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0 pointsburntsushi3y ago0 comments

> But I can basically demonstrate other language features that are not working with closures, such as generics -- you just can't have generic closures in the same way as you have generic functions, even in a synchronous code.

Generics and closures work just fine:

    use std::ops::Add;

    fn adder<T: Copy + Add<Output=T> + 'static>(x: T) -> impl Fn(T) -> Box<dyn Fn(T) -> T> {
        move |y| Box::new(move |z| y + x + z)
    }

    fn main() {
        let add23 = adder(2)(3);
        assert_eq!(10, add23(5));
        assert_eq!(13, add23(8));
    }

You keep making sensationalist generalizations. It's trivial to demonstrate that closures and generics work together just fine, as I've done above. Are there subtleties and complexities and other things that can make certain cases or areas tricky? Absolutely. But that's not the same as "not working."

0 comments

Hirrolot3y ago

Well, your example is not a generic closure, since inside `adder`, you already have a generic type `T`; it's introduced by the `fn` keyword. If we try to do the same for a closure, we would face a type error. This issue has been discussed on SO [1].

I haven't claimed that closures are not working, since well, they are working, but under a very limited set of circumstances. Again, I see nothing sensational, since the trickery of using closures has been discussed elsewhere.

[1] https://stackoverflow.com/questions/34814423/possible-to-def...

burntsushiOP3y ago

> Well, your example is not a generic closure, since inside `adder`, you already have a generic type `T`; it's introduced by the `fn` keyword. If we try to do the same for a closure, we would face a type error. This issue has been discussed on SO [1].

Of course it's generic. If you were to write the type out for the closure, it would be generic over 'T'. (Whether that type ever actually gets written out that way or not is a different matter.) As far as I can tell, what you're saying is that Rust doesn't support higher-rank polymorphism. Which is true (for types, but not for lifetimes). But that's not the same as "generics don't work with closures."

> I haven't claimed that closures are not working

You've said:

> Closures in Rust are fundamentally broken.

(which was completely unqualified and not to a "very limited set of circumstances")

and (emphasis mine)

> But I can basically demonstrate other language features that are not working with closures, such as generics

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