Piping only when the first system returned Some(T)

[mamekoro]

I'm trying to create a CombinatorSystem that combines two systems. The first system returns Option<T>, and if it is Some(T), the inner value T is passed to the second system as the first argument, In<T>. If it is None, the second system is not executed.

These two systems are, for example, as follows.

fn get_config_path() -> Option<PathBuf> { todo!() }
fn load_config(In(_path): In<PathBuf>) {}

To achieve this, I wrote the following code:

use std::{borrow::Cow, path::PathBuf};

use bevy::{
    ecs::system::{CombinatorSystem, Combine},
    prelude::*,
};

pub type AndThenPipeSystem<SystemA, SystemB> =
    CombinatorSystem<AndThenPipeMarker, SystemA, SystemB>;

#[doc(hidden)]
pub struct AndThenPipeMarker;

impl<SystemA, SystemB, T> Combine<SystemA, SystemB> for AndThenPipeMarker
where
    SystemA: System<Out = Option<T>>,
    SystemB: System,
    for<'a> SystemB::In: SystemInput<Inner<'a> = T>,
{
    type In = SystemA::In;
    type Out = Option<SystemB::Out>;

    fn combine(
        input: <Self::In as SystemInput>::Inner<'_>,
        a: impl FnOnce(SystemIn<'_, SystemA>) -> <SystemA as System>::Out,
        b: impl FnOnce(SystemIn<'_, SystemB>) -> <SystemB as System>::Out,
    ) -> Self::Out {
        a(input).map(b)
    }
}

fn get_config_path() -> Option<PathBuf> {
    todo!();
}

fn load_config(In(_path): In<PathBuf>) {}

fn main() {
    let mut app = App::new();

    app.add_systems(
        Startup,
        AndThenPipeSystem::new(get_config_path, load_config, Cow::Borrowed("AndThenPipe")),
    );
}

And the following error occurred: `CombinatorSystem<AndThenPipeMarker, ..., ...>` does not describe a valid system configuration

error[E0277]: `CombinatorSystem<AndThenPipeMarker, ..., ...>` does not describe a valid system configuration
   --> src/main.rs:43:9
    |
41  |     app.add_systems(
    |         ----------- required by a bound introduced by this call
42  |         Startup,
43  |         AndThenPipeSystem::new(get_config_path, load_config, Cow::Borrowed("AndThenPipe")),
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ invalid system configuration
    |
    = help: the trait `IntoScheduleConfigs<Box<(dyn bevy::prelude::System<In = (), Out = Result<(), BevyError>> + 'static)>, _>` is not implemented for `CombinatorSystem<AndThenPipeMarker, ..., ...>`
    = help: the following other types implement trait `IntoScheduleConfigs<T, Marker>`:
              `(S,)` implements `IntoScheduleConfigs<T, (ScheduleConfigTupleMarker, P)>`
              `(S0, S1)` implements `IntoScheduleConfigs<T, (ScheduleConfigTupleMarker, P0, P1)>`
              `(S0, S1, S2)` implements `IntoScheduleConfigs<T, (ScheduleConfigTupleMarker, P0, P1, P2)>`
              `(S0, S1, S2, S3)` implements `IntoScheduleConfigs<T, (ScheduleConfigTupleMarker, P0, P1, P2, P3)>`
              `(S0, S1, S2, S3, S4)` implements `IntoScheduleConfigs<T, (ScheduleConfigTupleMarker, P0, P1, P2, P3, P4)>`
              `(S0, S1, S2, S3, S4, S5)` implements `IntoScheduleConfigs<T, (ScheduleConfigTupleMarker, P0, P1, P2, P3, P4, P5)>`
              `(S0, S1, S2, S3, S4, S5, S6)` implements `IntoScheduleConfigs<T, (ScheduleConfigTupleMarker, P0, P1, P2, P3, P4, P5, P6)>`
              `(S0, S1, S2, S3, S4, S5, S6, S7)` implements `IntoScheduleConfigs<T, (ScheduleConfigTupleMarker, P0, P1, P2, P3, P4, P5, P6, P7)>`
            and 14 others
note: required by a bound in `bevy::prelude::App::add_systems`
   --> /home/rust/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/bevy_app-0.16.1/src/app.rs:304:23
    |
301 |     pub fn add_systems<M>(
    |            ----------- required by a bound in this associated function
...
304 |         systems: impl IntoScheduleConfigs<ScheduleSystem, M>,
    |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `App::add_systems`
    = note: the full name for the type has been written to '/home/rust/aaa/target/debug/deps/aaa-f180d0e646ad9074.long-type-14913119345994110843.txt'
    = note: consider using `--verbose` to print the full type name to the console

How can I implement it correctly? What am I missing?

