Enums and Matching Pattern

• Enums:

  • Enums (short for "enumerations") are types that allow you to define a value that can be one of several variants.

  • Multiple variants types in single enum:

    enum Message {
      Quit,                       // No data
      Move { x: i32, y: i32 },    // Named fields
      Write(String),              // Single unnamed field
      ChangeColor(u8, u8, u8),    // Multiple unnamed fields
    }

• Matching Pattern:

  • Matching pattern syntax allows you to compare a value against a series of patterns and execute code based on the matching pattern.

  • You can destructure complex data types like structs, enums, and tuples in a match.

  • You can add conditions to patterns using if guards.

• Reference:

  • Enums

  • Pattern syntax

enums1.rs

#[derive(Debug)]
enum Message {
    // Define a few types of messages as used below.
    Resize,
    Move,
    Echo,
    ChangeColor,
    Quit,
}

fn main() {
    println!("{:?}", Message::Resize);
    println!("{:?}", Message::Move);
    println!("{:?}", Message::Echo);
    println!("{:?}", Message::ChangeColor);
    println!("{:?}", Message::Quit);
}

• In this exercise we need to define types of messages as used in the main function.

• So we simply add all of the types available in the main function into enum definition.

enums2.rs

#[derive(Debug)]
struct Point {
    x: u64,
    y: u64,
}

#[derive(Debug)]
enum Message {
    // Define the different variants used below.
    Resize { width: i32, height: i32 },
    Move(Point),
    Echo(String),
    ChangeColor(i32, i32, i32),
    Quit,
}

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

fn main() {
    let messages = [
        Message::Resize {
            width: 10,
            height: 30,
        },
        Message::Move(Point { x: 10, y: 15 }),
        Message::Echo(String::from("hello world")),
        Message::ChangeColor(200, 255, 255),
        Message::Quit,
    ];

    for message in &messages {
        message.call();
    }
}

• We can have multiple variants types in single enum.

• In this task we need to write 5 variants.

  enum Message {
    Resize { width: i32, height: i32 }, // with named field data like a struct
    Move(Point),  // has a struct data 
    Echo(String), // has a single String data
    ChangeColor(i32, i32, i32), // has 3 data like  a tuple
    Quit, // no data
  }

enums3.rs

struct Point {
    x: u64,
    y: u64,
}

enum Message {
    Resize { width: u64, height: u64 },
    Move(Point),
    Echo(String),
    ChangeColor(u8, u8, u8),
    Quit,
}

struct State {
    width: u64,
    height: u64,
    position: Point,
    message: String,
    // RGB color composed of red, green and blue.
    color: (u8, u8, u8),
    quit: bool,
}

impl State {
    fn resize(&mut self, width: u64, height: u64) {
        self.width = width;
        self.height = height;
    }

    fn move_position(&mut self, point: Point) {
        self.position = point;
    }

    fn echo(&mut self, s: String) {
        self.message = s;
    }

    fn change_color(&mut self, red: u8, green: u8, blue: u8) {
        self.color = (red, green, blue);
    }

    fn quit(&mut self) {
        self.quit = true;
    }

    fn process(&mut self, message: Message) {
        // TODO: Create a match expression to process the different message
        // variants using the methods defined above.
        match message {
            Message::Resize {
                width: w,
                height: h,
            } => self.resize(w, h),
            Message::Move(p) => self.move_position(p),
            Message::Echo(s) => self.echo(s),
            Message::ChangeColor(r, g, b) => self.change_color(r, g, b),
            Message::Quit => self.quit(),
        }
    }
}

fn main() {
    // You can optionally experiment here.
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_match_message_call() {
        let mut state = State {
            width: 0,
            height: 0,
            position: Point { x: 0, y: 0 },
            message: String::from("hello world"),
            color: (0, 0, 0),
            quit: false,
        };

        state.process(Message::Resize {
            width: 10,
            height: 30,
        });
        state.process(Message::Move(Point { x: 10, y: 15 }));
        state.process(Message::Echo(String::from("Hello world!")));
        state.process(Message::ChangeColor(255, 0, 255));
        state.process(Message::Quit);

        assert_eq!(state.width, 10);
        assert_eq!(state.height, 30);
        assert_eq!(state.position.x, 10);
        assert_eq!(state.position.y, 15);
        assert_eq!(state.message, "Hello world!");
        assert_eq!(state.color, (255, 0, 255));
        assert!(state.quit);
    }
}

• In this exercise we need to create matching pattern syntax for all the available variants in Message enum and call it's respective method.

• For each variants that have data, we want to catch or get that data so we can use it as arguments when calling it's respective method.

  match message {
    // bind width -> w and height -> h
    Message::Resize { width: w, height: h } => self.resize(w, h),
    // bind Point -> p
    Message::Move(p) => self.move_position(p),
    // bind String -> s
    Message::Echo(s) => self.echo(s),
    // bind first value -> r
    // bind second value -> r
    // bind third value -> r
    Message::ChangeColor(r, g, b) => self.change_color(r, g, b),
    Message::Quit => self.quit(),
  }

• This pattern syntax will bind data in the message enum variants into the defined variables.