Implementing a state engine using instance behaviour

State engines are a pretty useful, but underutilised programming technique. Most common ways of implementing them tend to give pretty messy code.

Part of the problem is of course the impedance mismatch between the programming languages we use and the state engine. Even in object oriented languages we normally see state engines implemented in terms of a switch statement which grows to gargantuan proportions.

What I'm going to do is to show how to neaten these implementations up by making use of an advanced object-oriented technique called instance behaviour.