wiki/content/20200902150714-enums.md

date	id	title
20200902	b95f09af-2cd0-46f2-872e-3cb542d2b2e9	Rust enums

Basics

Rust also supports enums.

use std::fmt::Debug;

#[derive(Debug)]
enum Message {
    Write(String),
}

fn main() {
    let m = Message::Write(String::from("Hello!"));
    println!("{:?}", m)
}

Methods

As with structs, methods can be defined as well:

use std::fmt::Debug;

#[derive(Debug)]
enum Message {
    Write(String),
}

impl Message {
    fn call(&self) {
        println!("tralala");
        println!("{:?}", self)
    }
}

fn main() {
    let m = Message::Write(String::from("Hello!"));
    m.call();
}

Standard library enums

Option

Instead of `null` Rust has the Option enum built in:

fn main() {
    let some_number = Some(5);
    let some_string = Some("a string");

    let absent_number: Option<i32> = None;
}

The cool thing about using Option is that `Option<T>` and `T` are different types, therefore the programmer has to define if NULL is acceptable beforehand. The following won't run:

fn main() {
    let x: i8 = 5;
    let y: Option<i8> = Some(5);

    let sum = x + y;
}

Huzzah, No more having to worry about incorrectly assuming a not null value!

Match control flow operator

Think of `switch`, but for enums.

enum Coin {
    Penny,
    Nickel,
    Dime,
    Quarter,
}

fn value_in_cents(coin: Coin) -> u8 {
    match coin {
        Coin::Penny => {
            println!("Lucky penny!");
            1
        }
        Coin::Nickel => 5,
        Coin::Dime => 10,
        Coin::Quarter => 25,
    }
}

fn main() {}

Patterns that bind to values

Another useful feature of match arms is that they can bind to the parts of the values that match the pattern. This is how we can extract values out of enum variants.

#[derive(Debug)]
enum UsState {
    Alabama,
    Alaska,
    // --snip--
}

enum Coin {
    Penny,
    Nickel,
    Dime,
    Quarter(UsState),
}

fn value_in_cents(coin: Coin) -> u8 {
    match coin {
        Coin::Penny => 1,
        Coin::Nickel => 5,
        Coin::Dime => 10,
        Coin::Quarter(state) => {
            println!("State quarter from {:?}!", state);
            25
        }
    }
}

fn main() {
    value_in_cents(Coin::Quarter(UsState::Alaska));
}

Matching with Option<T>

`match` also plays nice with `Option<t>`:

fn main() {
    fn plus_one(x: Option<i32>) -> Option<i32> {
        match x {
            None => None,
            Some(i) => Some(i + 1),
        }
    }

    let five = Some(5);
    let six = plus_one(five);
    let none = plus_one(None);
}

The _ Placeholder

Rust also has a pattern we can use when we don’t want to list all possible values:

fn main() {
    let some_u8_value = 0u8;
    match some_u8_value {
        1 => println!("one"),
        3 => println!("three"),
        5 => println!("five"),
        7 => println!("seven"),
        _ => (),
    }
}

if let syntax suger

fn main() {
    let some_u8_value = Some(0u8);
    match some_u8_value {
        Some(3) => println!("three"),
        _ => (),
    }
}

is the same as

fn main() {
    let some_u8_value = Some(0u8);
    if let Some(3) = some_u8_value {
        println!("three");
    }
}

and

#[derive(Debug)]
enum UsState {
    Alabama,
    Alaska,
    // --snip--
}

enum Coin {
    Penny,
    Nickel,
    Dime,
    Quarter(UsState),
}

fn main() {
    let coin = Coin::Penny;
    let mut count = 0;
    match coin {
        Coin::Quarter(state) => println!("State quarter from {:?}!", state),
        _ => count += 1,
    }
}

is the same as

#[derive(Debug)]
enum UsState {
    Alabama,
    Alaska,
    // --snip--
}

enum Coin {
    Penny,
    Nickel,
    Dime,
    Quarter(UsState),
}

fn main() {
    let coin = Coin::Penny;
    let mut count = 0;
    if let Coin::Quarter(state) = coin {
        println!("State quarter from {:?}!", state);
    } else {
        count += 1;
    }
}