Enums

A type that can be exactly one of several named options.

Subscription status, payment state, instruction types, any fixed set of possibilities.

enum Status {
Active,
Expired,
Pending,
}

One of the named options inside an enum.

Creating or matching on an enum value. Each variant is one possibility.

Status::Active
Status::Expired

How you refer to a specific enum variant. Type name, then ::, then the variant name.

Every time you create or compare an enum value.

let s = Status::Active;
let d = Direction::North;

An integer Rust assigns to each variant automatically, starting at 0.

Lets you convert a variant to a number with as.

Direction::North as u8 // 0
Direction::South as u8 // 1

An enum variant with no data. Just a named state.

When one option simply means 'this happened' with no extra information.

enum Command {
Quit, // unit variant, no data
Pause,
}

An enum variant that holds unnamed values in parentheses.

When a variant needs to carry data but named fields are unnecessary.

enum Message {
Write(String),
Move(i32, i32),
}

An enum variant with named fields in curly braces.

When the data inside a variant is complex enough to benefit from names.

enum Message {
Move { x: i32, y: i32 },
}

Pulling data out of an enum variant right inside a match arm.

When your enum variants carry data you need to use.

match msg {
Message::Move { x, y } =>
println!("{} {}", x, y),
}