rust_book/ch10/traits/src/main.rs

// trait defines functionalitity a particular type has and
// can share with other types. Can use traits to define shared
// behavior in an abstract way. Can use trait bounds to specify
// that a generic type can be any type that has certain behavior

use std::fmt::Display;

// Like Haskell's deriving
// Except orphans aren't allowed at all
pub trait Summary {
    // fn summarize(&self) -> String; // Structs must provide own implementation
    fn summarize(&self) -> String {
        String::from("(Read more...)")
    }
}

pub struct NewsArticle {
    pub headline: String,
    pub location: String,
    pub author: String,
    pub content: String,
}

impl Summary for NewsArticle {
    fn summarize(&self) -> String {
        format!("{}, by {} ({})", self.headline, self.author, self.location)
    }
}

pub struct Tweet {
    pub username: String,
    pub content: String,
    pub reply: bool,
    pub retweet: bool,
}

impl Summary for Tweet {
    fn summarize(&self) -> String {
        format!("{}: {}", self.username, self.content)
    }
}

// Should use lifetime annotations when using a reference
// in a variable
struct ImportantExcerpt<'a> {
    part: &'a str,
}

fn main() {
    let tweet = Tweet {
        username: String::from("horse_ebooks"),
        content: String::from("of course, as you probably already know, people"),
        reply: false,
        retweet: false,
    };

    println!("1 new tweet: {}", tweet.summarize());

    // The next lines fail as borrowing
    // let r;

    // {
    //     let x = 5;
    //     r = &x;
    // }

    // println!("r: {}", r);

    let x = 5;
    let r = &x;
    println!("r: {r}");

    let string1 = String::from("abcd");
    let string2 = "xyz";

    // need a lifetime reference for situations like this
    // &i32: a regular reference
    // &'a i32: a reference with an explicit lifetime
    // &'a mut i32: a mutable reference with an explicit lifetime
    let result = longest(string1.as_str(), string2);
    println!("The longest string is {result}");

    let string1 = String::from("long string is long");
    // let result;

    {
        let string2 = String::from("xyz");
        let result = longest(string1.as_str(), string2.as_str());
        // result = longest(string1.as_str(), string2.as_str());
        println!("The longest string is {result}");
    }

    // println!("The longest string is {result}");

    // the static lifetime
    // Can life for the duration of the program
    let s: &'static str = "I have a static lifetime";
}

// fn notify<T: Summary>(item1: &T, item2: &T) {}
fn notify(item: &(impl Summary + std::fmt::Display)) {}
fn notify2<T: Summary + std::fmt::Display>(item: &T) {}

fn some_function<T, U>(t: &T, u: &U) -> i32
where
    // Similar to haskell :)
    T: std::fmt::Display + Clone,
    U: Clone + std::fmt::Debug,
{
    2
}

// impl<T: Display> ToString for T {}

// <'a> and 'a dispersed in the function parameters are
// lifetime annotations. Lifetimes annotations are needed because
// Rust can't tell whether the reference being returned refers to x or y
// Feels like a similar vibe to Haskell `forall a`.
fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
    if x.len() > y.len() {
        x
    } else {
        y
    }
}

// lifetime elision rules. Some functions that have common patterns
// with references passed as a parameter have lifetime annotations
// passed implicitly due to soooooo many functions having this
// lifetime annotation pattern used.
//
// Lifetimes on function/method parameters called input lifetimes.
// Lifetimes on return values output lifetimes
//
// 3 Rules for lifetime functions
// 1: Compiler assigns different lifetime parameter to each lifetime
//    in each input type
//    fn foo(x: &i32) becomes fn foo<'a>(x: &'a i32)
//    fn foo(x: &i32, y: &i32) becomes fn foo<'a, 'b>(x: &'a i32, y: &'b i32)
//    fn foo(x: &ImportantExcerpt) becomes fn foo<'a, 'b>(x: &'a ImportantExcerpt<'b>)
// 2: If exactly one input lifetime paramter, lifetime applied to all output
//    lifetime parameters
// 3: If multiple input lifetime parameters, but one of them is &self or &mut self,
//    lifetime of self assigned to all output lifetime parameters

// Lifetime annotations in method definitions

impl<'a> ImportantExcerpt<'a> {
    fn level(&self) -> i32 {
        3
    }

    // example of third lifetime elision rule
    fn announce_and_return_part(&self, announcement: &str) -> &str {
        println!("Attention Please: {announcement}");
        self.part
    }
}

// Generic Type Paramters, trait bounds, and lifetimes together
fn longest_with_an_announcement<'a, T>(x: &'a str, y: &'a str, ann: T) -> &'a str
where
    T: Display,
{
    println!("Announcement!: {ann}");
    if x.len() > y.len() {
        x
    } else {
        y
    }
}