State machines in practice

In this talk and live coding session we will see how state machines can help us with problems that often crop up in day-to-day development.

Code: https://github.com/pwm/fsm-talk

Transcript

1. State machines and other ramblings
2. Problem-solution ordering
3. Bottom-up vs. Top-down
4. Let's introduce the problem first:
5. Has any of you ever seen a variable, object property or database field named status, state or similar? Let's introduce the problem first:
6. Let’s write some code…
7. The problem: We don't control how state changes!
8. What is state?
9. "The particular condition that someone or something is in at a specific time." (Oxford Dictionaries)
10. "The particular condition that someone or something is in at a specific time." (Oxford Dictionaries)
11. State is a function of time !
12. S1 S6 S2 S5 Time Set of possible states t1 t2 t3 t4 t5 t6 t7 S4 S3
13. S1 S6 S2 S5 Time Set of possible states t1 t2 t3 t4 t5 t6 t7 S4 S3
14. S1 S6 S2 S5 Time Set of possible states t1 t2 t3 t4 t5 t6 t7 S4 S3
15. S1 S6 S2 S5 Time Set of possible states t1 t2 t3 t4 t5 t6 t7 S4 S3
16. S1 S6 S2 S5 Time Set of possible states t1 t2 t3 t4 t5 t6 t7 S4 S3
17. S1 S6 S2 S5 Time Set of possible states t1 t2 t3 t4 t5 t6 t7 S4 S3
18. S1 S6 S2 S5 Time Set of possible states t1 t2 t3 t4 t5 t6 t7 S4 S3
19. S1 S6 S2 S5 Time Set of possible states t1 t2 t3 t4 t5 t6 t7 S4 S3
20. Claim: Much grief in software development arises from the inability to control state, i.e. from uncontrolled mutation of state.
21. Claim: Much grief in software development arises from the inability to control state, i.e. from uncontrolled mutation of state. Proof: What do we do when we step through a program to debug a problem? We pause time to observe state.
22. Finite state machines (FSM) can help us bringing state changes under control.
23. FSMs define how we can move between states ie. what is a valid move and what is an invalid move.
24. Without a FSM in place, every move can be seen as a valid move!
25. Arrows between states represent events/actions.
26. So how do we move between states?
27. So how do we move between states? The next state is a function of the current state and the event.
28. So how do we move between states? The next state is a function of the current state and the event. (State, Event) ! State
29. The next state is a function of the current state and the event. (State, Event) ! State This is called the transition function. So how do we move between states?
30. Let’s write some code…