% Enums

An enum in Rust is a type that represents data that is one of several possible variants. Each variant in the enum can optionally have data associated with it:

enum Message {
    Quit,
    ChangeColor(i32, i32, i32),
    Move { x: i32, y: i32 },
    Write(String),
}

The syntax for defining variants resembles the syntaxes used to define structs: you can have variants with no data (like unit-like structs), variants with named data, and variants with unnamed data (like tuple structs). Unlike separate struct definitions, however, an enum is a single type. A value of the enum can match any of the variants. For this reason, an enum is sometimes called a ‘sum type’: the set of possible values of the enum is the sum of the sets of possible values for each variant.

We use the :: syntax to use the name of each variant: they’re scoped by the name of the enum itself. This allows both of these to work:

# enum Message {
#     Move { x: i32, y: i32 },
# }
let x: Message = Message::Move { x: 3, y: 4 };

enum BoardGameTurn {
    Move { squares: i32 },
    Pass,
}

let y: BoardGameTurn = BoardGameTurn::Move { squares: 1 };

Both variants are named Move, but since they’re scoped to the name of the enum, they can both be used without conflict.

A value of an enum type contains information about which variant it is, in addition to any data associated with that variant. This is sometimes referred to as a ‘tagged union’, since the data includes a ‘tag’ indicating what type it is. The compiler uses this information to enforce that you’re accessing the data in the enum safely. For instance, you can’t simply try to destructure a value as if it were one of the possible variants:

fn process_color_change(msg: Message) {
    let Message::ChangeColor(r, g, b) = msg; // This causes a compile-time error.
}

Not supporting these operations may seem rather limiting, but it’s a limitation which we can overcome. There are two ways: by implementing equality ourselves, or by pattern matching variants with match expressions, which you’ll learn in the next section. We don’t know enough about Rust to implement equality yet, but we’ll find out in the traits section.

Constructors as functions

An enum constructor can also be used like a function. For example:

# enum Message {
# Write(String),
# }
let m = Message::Write("Hello, world".to_string());

is the same as

# enum Message {
# Write(String),
# }
fn foo(x: String) -> Message {
    Message::Write(x)
}

let x = foo("Hello, world".to_string());

This is not immediately useful to us, but when we get to closures, we’ll talk about passing functions as arguments to other functions. For example, with iterators, we can do this to convert a vector of Strings into a vector of Message::Writes:

# enum Message {
# Write(String),
# }

let v = vec!["Hello".to_string(), "World".to_string()];

let v1: Vec<Message> = v.into_iter().map(Message::Write).collect();