I really like the optional system param thing, I was wondering about that myself.

I'm less sold on having a single skipping wrapper; however if we did decide to go with it my vote would be to call it When<T> (eg. When<Res<MyResource>> or When<Single<Window>> or When<Populated<MyComponent>>) to indicate that this system runs when the parameter is valid.

Just having a few separate wrappers with this functionality built in seems fine. Having a dedicated When<T> wrapper does increase flexibility somewhat... There are situations where users may want their app to panic if they don't have exactly one entity with a single value, I think I've had situations where I'd have liked to enforce that invariant before.

Just so we are all on the same page, your proposed semantics would basically be:

1. Every system parameter errors by default when invalid.
2. The Option<T> wrapper turns an error into a successful None parameter.
3. The When<T> wrapper skips the system when it encounters an error.
4. The fallback error handler lets users further customize how unwrapped errors are handled.

I don't hate that. One slightly awkward result is the behavior of Option<When<T>> and When<Option<T>>. Should the former skip?

Originally posted by @NthTensor in #18504 (comment)