How to receive a closure and pass an inner closure to it?

[bzdjsmyhm]

As in JavaScript, we can create a closure in a function, and then pass it to another closure in the argument list, like this:

function foo(f) {
    f(() => {});
}

foo((g) => {
    g();
});

How to implement that with generic types in Rust?

[kpreid]

Unfortunately, there is no “perfect” way to do this, but this is the closest reasonable thing you can write:

fn foo<F>(f: F)
where
    F: Fn(&dyn Fn()),
{
    f(&|| {});
}

fn main() {
    foo(|g| {
        g();
    });
}

The & is to allow using dyn, and the dyn is to avoid having to give a name for the specific type of the closure that foo passes to f (which is currently impossible^[1]).

---

1. on stable Rust, but might be possible with TAIT ↩︎

[bzdjsmyhm]

Thanks! I now try to use an older way to solve it: use a struct to capture the envirenment explicitly and implement an assosiated function to do what the original inner closure does....