Enums

Declaration

An enum defines a type with a fixed set of variants.

enum Direction {
  North,
  East,
  West,
  South,
}

Variants with data

Variants can carry data. A tuple variant has positional fields:

enum IpAddress {
  V4(int, int, int, int),
  V6(string),
}

let home = IpAddress.V4(127, 0, 0, 1)
let loopback = IpAddress.V6("::1")

A struct variant has named fields:

enum Shape {
  Circle { radius: float64 },
  Rectangle { width: float64, height: float64 },
}

let c = Shape.Circle { radius: 5.0 }
let r = Shape.Rectangle { width: 10.0, height: 20.0 }

Variants can be mixed in a single enum:

enum Event {
  Ready,
  KeyPress(rune),
  Click { x: int, y: int },
}

Generic enums

Enums accept type parameters:

enum Cache<T> {
  Hit(T),
  Miss,
}

let found = Cache.Hit("hello")

Option, Result, and Partial in the prelude are generic enums.