Variants

Construct and use enum variants.

Working with variants

Enum values are passed to functions, returned, and taken apart with match. match checks at compile time that every variant is handled:

fn ters(y: Yon) -> Yon {
    match y {
        Yon::Kuzey => Yon::Guney,
        Yon::Guney => Yon::Kuzey,
        Yon::Dogu => Yon::Bati,
        Yon::Bati => Yon::Dogu,
    }
}

If you add a new variant to the enum, every match that doesn't handle it fails to compile, so no case is forgotten.

Shortening with use, and methods

Methods are added to enums with impl too. To reduce long names, the variants can be brought into scope:

impl Yon {
    fn yatay_mi(&self) -> bool {
        use Yon::*;
        matches!(self, Dogu | Bati)
    }
}

println!("{}", Yon::Dogu.yatay_mi());   // true

The matches! macro returns, as a bool, whether a value fits a pattern.

Exercise Opposite direction

Write fn ters(y: Yon) -> Yon for the Yon enum with match. Print the results of ters(Yon::Kuzey) and ters(Yon::Dogu) with {:?}, separated by a space.

Show example solution

#[derive(Debug)]
enum Yon {
    Kuzey,
    Guney,
    Dogu,
    Bati,
}

fn ters(y: Yon) -> Yon {
    match y {
        Yon::Kuzey => Yon::Guney,
        Yon::Guney => Yon::Kuzey,
        Yon::Dogu => Yon::Bati,
        Yon::Bati => Yon::Dogu,
    }
}

fn main() {
    println!("{:?} {:?}", ters(Yon::Kuzey), ters(Yon::Dogu));
}

Test yourself

What is needed to write <code>Yon::Kuzey == Yon::Kuzey</code>?
- #[derive(PartialEq)]
- #[derive(Debug)]
- Nothing
- impl Yon
Show answer

#[derive(PartialEq)] — The == operator needs the PartialEq trait; derive generates it automatically.

Pattern check

let y = Yon::Bati;
let yatay = ____(y, Yon::Dogu | Yon::Bati);

Show answer

let y = Yon::Bati;
let yatay = matches!(y, Yon::Dogu | Yon::Bati);

Syntax and role

match y { .. }

Branching by variant
matches!(y, ..)

bool pattern test
use Yon::*

Brings variants into scope
#[derive(PartialEq)]

== support

Next lesson: Variants with data

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