How to define function that returns Struct

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>

?

Does this answer your question? [How do I store a closure in a struct in Rust?](https://stackoverflow.com/questions/27831944/how-do-i-store-a-closure-in-a-struct-in-rust) — kotatsuyaki, Jun 06 '22 at 06:11
You are aware that `Fn(i32) -> i32` is a trait, not a type, right? — jthulhu, Jun 06 '22 at 06:21

Finomnis · Answer 1 · 2022-06-07T07:49:04.457

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));
}

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));
}

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

How to define function that returns Struct

1 Answers1