Behaviors as Values¶

In most actor frameworks, an actor is defined by subclassing a base class and overriding a receive method. Casty takes a different approach: behaviors are values, not classes.

A behavior is a frozen dataclass that describes how an actor processes messages. You compose behaviors using the Behaviors factory:

Factory	Purpose
`Behaviors.receive(handler)`	Create a behavior from an async message handler `(ctx, msg) -> Behavior`
`Behaviors.setup(factory)`	Run initialization logic with access to `ActorContext`, then return the real behavior
`Behaviors.ignore()`	Accept any message and keep the current behavior — useful for terminal states
`Behaviors.same()`	Keep the current behavior unchanged (returned from a message handler)
`Behaviors.stopped()`	Stop the actor gracefully
`Behaviors.unhandled()`	Signal that the message was not handled
`Behaviors.restart()`	Explicitly restart the actor
`Behaviors.supervise(behavior, strategy)`	Wrap a behavior with a supervision strategy
`Behaviors.with_lifecycle(behavior, ...)`	Attach lifecycle hooks (pre_start, post_stop, etc.)
`Behaviors.event_sourced(...)`	Persist actor state as a sequence of events
`Behaviors.persisted(events)`	Return from a command handler to persist events and update state
`Behaviors.sharded(entity_factory, ...)`	Distribute entities across cluster nodes via sharding
`Behaviors.singleton(factory)`	Run exactly one actor instance across the cluster
`Behaviors.broadcasted(behavior)`	Enable broadcasting messages to all instances across the cluster
`Behaviors.discoverable(behavior, key=...)`	Auto-register the actor with the cluster receptionist on spawn
`Behaviors.spy(behavior, observer)`	Observe all messages an actor processes

Because behaviors are values, they compose naturally. A behavior can be wrapped with supervision, decorated with lifecycle hooks, and backed by event sourcing — all through function composition, not class inheritance.

Next: Functional State