06_enums_and_pattern_matching

Enums and Pattern Matching

The notes reflect the topics in Chapter 6

Enum is not the enum in C++ or Enum in Python. If it has to be state in the term of C++, it’s like a combination of std::variant + class enum + std::tuple. To state in other word, enums contains variants of types that can holds other data.

#[derive(Debug)]
enum Message { // (1)
    Quit,
    Move { x: i32, y: i32 },
    Write(String),
    ChangeColor(i32, i32, i32),
}
 
impl Message {
    fn call(&self) {
        println!("The message is {:?}", self)
    }
}
 
fn main() {
    let m = Message::Write(String::from("hello"));
    m.call();
}

1. This is similar to

   struct QuitMessage; // unit struct
   struct MoveMessage {
       x: i32,
       y: i32,
   }
   struct WriteMessage(String); // tuple struct
   struct ChangeColorMessage(i32, i32, i32); // tuple struct

   but the data stored can not be retrieved trivially, like:

   let m = Message::Move(6, 9);
   println!(m.x);

   would not compile. This is because the type of m is Message, not a custom type. To get the data out you’ll need some match.

Another example of pattern matching

#[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
        }
        // (1)
    }
}
 
fn main() {
    value_in_cents(Coin::Quarter(UsState::Alaska));
 
    // Showcase the usage of if let
    let coin = Coin::Penny;
    let mut count = 0;
    if let Coin::Quarter(state) = coin {
        println!("State quarter from {:?}!", state);
    } else {
        count += 1;
    }
}
 
 

1. matches are exhaustive. You can use others => foo(others) or _ => 42 to handle the default cases.

The Option enum

The Option in stdlib looks like this:

enum Option<T> {
    None,
    Some(T),
}
 
// Use like this
fn main() {
    let some_number = Some(5);
    let some_string = Some("a string");
 
    let absent_number: Option<i32> = None;
}
}

Rant time (about C++)

Enjoy my freshed made meme:

See also the great blog post: std::visit is everything wrong with modern C++