# @suaveplan/fsm - Typed finite state machine with guards, actions, history tracking, hierarchical states, parallel regions, deferred events, and OTel instrumentation

## Summary

Typed, generic finite state machine (FSM) for TypeScript. Ships a synchronous-by-default interpreter with guard predicates, ordered transition actions, context threading, entry/exit lifecycle hooks, history recording, and OTel instrumentation. Zero peer dependencies beyond `@suaveplan/error`, `@suaveplan/events`, and `@suaveplan/types`.

<!-- begin:auto-docs -->
## Documentation tree

- [interpreter](./src/core/interpreter/fsm-interpreter.md) — <!-- begin:auto-docs --> _(no submodules)_ <!-- end:auto-docs -->
  - [fsm-interpreter](./src/core/interpreter/fsm-interpreter.md) — <!-- begin:auto-docs --> _(no submodules)_ <!-- end:auto-docs -->
- [errors](./src/errors/fsm-error.md) — <!-- begin:auto-docs --> _(no submodules)_ <!-- end:auto-docs -->
  - [fsm-error](./src/errors/fsm-error.md) — <!-- begin:auto-docs --> _(no submodules)_ <!-- end:auto-docs -->
- [context-helpers](./src/utils/context-helpers.md) — Provides ergonomic factory helpers for building context-update and side-effect actions that can be slotted directly int…
- [definition-validator](./src/utils/definition-validator.md) — Validates an `FSMDefinition` object before the interpreter begins executing. Early structural validation prevents subtl…
<!-- end:auto-docs -->  - [fsm-interpreter](./src/core/interpreter/fsm-interpreter.md) — <!-- begin:auto-docs --> _(no submodules)_- [errors](./src/errors/fsm-error.md) — <!-- begin:auto-docs --> _(no submodules)_- [fsm-error](./src/errors/fsm-error.md) — <!-- begin:auto-docs --> _(no submodules)_- [context-helpers](./src/utils/context-helpers.md) — Provides ergonomic factory helpers for building context-update and side-effect actions that can be slotted directly int…
- [definition-validator](./src/utils/definition-validator.md) — Validates an `FSMDefinition` object before the interpreter begins executing. Early structural validation prevents subtl…
- [fsm-interpreter](./src/core/interpreter/fsm-interpreter.md) — <!-- begin:auto-docs --> _(no submodules)_- [errors](./src/errors/fsm-error.md) — <!-- begin:auto-docs --> _(no submodules)_- [fsm-error](./src/errors/fsm-error.md) — <!-- begin:auto-docs --> _(no submodules)_- [context-helpers](./src/utils/context-helpers.md) — Provides ergonomic factory helpers for building context-update and side-effect actions that can be slotted directly int…
- [definition-validator](./src/utils/definition-validator.md) — Validates an `FSMDefinition` object before the interpreter begins executing. Early structural validation prevents subtl…
- [fsm-interpreter](./src/core/interpreter/fsm-interpreter.md) — <!-- begin:auto-docs --> _(no submodules)_- [errors](./src/errors/fsm-error.md) — <!-- begin:auto-docs --> _(no submodules)_- [fsm-error](./src/errors/fsm-error.md) — <!-- begin:auto-docs --> _(no submodules)_- [context-helpers](./src/utils/context-helpers.md) — Provides ergonomic factory helpers for building context-update and side-effect actions that can be slotted directly int…
- [definition-validator](./src/utils/definition-validator.md) — Validates an `FSMDefinition` object before the interpreter begins executing. Early structural validation prevents subtl…
- [fsm-interpreter](./src/core/interpreter/fsm-interpreter.md) — <!-- begin:auto-docs --> _(no submodules)_- [errors](./src/errors/fsm-error.md) — <!-- begin:auto-docs --> _(no submodules)_  - [fsm-error](./src/errors/fsm-error.md) — <!-- begin:auto-docs --> _(no submodules)_- [context-helpers](./src/utils/context-helpers.md) — Provides ergonomic factory helpers for building context-update and side-effect actions that can be slotted directly int…
- [definition-validator](./src/utils/definition-validator.md) — Validates an `FSMDefinition` object before the interpreter begins executing. Early structural validation prevents subtl…

