let x = 5;
    let y = 10;

    let ordering = cmp(x, y); // ordering: Ordering

    if ordering == Ordering::Less {
        println!("less");
    } else if ordering == Ordering::Greater {
        println!("greater");
    } else if ordering == Ordering::Equal {
        println!("equal");
    }
}

The :: symbol is used to indicate a namespace. In this case, Ordering lives in the cmp submodule of the std module. We'll talk more about modules later in the guide. For now, all you need to know is that you can use things from the standard library if you need them.

Okay, let's talk about the actual code in the example. cmp is a function that compares two things, and returns an Ordering. We return either Ordering::Less, Ordering::Greater, or Ordering::Equal, depending on if the two values are less, greater, or equal. Note that each variant of the enum is namespaced under the enum itself: it's Ordering::Greater not Greater.

The ordering variable has the type Ordering, and so contains one of the three values. We then do a bunch of if/else comparisons to check which one it is.

This Ordering::Greater notation is too long. Let's use use to import the enum variants instead. This will avoid full scoping:

use std::cmp::Ordering::{self, Equal, Less, Greater};

fn cmp(a: i32, b: i32) -> Ordering {
    if a < b { Less }
    else if a > b { Greater }
    else { Equal }
}

fn main() {
    let x = 5;
    let y = 10;

    let ordering = cmp(x, y); // ordering: Ordering

    if ordering == Less { println!("less"); }
    else if ordering == Greater { println!("greater"); }
    else if ordering == Equal { println!("equal"); }
}

Importing variants is convenient and compact, but can also cause name conflicts, so do this with caution. It's considered good style to rarely import variants for this reason.

As you can see, enums are quite a powerful tool for data representation, and are even more useful when they're generic across types. Before we get to generics, though, let's talk about how to use them with pattern matching, a tool that will let us deconstruct this sum type (the type theory term for enums) in a very elegant way and avoid all these messy if/elses.