[gwafotapa]

I believe you're at least missing an implementation of IntoSystem for AndThenPipeSystem.

That being said, your implementation of Combine looks wrong to me. Combine::combine is supposed to return a system and your implementation returns an Option<SystemB::Out>.

Finally, I don't know if you're aware but you can achieve what you want with IntoSystem::pipe if you're willing to have your second system accept an Option and return right away in case it is None like so:

fn get_config_path() -> Option<PathBuf> {
    todo!()
}

fn load_config(In(maybe_path): In<Option<PathBuf>>) {
    let Some(_path) = maybe_path else {
        return;
    };

    todo!()
}

fn main() {
    let mut app = App::new();
    app.add_systems(Startup, get_config_path.pipe(load_config));
}

[mamekoro]

I believe you're at least missing an implementation of IntoSystem for AndThenPipeSystem.

Implementing IntoSystem for AndThenPipeSystem causes the E0210 warning because AndThenPipeSystem is a type alias to CombinatorSystem, which is a foreign type.

Instead, I created a new struct IntoAndThenPipeSystem based on IntoPipeSystem, and passed its instance to app.add_systems(...), but I got a similar error: error[E0277]: `IntoAndThenPipeSystem<fn() -> ... {get_config_path}, ...>` does not describe a valid system configuration.

Do you know what a valid system configuration means?

That being said, your implementation of Combine looks wrong to me. Combine::combine is supposed to return a system and your implementation returns an OptionSystemB::Out.

AndMarker and similar types implement Combine, and they have an associated type defined as type Out = bool;, so I think type Out can be a non-system type.

Finally, I don't know if you're aware but you can achieve what you want with IntoSystem::pipe if you're willing to have your second system accept an Option and return right away in case it is None like so:

I'm aware of let-else, but when a system receives an Option<T>, I have to test for both Some(T) and None, which increases the amount of unit test code. Ideally, I would like to receive just T.

[gwafotapa]

AndMarker and similar types implement Combine, and they have an associated type defined as type Out = bool;, so I think type Out can be a non-system type.

You're right. Combine::combine is supposed to return the output of the combined system, not the combined system. I mixed it up.

Do you know what a valid system configuration means?

Not exactly but something came to mind. I think the issue is that your system returns an output. If you look at the implementors of IntoScheduleConfigs, the systems return (), Result<(), BevyError> or <fn() -> ! as FnRet>::Output. To prove my point, the following code will return the same error of an invalid configuration system

fn my_function() -> u32 {
    0
}

fn main() {
    let mut app = App::new();
    app.add_systems(Startup, my_function);
}

Combine is used for systems meant to be used with run_if but that doesn't fit your case as you need to inject the output into the other system. However, since what you want is so close to IntoSystem::pipe, maybe you could take its source code and modify it for AndThenPipeSystem.

[mamekoro]

Inspired by your example of my_function, I added .map(drop) to the instance of IntoAndThenPipeSystem and passed it to add_systems(...), and it worked correctly! Thank you so much!

[mamekoro]

app.add_systems(
    Startup,
    IntoAndThenPipeSystem::new(get_config_path, load_config).map(drop),
);

Refer to IntoPipeSystem (v0.16.1) to implement IntoAndThenPipeSystem.

[gwafotapa]

That .map(drop) looks weird. You could have IntoAndThenPipeSystem return () directly or Result<(), BevyError>.

[mamekoro]

Returning Option<SystemB::Out> allows the combined system to be connected to another pipe.

Since this could be useful in my project, I think I will leave it as is. There is also an example of using .map(drop) in the documentation for IntoSystem::map.