Road to State Machines IV - But How Do We Let Data Influence Transitions Without Turning Every Value into Another State?

In the fourth part of his series on state machines, the author explores how to incorporate data-dependent conditions into transition logic without inflating the number of states. By introducing the concept of 'guards'—Boolean conditions that must be satisfied for a transition to occur—the author demonstrates how to keep the state machine model clean and maintainable. Instead of creating complex, redundant states like 'PAID_WITH_SUPPORTED_ADDRESS', the implementation uses a structured transition table that evaluates guards against the order's existing data. This approach, characteristic of Extended Finite State Machines (EFSM), allows for precise control over lifecycle movements while keeping the business logic decoupled from the state definitions. The article concludes by emphasizing that guards should remain side-effect-free, ensuring that the read path (checking available actions) and the write path (executing transitions) remain consistent and predictable.