tyfsm (wip)

simple and typesafe finite-state machines. Inspired by zustand and xstate.

Background

I like automata-based programming. Certain problems are solved elegantly when modelled as finite-state machines, particularly those related to the logic of user interfaces. It's a great tool in a programmer's toolbox.

The most popular js/ts fsm library is XState. It's pretty good but verbose and difficult to learn, it's a big investment to drop in a team setting. Furthermore, type-safety in XState seems like an afterthought.

For me one of the biggest benefits of finite-state machines is that they enable you to make illegal states unrepresentable, and a large chunk of that is missing from xstate. So that's why I built this library

Features

tyfsm's state machines are centered around leveraging the type-narrowing features of Typescript's discriminated unions.

The core of the state machine is a discriminated union, where each state is a single variant of the union. This gives us neat type-safety narrowing features.

Example

You define your state machine in Typescript's type system:

type WebsocketMachine = StateMachine<
  // First define your states and the data each state carries
  {
    idle: {
      addr: string;
    };
    connecting: {
      socket: WebSocket;
      addr: string;
    };
    connected: {
      socket: WebSocket;
      addr: string;
    };
    error: {
      errorMessage: string;
    };
  },
  // Now define the valid transitions between states
  {
    idle: ["connecting"];
    connecting: ["error", "connected"];
    connected: ["idle", "error"];
    error: ["idle"];
  }
>;

// Write a utility to make selecting states easy
type State<K extends WebsocketMachine["allStates"]> = SelectStates<
  WebsocketMachine,
  K
>;

// Now define actions for the machine
type Actions = {
  // This action can only be called in the `idle` state and transitions
  // the machine into the `connecting` state
  connect: (state: State<"idle">) => State<"connecting">;

  // This action can only be called in the `connecting` or `connected` state and transitions
  // the machine into the `idle` state
  disconnect: (state: State<"connecting" | "connected">) => State<"idle">;
};

Once you've modelled your state machine, you can create a store in a similar way like in zustand:

// Create the initial state
const initial: State<"idle"> = {
  kind: "idle",
  addr: "ws://localhost:8302",
};

export const useWebsocketStore = create<WebsocketMachine, Actions>(
  initial,
  // Create the machine's actions:
  //
  // `get` is a function that returns the current state of the machine
  // `transition` is a function that transitions the machine, with the following parameters:
  //    * the current state
  //    * the next state
  //    * the data for the next state
  (get, transition) => ({
    connect(idleState) {
      const socket = new WebSocket(idleState.addr);

      socket.addEventListener("error", () => {
        // Get the current state of the machine, it is important to not use
        // `idleState` from the above scope because the state may have changed in
        // between the time the outer function returns and this callback runs.
        const currentState = get();
        if (currentState.kind === "connecting") {
          transition(currentState.kind, "error", {
            errorMessage: "Failed to connect",
          });
        }
      });

      socket.addEventListener("open", () => {
        // Same treatment as above
        const currentState = get();
        if (currentState.kind === "connecting") {
          transition(currentState.kind, "connected", {
            socket,
            addr: currentState.addr,
          });
        }
      });

      return transition(idleState.kind, "connecting", {
        socket,
        addr: idleState.addr,
      });
    },
    disconnect(state) {
      state.socket.close();
      return transition(state.kind, "idle", {
        addr: state.addr,
      });
    },
  })
);

Now you can use it in React:

const App = () => {
  const { state, actions } = useWebsocketStore();

  switch (state.kind) {
    case "idle": {
      return <button onClick={() => actions.connect(state)}>Connect</button>;
    }
    case "connecting": {
      return <p>Connecting...</p>;
    }
    case "connected": {
      return (
        <button onClick={() => actions.disconnect(state)}>Disconnect</button>
      );
    }
    case "error": {
      return <p>Something went wrong: {state.errorMessage}</p>;
    }
  }
};

Note that all the actions are type-safe. You can only call actions.connect(state) when state is in the idle state. Similarly, the errorMessage property is only available on the state object when the machine is in the error state.

todo

• explore hierarchical designs
• transaction functions for async actions so state doesn't unexpectedly change