Blowing in the wind

fn main() {
    // associated types
    //
    let point = Point(x, y);

    println!("Point X:{}, Y:{}", &x, &y);
    println!("Exist?:{}", point.exist(&x, &y));

    println!("Point-X:{}", point.v_axis());
    println!("Point-X:{}", point.h_axis());

    new_point(&point);
}

struct Point(i32, i32);

trait Position {
    type X;         // to use associated types
    type Y;

    fn exist(&self, _: &Self::X, _: &Self::Y) -> bool;
    fn h_axis(&self) -> i32;
    fn v_axis(&self) -> i32;
}

impl Position for Point {
    type X = i32;
    type Y = i32;

    fn exist(&self, x: &Self::X, y: &Self::Y) -> bool {
        (&self.0 == x) && (&self.1 == y)
    }

    fn h_axis(&self) -> Self::X {
        self.0
    }

    fn v_axis(&self) -> Self::Y {
        self.1
    }
}

fn new_point<Z: Position>(point: &Z) {                          // you don't need specify X and Y anymore
    println!("POINT:({},{})", point.v_axis(), point.h_axis())   // like "fn new_point<X, Y, Z>(point: &Z) where Z: Position<X, Y>"
}

struct Cardinal;
struct BlueJay;
struct Turkey;

trait Red {}
trait Blue {}
trait Yellow {}

impl Red for Cardinal {}
impl Blue for BlueJay {}
impl Yellow for Turkey {}

// These functions are only valid for types which implement these
// traits. The fact that the traits are empty is irrelevant.
fn red<T: Red>(_: &T)   -> &'static str { "red" }
fn blue<T: Blue>(_: &T) -> &'static str { "blue" }
fn yellow<T: Yellow>(_: &T) -> &'static str { "yellow" }

fn main() {
    let cardinal = Cardinal;
    let blue_jay = BlueJay;
    let turkey   = Turkey;

    // `red()` won't work on a blue jay nor vice versa
    // because of the bounds.
    println!("A cardinal is {}", red(&cardinal));
    println!("A blue jay is {}", blue(&blue_jay));
    println!("A turkey is {}", yellow(&turkey));
}

// A trait which implements the print marker: `{:?}`.
use std::fmt::Debug;

trait HasArea {
    fn area(&self) -> f64;
}

impl HasArea for Rectangle {
    fn area(&self) -> f64 {
        self.length * self.height
    }
}

impl HasArea for Triangle {
    fn area(&self) -> f64 {
        (self.length * self.height) / 2.0
    }
}

#[derive(Debug)]
struct Rectangle {
    length: f64,
    height: f64,
}

#[derive(Debug)]
struct Triangle {
    length: f64,
    height: f64,
}

// The generic `T` must implement `Debug`. Regardless
// of the type, this will work properly.
fn print_debug(t: &T) {
    println!("{:?}", t);
}

// `T` must implement `HasArea`. Any type which meets
// the bound can access `HasArea`'s function `area`.
fn area<t>(t: &T) -> f64
where
    T: HasArea,
{
    t.area()
}

fn main() {
    let rectangle = Rectangle {
        length: 3.0,
        height: 4.0,
    };
    let triangle = Triangle {
        length: 3.0,
        height: 5.0,
    };

    print_debug(&rectangle);
    println!("Area: {}", area(&rectangle));

    print_debug(&triangle);
    println!("Area: {}", area(&triangle));
}

use std::ops::Deref;

struct MyBox<T>(T);

impl MyBox<T> {      // original smart pointer
    fn new(x: T) -> MyBox {
        MyBox(x)
    }
}

impl Deref for MyBox {    // deref coercion
    type Target = T;    // to define return value type T for the Deref "Target"
                        // in the DeRef trait : fn deref(&self) -> &Self::Target;

    fn deref(&self) -> &T {
        &self.0
    }
}

fn hello(name: &str) {          // "&" is needed to get a known size of the argument at compile-time
    println!("Hello, {}!", name);
}

fn main() {
    let x = 5;
    let y = MyBox::new(x);

    assert_eq!(5, x);
    assert_eq!(5, *y);  // to success compile, Deref definition should be defined

    println!("x = {}, y = {}", x, *y);

    let m = MyBox::new(String::from("Rust"));
    hello(&m);
}

fn main(){
    println!("{}", hoge()());
}

fn hoge() -> Box<dyn Fn() -> String> {
    Box::new(|| -> String {
        String::from("ほげー！")
    })
}

    let abc = hoge();
    println!("{}", abc());

    let fn_plain = create_fn(); // return a closure
    
    fn_plain();

と

    create_fn()();

は等価です