Why is it possible to pass a function as parameter to the add_system() function?

[Koettlitz]

The add_system() fn of the App struct expects a parameter of type impl IntoSystemDescriptor<Params>. How does a function match this type? I looked up the Implementors of IntoSystemDescriptor but I can't find any of the Fn-types. Can someone explain how this works?

[bjorn3]

There is a impl<Params, S> IntoSystemDescriptor<Params> for S where S: IntoSystem<(), (), Params> implementation. IntoSystem is implemented for all types implementing SystemParamFunction which is implemented for functions.

[Koettlitz]

Oh I just looked at the implementors of SystemParamFunction. It looks scary!

[bjorn3]

Yeah, the combination of handling the implementation using a macro and (ab)using the trait system of rustc (earlier iterations caused the compiler to crash) is indeed scary!

[Koettlitz]

But why does the following code not compile?

use bevy::prelude::{App, Commands, In};

fn main() {
    App::new().add_system(my_sys).run();
}

fn my_sys(_: In<usize>, _: Commands) {
    println!("Hugelbugel");
}

Why does it not match this Blanket implementation of SystemParamFunction?

[bjorn3]

In is only allowed when you chain two systems together I believe. It will take the value of whatever the previous system in the chain returned.

[NickLarsenNZ]

I had the same question after realising I could put the params in any order (wondering how that was valid). Thanks for the answer @bjorn3