## Install

```sh
bun add @suaveplan/fsm
```

## Quick Start

```ts
import { createMachine, assign } from '@suaveplan/fsm';

interface TrafficCtx { count: number }

const traffic = createMachine<TrafficCtx>({
  id: 'traffic',
  initial: 'red',
  context: { count: 0 },
  states: {
    red:    { on: { NEXT: { target: 'green' } } },
    green:  { on: { NEXT: { target: 'yellow' } } },
    yellow: {
      on: {
        NEXT: {
          target: 'red',
          actions: [assign<TrafficCtx>(ctx => ({ count: ctx.count + 1 }))],
        },
      },
    },
  },
});

traffic.send('NEXT'); // → green
traffic.send('NEXT'); // → yellow
const state = traffic.send('NEXT'); // → red
console.log(state.context.count); // 1
console.log(state.history.length); // 3
```

## Modules

| Module | Path | Description |
|--------|------|-------------|
| Root | `@suaveplan/fsm` | `createMachine`, types, `assign`, `pure`, `validateDefinition` |
| Interpreter | `@suaveplan/fsm/interpreter` | `FSMInterpreter`, `createMachine` |
| Errors | `@suaveplan/fsm/errors` | `FSMError`, `FSM_CODES` |

## Core Concepts

### `createMachine(definition)`

Factory function returning an `FSMInterpreter` instance. The definition is validated eagerly — malformed definitions throw `FSMError(INVALID_DEFINITION)` before any events are processed.

```ts
interface FSMDefinition<TContext> {
  id?: string;         // used in OTel metric labels
  initial: string;     // must exist in states
  context: TContext;   // initial extended context
  states: Record<string, StateDefinition<TContext>>;
}
```

### Guards

Guard functions on a transition prevent it from firing when they return `false`. If multiple transitions are declared for the same event (as an array), the first one whose guard returns `true` is taken.

```ts
states: {
  locked: {
    on: {
      UNLOCK: [
        { target: 'open',   guard: (ctx, evt) => evt.payload === ctx.pin },
        { target: 'locked', guard: () => true },   // fallthrough (stay locked)
      ],
    },
  },
}
```

### Actions

Transition actions are called with the accumulated context. Returning a new context replaces it; returning `void` leaves it unchanged.

```ts
import { assign, pure } from '@suaveplan/fsm';

const bump   = assign<Ctx>(ctx => ({ ...ctx, count: ctx.count + 1 }));
const logEvt = pure<Ctx>((ctx, evt) => console.log(evt.type, ctx));
```

### Entry / Exit lifecycle

```ts
states: {
  loading: {
    entry: ctx => ({ ...ctx, loading: true }),
    exit:  ctx => ({ ...ctx, loading: false }),
    on: { DONE: { target: 'idle' } },
  },
}
```

### Final states

Entering a state with `type: 'final'` sets `state.done = true`.

### Subscriptions

```ts
const unsub = machine.onTransition(state => {
  console.log('now in', state.value, 'done?', state.done);
});
// later:
unsub();
```

### `can()` and `matches()`

```ts
machine.can('NEXT');      // true if the event is declared in current state
machine.matches('green'); // true if currently in 'green'
```

### `reset()`

Resets to initial state, original context, and clears history.

## OTel Metrics

| Name | Type | Attributes |
|------|------|------------|
| `fsm.transition.total` | Counter | `id`, `from`, `to`, `event` |
| `fsm.transition.duration_ms` | Histogram | `id`, `from`, `to` |

## Error Codes

| Code | Meaning |
|------|---------|
| `FSM.INVALID_DEFINITION` | Structural error in definition (missing initial, unknown target, etc.) |
| `FSM.INVALID_TRANSITION` | No matching transition for event in current state |
| `FSM.INVALID_STATE` | Current state not found in definition |
| `FSM.GUARD_FAILED` | All guards for an event returned false |
| `FSM.ACTION_ERROR` | An action or lifecycle function threw |

## Module Docs

- [`src/core/interpreter/fsm-interpreter.md`](src/core/interpreter/fsm-interpreter.md)
- [`src/errors/fsm-error.md`](src/errors/fsm-error.md)
- [`src/utils/definition-validator.md`](src/utils/definition-validator.md)
- [`src/utils/context-helpers.md`](src/utils/context-helpers.md)