Go: Redefining For Loop Variable Semantics

Re:Go: Redefining For Loop Variable Semantics by littlestymaar on Tuesday October 11, 2022 @01:25PM (#33166099)

Wow, Python and Go have it too? It was the biggest wart of pre-ES6 JavaScript, I never imagined other languages had it too (and it's honestly very disappointing for Go, since it's much more recent and we had plenty of hindsight when it was created…)

Edit: looks like Rust does it right: https://play.rust-lang.org/?version=stable&mode=debug&editio...

[ Parent | on HN ]

Re:Go: Redefining For Loop Variable Semantics by kevincox on Tuesday October 11, 2022 @03:00PM (#33167631)

Not quite how you tested it. The `move` is actually copying the variable:

    fn main() {
        let mut functions = vec![];
    
        let mut i = 0;
        functions.push(Box::new(move || println!("Hello {i}")) as Box<dyn Fn()>);
        i = 1;
        functions.push(Box::new(move || println!("Hello {i}")));
        i = 2;
        functions.push(Box::new(move || println!("Hello {i}")));
        
        for func in functions {
            func();
        }
    }

But the most equivalent code would be the following which doesn't compile:

    fn main() {
        let mut functions = vec![];
    
        for i in 0..10 {
            functions.push(Box::new(|| println!("Hello {i}")))
        }
        
        for func in functions {
            func();
        }
    }

[ Parent | on HN ]

Re:Go: Redefining For Loop Variable Semantics by littlestymaar on Tuesday October 11, 2022 @04:55PM (#33169128)

That's the point: the misleading code doesn't compile because the `i` variable scope in limited to the current execution of the loop (and that's why you can't borrow it for `functions`'s lifetime), which is exactly how ES6 fixed this in JavaScript.

[ Parent | on HN ]