Traits and Generics

fn first<T>(xs: &[T]) -> Option<&T> { xs.first() }
first(&[10, 20]).copied()  // Some(10)

fn stringify<T: std::fmt::Display>(v: T) -> String {
    format!("{v}")
}
stringify(42)  // "42"

fn nums() -> impl Iterator<Item = i32> {
    [1, 2, 3].into_iter()
}
nums().sum::<i32>()  // 6

fn max_of<T>(a: T, b: T) -> T
where
    T: PartialOrd,
{
    if a >= b { a } else { b }
}
max_of(3, 5)  // 5

trait Greet {
    fn greet(&self) -> String;
}
impl Greet for &str {
    fn greet(&self) -> String { format!("hi {self}") }
}
"Ada".greet()  // "hi Ada"

fn len(s: &dyn AsRef<str>) -> usize { s.as_ref().len() }
len(&"ab")  // 2

let s: String = String::from("x");
let t: String = "y".into();
s.len() + t.len()  // 2

fn chars<S: AsRef<str>>(s: S) -> usize {
    s.as_ref().chars().count()
}
chars(String::from("hi"))  // 2

#[derive(Default)]
struct C { n: i32 }
C::default().n  // 0

#[derive(Clone)]
struct P { x: i32 }
P { x: 1 }.clone().x  // 1

let v: Vec<_> = [1, 2, 3].iter().map(|x| x * 2).collect();
v  // [2, 4, 6]

fn show<T: std::fmt::Debug + Clone>(v: T) -> String {
    format!("{:?}", v.clone())
}
show(1)  // "1"

trait Bag {
    type Item;
    fn first(&self) -> Option<&Self::Item>;
}
impl Bag for Vec<i32> {
    type Item = i32;
    fn first(&self) -> Option<&i32> { self.get(0) }
}
vec![9, 8].first()  // Some(9)

use std::marker::PhantomData;
struct Id<T> { n: u64, _t: PhantomData<T> }
Id::<u8> { n: 1, _t: PhantomData }.n  // 1

fn first_n<const N: usize>(a: [i32; N]) -> i32 { a[0] }
first_n([7, 8, 9])  // 7