Rust study : As input parameters

closure_fn.rs

/*
The rust by example ko https://hanbum.gitbooks.io/rustbyexample/content/fn/closures/input_parameters.html
compiled on https://play.rust-lang.org/
Fn 클로저는 참조로 획득 (&T)
FnMut 클로저는 가변참조로 획득 (&mut T)
FnOnce 클로저는 값으로 획득 (T)
...
이해가 잘 안되네..
*/

fn apply<F>(f: F) where F: FnOnce() {
    f()
}

fn apply_to_3<F>(f: F) -> i32 where
    F: Fn(i32) -> i32 {
        f(3)
}

fn main() {
    use std::mem;
    
    let greeting = String::from("hello");
    let mut farewell = String::from("goodbye");
    
    let diary = || {
        println!("I said {} ", greeting);
        
        farewell.push_str("!!!");
        println!("Then I screamed {}", farewell);
        
        //mem::drop(farewell);
    };
    
    apply(diary);
    //apply(diary);
    
    let double = |x| 2 * x;
    
    println!("3 double = {}", apply_to_3(double));
}