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# Objective Error handling in bevy is hard. See for reference #11562, #10874 and #12660. The goal of this PR is to make it better, by allowing users to optionally return `Result` from systems as outlined by Cart in <#14275 (comment)>. ## Solution This PR introduces a new `ScheuleSystem` type to represent systems that can be added to schedules. Instances of this type contain either an infallible `BoxedSystem<(), ()>` or a fallible `BoxedSystem<(), Result>`. `ScheuleSystem` implements `System<In = (), Out = Result>` and replaces all uses of `BoxedSystem` in schedules. The async executor now receives a result after executing a system, which for infallible systems is always `Ok(())`. Currently it ignores this result, but more useful error handling could also be implemented. Aliases for `Error` and `Result` have been added to the `bevy_ecs` prelude, as well as const `OK` which new users may find more friendly than `Ok(())`. ## Testing - Currently there are not actual semantics changes that really require new tests, but I added a basic one just to make sure we don't break stuff in the future. - The behavior of existing systems is totally unchanged, including logging. - All of the existing systems tests pass, and I have not noticed anything strange while playing with the examples ## Showcase The following minimal example prints "hello world" once, then completes. ```rust use bevy::prelude::*; fn main() { App::new().add_systems(Update, hello_world_system).run(); } fn hello_world_system() -> Result { println!("hello world"); Err("string")?; println!("goodbye world"); OK } ``` ## Migration Guide This change should be pretty much non-breaking, except for users who have implemented their own custom executors. Those users should use `ScheduleSystem` in place of `BoxedSystem<(), ()>` and import the `System` trait where needed. They can choose to do whatever they wish with the result. ## Current Work + [x] Fix tests & doc comments + [x] Write more tests + [x] Add examples + [X] Draft release notes ## Draft Release Notes As of this release, systems can now return results. First a bit of background: Bevy has hisotrically expected systems to return the empty type `()`. While this makes sense in the context of the ecs, it's at odds with how error handling is typically done in rust: returning `Result::Error` to indicate failure, and using the short-circuiting `?` operator to propagate that error up the call stack to where it can be properly handled. Users of functional languages will tell you this is called "monadic error handling". Not being able to return `Results` from systems left bevy users with a quandry. They could add custom error handling logic to every system, or manually pipe every system into an error handler, or perhaps sidestep the issue with some combination of fallible assignents, logging, macros, and early returns. Often, users would just litter their systems with unwraps and possible panics. While any one of these approaches might be fine for a particular user, each of them has their own drawbacks, and none makes good use of the language. Serious issues could also arrise when two different crates used by the same project made different choices about error handling. Now, by returning results, systems can defer error handling to the application itself. It looks like this: ```rust // Previous, handling internally app.add_systems(my_system) fn my_system(window: Query<&Window>) { let Ok(window) = query.get_single() else { return; }; // ... do something to the window here } // Previous, handling externally app.add_systems(my_system.pipe(my_error_handler)) fn my_system(window: Query<&Window>) -> Result<(), impl Error> { let window = query.get_single()?; // ... do something to the window here Ok(()) } // Previous, panicking app.add_systems(my_system) fn my_system(window: Query<&Window>) { let window = query.single(); // ... do something to the window here } // Now app.add_systems(my_system) fn my_system(window: Query<&Window>) -> Result { let window = query.get_single()?; // ... do something to the window here Ok(()) } ``` There are currently some limitations. Systems must either return `()` or `Result<(), Box<dyn Error + Send + Sync + 'static>>`, with no in-between. Results are also ignored by default, and though implementing a custom handler is possible, it involves writing your own custom ecs executor (which is *not* recomended). Systems should return errors when they cannot perform their normal behavior. In turn, errors returned to the executor while running the schedule will (eventually) be treated as unexpected. Users and library authors should prefer to return errors for anything that disrupts the normal expected behavior of a system, and should only handle expected cases internally. We have big plans for improving error handling further: + Allowing users to change the error handling logic of the default executors. + Adding source tracking and optional backtraces to errors. + Possibly adding tracing-levels (Error/Warn/Info/Debug/Trace) to errors. + Generally making the default error logging more helpful and inteligent. + Adding monadic system combininators for fallible systems. + Possibly removing all panicking variants from our api. --------- Co-authored-by: Zachary Harrold <zac@harrold.com.au>
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