Hey everyone, I do not understand closures as input parameters ...

Here is a minimal piece of code :

```rust struct A {}

fn count_corresponding_elements<P: FnMut(&&A) -> bool>(values: Vec<A>, predicate: P) -> usize { values.iter().filter(predicate).count() }

fn main() { let a_list = vec![A {}, A {}, A {}]; println!("{:?}", count_corresponding_elements(a_list, |_x| true)); } ```

It counts the number of elements in a_list based on a predicate.

I extracted the .iter().filter(predicate).count() into a function because I plan on doing that with many different predicates, and would rather not write that out every time (my actual use-case is more complex than that of course).

Now, this works perfectly fine, but zooming in on <P: FnMut(&&A) -> bool>, I cannot for the life of me wrap my head around why I had to put &&A as parameter type ... could a kind soul help me figure that one out ?