Polystate: Composable Finite State Machines

Polystate's Core Design Philosophy

1. Record the state machine's status at the type level.

2. Achieve composable state machines through type composition.

Practical Effects of Polystate

1. Define the program's overall behavior through compositional declarations. This means we gain the ability to specify the program's overall behavior at the type level. This significantly improves the correctness of imperative program structures. This programming style also encourages us to redesign the program's state from the perspective of types and composition, thereby enhancing code composability.

2. Build complex state machines by composing simple states. For the first time, we can achieve semantic-level code reuse through type composition. In other words, we have found a way to express semantic-level code reuse at the type level. This approach achieves three effects simultaneously: conciseness, correctness, and safety.

3. Automatically generate state diagrams. Since the program's overall behavior is determined by declarations, polystate can automatically generate state diagrams. Users can intuitively understand the program's overall behavior through these diagrams.

I'm guessing you're not a native English speaker. Your descriptions could use some work. A few examples:

"For the first time, we can achieve semantic-level code reuse through type composition." is, to me, mostly meaningless. This almost certainly isn't the first time someone has done whatever it is you are claiming. What is *semantic-level" code reuse? Calling a library function is code reuse, and I expect that function to have the same semantics every time I call it. Why is type composition necessary to achieve this?

"Define the program's overall behavior through compositional declarations. This means we gain the ability to specify the program's overall behavior at the type level." How does the specifying behavior at the type level follow from composition? I can use composition, at the value level, just fine without types (e.g. in Javascript).

Yes the expression may not be precise enough, but the example should be accurate enough.

1, https://github.com/sdzx-1/polystate?tab=readme-ov-file#2-imp...

This shows what composition means, and even complex nested selects are described quite precisely by type.

2, https://github.com/sdzx-1/polystate?tab=readme-ov-file#1-com...

Yes, this effect can be achieved without using composite types. But if it is convenient and easy to achieve this effect through composite types, is it worth it?

I have to admit I'm not very interested in reading the code. I'm not likely to use this library so I'm more interested in a conceptual understanding than in the details that code requires.

Regarding composition, there are at least three ways to compose FSMs:

* Sequential: when one FSM finishes (reaches a final state) it transitions to the start state of another FSM.

* Parallel: two FSMs transition in parallel from the same input

* Nested: when a FSM reaches a certain state, another FSM starts responding to the input. When the second FSM reaches a final state, control returns to the original FSM.

It would help your description if you were clear about the kinds of composition your library supports. The terminology of FSMs is quite consistent and well defined, so I think you should be able to use it to describe what the library does.

Is there a way to combine:

Higher order finite state machines that require other states as parameters to work.

https://github.com/sdzx-1/ray-game/blob/master/src/select.zi...

The select, inside, and hover states here are all high-level states, and all require two state parameters. And these three states form a small state machine for handling mouse interactive selection.

Can I think of this way of using higher-order state machines as a kind of composition? A semantic composition.