Passing trait's funcion as a parameter to another function

I have a function that accepts a function as an argument.
The function activates the function it accepts on another argument it’s getting.
Can I pass a trait’s function as the argument?

trait A {
    fn behave(&self, s: String) -> String;
}

struct B; 
impl A for B {
    fn behave(&self, s: String) -> String {
        return s.to_ascii_uppercase();
    }
}

fn doSomething(s: String,f: fn(String) -> String) -> String {
    return f(s)
}

fn useA(a: impl A) {
    println!("{}", a.behave("small".to_string()));
}

fn main() {
    let b = Box::new(B); 
    useA(*b);
    doSomething("small".to_string(), b.behave);  <--- dosent work
}

The code in the last line produces the following error:
error[E0609]: no field behave on type Box

my question is can I achieve this kind of functionality.
I’ve tried also static function inside the trait’s definition and did not succeed.

Thanks!

>Solution :

First, don’t use the function pointer type fn unless you absolutely need to. In nearly all cases, you’ll use the Fn/FnMut/FnOnce traits. You will need to decide which one you need based on your situation. For example, Option::map uses FnOnce, while Iterator::map uses FnMut. Fn is usually used when you want to store the closure and use it immutably. In this case, you would use FnOnce.

fn doSomething<F: FnOnce(String) -> String>(s: String, f: F) -> String {
    return f(s)
}

Second, you can’t create a partially applied function like with b.behave. Instead, you can make a closure.

let func = |s| b.behave(s);
doSomething("small".to_string(), func);