rust_book/ch13/closures/src/main.rs

// Closures are anonymous functions. Can save in variables or pass
// to other functions. Closures can capture the values from the scope
// they're defined in.

use std::thread;
use std::time::Duration;

#[derive(Debug, PartialEq, Copy, Clone)]
enum ShirtColor {
    Red,
    Blue,
}

struct Inventory {
    shirts: Vec<ShirtColor>,
}

impl Inventory {
    fn giveaway(&self, user_preference: Option<ShirtColor>) -> ShirtColor {
        user_preference.unwrap_or_else(|| self.most_stocked())
    }

    fn most_stocked(&self) -> ShirtColor {
        let mut num_red = 0;
        let mut num_blue = 0;

        for color in &self.shirts {
            match color {
                ShirtColor::Red => num_red += 1,
                ShirtColor::Blue => num_blue += 1,
            }
        }

        if num_red > num_blue {
            ShirtColor::Red
        } else {
            ShirtColor::Blue
        }
    }
}

#[derive(Debug)]
struct Rectangle {
    width: u32,
    height: u32,
}

fn main() {
    let store = Inventory {
        shirts: vec![ShirtColor::Blue, ShirtColor::Red, ShirtColor::Blue],
    };

    let user_pref1 = Some(ShirtColor::Red);
    let giveaway1 = store.giveaway(user_pref1);
    println!(
        "The user with preference {:?} gets {:?}",
        user_pref1, giveaway1
    );

    let user_pref2 = None;
    let giveaway2 = store.giveaway(user_pref2);
    println!(
        "The user with preference {:?} gets {:?}",
        user_pref2, giveaway2
    );

    // closure type inference and annotation
    let expensive_closure = |num: u32| -> u32 {
        println!("calculating slowly...");
        thread::sleep(Duration::from_secs(2));
        num
    };

    // similarity between closures and functions
    fn add_one_v1(x: u32) -> u32 {
        x + 1
    }
    let add_one_v2 = |x: u32| -> u32 { x + 1 };

    // capturing references or moving ownership
    // closures capture values from env in 3 ways
    // 1) borrowing immutably
    // 2) borrowing mutably
    // 3) taking ownership
    let list = vec![1, 2, 3];
    println!("Before defining closure: {list:?}");

    let only_borrows = || println!("From closure: {list:?}");

    println!("Before calling closure: {list:?}");
    only_borrows();
    println!("After calling closure: {list:?}");

    // change closure body to push instead of print
    let mut list = vec![1, 2, 3, 4];
    println!("Before defining closure: {list:?}");

    let mut borrows_mutably = || list.push(7);

    // println!("Before calling closure: {list:?}");
    borrows_mutably();
    println!("After calling closure: {list:?}");

    // If want to force closure to take ownership of values it uses even
    // tho closure doesn't strictly need ownership, use the `move` keyword
    let list = vec![1, 2, 3];
    println!("Before defining closure: {list:?}");

    thread::spawn(move || println!("From thread: {list:?}"))
        .join()
        .unwrap();

    // Moving captured values out of the closures and the Fn traits
    // Once a closure has captured a reference or captured ownership of a value
    // from the env where closure is defined (affecting what is moved into the
    // closure), the code in the body of the closure defines what happens to
    // the references or values when the closure is evaluated later (affecting
    // what is moved out of the closure). Closure body can do any of the following:
    // 1) move captured value out of the closure
    // 2) mutate captured value
    // 3) neither move nor mutate the value
    // 4) capture nothing from the env to begin with
    //
    // Closures will automatically implement some or all of the following
    // traits in an additive fashing depending on the closure's body
    // handles the values
    // 1) FnOnce applies to all closures that can be called once.
    //    All closures implement at least this trait. A closure that
    //    moves captured values out of its body will only implement
    //    FnOnce and one of the other Fn traits, because it can only
    //    by called once.
    // 2) FnMut applies to closures that don't move captured values out
    //    of their body but might mutate the captured values. These
    //    closures can't be called more than once.
    // 3) Fn applies to closures that don't move captured values out of
    //    their body and that don't mutate captured values, and closures
    //    that capture nothing from their environment. These closures can
    //    be called more than once without mutating their environment. This
    //    is nice for calling a closure multiple times concurrently.

    // sort_by_key uses FnMut
    let mut list = [
        Rectangle {
            width: 10,
            height: 1,
        },
        Rectangle {
            width: 3,
            height: 5,
        },
        Rectangle {
            width: 7,
            height: 12,
        },
    ];

    // let mut sort_operations = vec![];
    // let value = String::from("closure called");
    let mut num_sort_operations = 0;

    // list.sort_by_key(|r| r.width);
    list.sort_by_key(|r| {
        // sort_operations.push(value);
        num_sort_operations += 1;
        r.width
    });
    println!("{list:#?}");

    // closures must name captured lifetimes
    // let s_own = String::from("Hello world");
    // let cloner = make_a_cloner(&s_own);
    // drop(s_own);
    // cloner();
}

// from closures must name captured lifetimes
// Need to add a lifetime to tell Rust that the closure
// returned from make_a_cloner must not live longer than s_ref.
// The `+ 'a` in the return type indicates that the closure
// must live no longer than 'a.
// fn make_a_cloner<'a>(s_ref: &'a str) -> impl Fn() -> String + 'a {
// move || s_ref.to_string()
// }
// Can make this function more concise with the lifetime elision rules
fn make_a_cloner(s_ref: &str) -> impl Fn() -> String + '_ {
    move || s_ref.to_string()
}

// The implementation for unwrap_or_else
// Uses FnOnce
// impl<T> Option<T> {
//     pub fn unwrap_or_else<F>(self, f: F) -> T
//     where
//         F: FnOnce() -> T,
//     {
//         match self {
//             Some(x) => x,
//             None => f(),
//         }
//     }
// }