Generics, System Parameters, and Type Erasure

Bevy’s pleasant API is powered by a conversion pipeline:

ordinary function
→ IntoSystem
→ FunctionSystem<Marker, F>
→ SystemParam tuple state
→ boxed ScheduleSystem

SystemParam uses generic associated types to distinguish long-lived parameter state from the short-lived item borrowed for one run. Query<'w, 's, D, F> is generic over selected data and filtering policy; D also determines whether access is read-only.

Bundles recursively describe groups of components. Plugins are generic composition at application construction time. Assets retain Handle<A> and Assets<A> so an image handle cannot accidentally retrieve a mesh.

At the schedule boundary, unlike concrete function and parameter tuples must share a collection. Bevy erases them behind dyn System, retaining virtual operations for access metadata, initialization, running, deferred application, and type identity.

This is the recurring pattern:

Use generics to derive exact behavior and safety; erase the final system only when heterogeneous runtime scheduling requires it.