Traits: methods + `Self` + access

Four ‘trait’ point of a trait

A trait is:

a collection of methods
defined for an unknown type:Self
They can access other methods declared in the same trait.
Traits can be implemented for any data type.

In the example below, we define Animal, a group of methods. The Animal trait is then implemented for the Sheep data type, allowing the use of methods from Animal with a Sheep.

struct Sheep { naked: bool, name: &'static str }

trait Animal {
    // Associated function signature; `Self` refers to the implementor type.
    fn new(name: &'static str) -> Self;

    // Method signatures; these will return a string.
    fn name(&self) -> &'static str;
    fn noise(&self) -> &'static str;

    // Traits can provide default method definitions.
    fn talk(&self) {
        println!("{} says {}", self.name(), self.noise());
    }
}

impl Sheep {
    fn is_naked(&self) -> bool {
        self.naked
    }

    fn shear(&mut self) {
        if self.is_naked() {
            // Implementor methods can use the implementor's trait methods.
            println!("{} is already naked...", self.name());
        } else {
            println!("{} gets a haircut!", self.name);

            self.naked = true;
        }
    }
}

// Implement the `Animal` trait for `Sheep`.
impl Animal for Sheep {
    // `Self` is the implementor type: `Sheep`.
    fn new(name: &'static str) -> Sheep {
        Sheep { name: name, naked: false }
    }

    fn name(&self) -> &'static str {
        self.name
    }

    fn noise(&self) -> &'static str {
        if self.is_naked() {
            "baaaaah?"
        } else {
            "baaaaah!"
        }
    }
    
    // Default trait methods can be overridden.
    fn talk(&self) {
        // For example, we can add some quiet contemplation.
        println!("{} pauses briefly... {}", self.name, self.noise());
    }
}

fn main() {
    // Type annotation is necessary in this case.
    let mut dolly: Sheep = Animal::new("Dolly");
    // TODO ^ Try removing the type annotations.

    dolly.talk();
    dolly.shear();
    dolly.talk();
}

Differentiate between definition and implementation

trait Animal {
    // Associated function signature; `Self` refers to the implementor type.
    fn new(name: &'static str) -> Self;
    }
impl Animal for Sheep {
    // `Self` is the implementor type: `Sheep`.
    fn new(name: &'static str) -> Sheep {
        Sheep { name: name, naked: false }
    }
}

Default trait methods can be overridden
Type annotation is necessary here

   Compiling playground v0.0.1 (/playground)
error[E0282]: type annotations needed
  --> src/main.rs:62:9
   |
62 |     let mut dolly = Animal::new("Dolly");
   |         ^^^^^^^^^
...
65 |     dolly.talk();
   |     ----- type must be known at this point
   |
help: consider giving `dolly` an explicit type
   |
62 |     let mut dolly: _ = Animal::new("Dolly");
   |                  +++

For more information about this error, try `rustc --explain E0282`.
error: could not compile `playground` due to previous error