Events and context

Two tiers serve two kinds of user (ADR-0002).

Dynamic tier (default)

The tier JSON machines, the CLI and persistence use:

• Context is serde_json::Value (DynContext).
• Events are Event { ty, payload } with a JSON payload.

Machine::from_json produces a machine over this tier, and MachineLogic::dynamic() registers the named logic. Actions receive &mut context and mutate it in place; guards receive & context.

Typed tier

The engine is generic over the context and event types:

• context C: Serialize + DeserializeOwned + Clone + Send + Sync + 'static
• event E: Serialize + DeserializeOwned + Send + 'static implementing EventType

The statechart! DSL and #[derive(TypedEvent)] / #[derive(TypedState)] (feature macros) generate typed enums for this tier, so the compiler checks that every event and state name is real.

Rollback needs a cloneable context

actionErrorPolicy: "rollback" restores the context, which requires C: Clone. Cheap-to-clone contexts (Arc, persistent collections) are recommended for large states.

Where the two tiers meet

Snapshots serialise context through serde in both tiers, so persistence and the Python compatibility story are tier-agnostic.