Enumerations

An enum defines a type whose value is exactly one of several named variants. The compiler knows the full set of variants, which is what makes match exhaustiveness checking possible.

enum Direction {
    North,
    South,
    East,
    West
}

let dir = Direction::North;

Simple Variants

The basic form lists the variant names, separated by commas:

enum Direction {
    North,
    South,
    East,
    West
}

By convention enum types and their variants use PascalCase. An enum introduces a distinct type: a value of one enum type is never interchangeable with a value of another, even if their variants look alike.

A variable of an enum type always holds exactly one variant — there is no "empty" or uninitialised enum value.

Referencing Variants

Refer to a variant through the enum name with the :: path separator:

let dir = Direction::North;
let opposite = Direction::South;

Exhaustive Matching

Because the compiler knows every variant, match can require that each one is handled. Omitting a case is a compile error, so adding a new variant forces every match over the enum to be revisited:

let label = match dir {
    Direction::North => "up",
    Direction::South => "down",
    Direction::East  => "right",
    Direction::West  => "left",
}

See match for the full pattern syntax, including guards and the _ wildcard.

Memory Layout

An enum without data is stored as a single integer tag — a small number identifying the variant. When no backing type is given, the compiler chooses the smallest integer type that can hold every variant, so a handful of variants typically occupy one byte:

enum Direction {     // 4 variants → fits in a uint8
    North,           // tag 0
    South,           // tag 1
    East,            // tag 2
    West             // tag 3
}

To pin the size and the numeric value of each variant — for serialisation or FFI — declare an explicit backing type.

Further Topics