The Floor Cleaner, Layered

In practice

The bottom layer owns the wheels: bump sensor fires, veer — a reflex in milliseconds, no goals consulted. The middle layer owns "go to the next waypoint": it remembers the current target, steers toward it through the bottom layer's reflexes, and tells upward when the target is reached. The top layer owns the plan — which rooms today, in what order — deliberating on the scale of minutes and commanding by handing down waypoints, never touching a wheel. Now read the design by what it spares. The reflex layer holds no concept of Tuesday's schedule; the planner holds no concept of wheel torque; each operates on its own abstraction of the world, delivered by its neighbors. Bump, veer, re-steer, arrive, report, re-plan — three clocks ticking at three speeds, one competent machine.