Pub type for Fn trait and as function argument

1

Hello,

Unfortunately I didn't find any documentation on this subject.

So I have a pub type MyType = dyn Fn(...).

But when I write this:

pub fn some_func<F>(arg: F) where F: MyType {
    ...
}

Then I get: cannot find trait MyType in this scope.

MyType and some_func are in the same module.

Thank you very much in advance for any help.

2

In Rust, traits are not types and viceversa.

3

Thank your for your time and help.

Is there a way or workaround to achieve what I'm trying to do ?

Thank you for your help.

4

You could create your own trait, and use it as a bound:

trait MyFnTrait: Fn(usize) -> usize {}

impl<T: Fn(usize) -> usize> MyFnTrait for T {}

fn call_with_one<F>(func: F) -> usize
    where F: MyFnTrait {
    func(1)
}

fn main() {
    let double = |x| x * 2;
    assert_eq!(call_with_one(double), 2);
}
5

Thank you for your help.

Just to make sure I understand, wouldn't it be easier / better to write:

trait MyFnTrait: Fn(usize) -> usize {}

impl MyFnTrait for T {}

fn call_with_one<F>(func: F) -> usize
    where F: MyFnTrait {
    func(1)
}

fn main() {
    let double = |x| x * 2;
    assert_eq!(call_with_one(double), 2);
}

Or is there a reason for "repeating" Fn(usize) -> size in

impl<T: Fn(usize) -> usize> MyFnTrait for T {}

Thank you for your help.

6

As food for thought, you can try it in the playground:

Read the compilation error and it would help you to understand more than what I could write here.

7

Oh okay I understand.

Thank you for this.