It's actually interesting that we're finally reaching a point where types systems in user interfaces are becoming incredibly appropriate. If we model the stall

It's actually interesting that we're finally reaching a point where types systems in user interfaces are becoming incredibly appropriate. If we model the stall warning as a boolean variable - then we are not able to signal the user about the fact that the warning system is in fact "inoperable". On the other hand if the warning system had a state which would say "inoperable", or at it's best return an error - "speed too low", then this warning would never have a state which could be incorrectly interpreted. This is exactly the same kind of mistake as a difference between NULL pointer and the 0 address you'd encounter while programming.