How to define function that returns Struct<Fn>

Question

I have a struct

struct A<F: Fn(i32)->i32>

How to define function which return it like

fn some_function() -> A<Fn(i32)->i32>

?

Answer 1

I think you need to use the fn type instead of the Fn trait:

struct A(fn(i32) -> i32);

fn some_function() -> A {
    let square = |x| x * x;
    A(square)
}

fn main() {
    let f = some_function();
    println!("{}", f.0(7));
}

49

---

You can of course also use the Fn trait, but then you need to use the impl keyword:

struct A<T: Fn(i32) -> i32>(T);

fn some_function() -> A<impl Fn(i32) -> i32> {
    let square = |x| x * x;
    A(square)
}

fn main() {
    let f = some_function();
    println!("{}", f.0(7));
}

49

impl here means "this is a type that I don't want to write here completely, but believe me, it implements Fn(i32) -> i32".