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Actors and Messages

In the actor model, an actor is the fundamental unit of computation. When an actor receives a message, it can do exactly three things:

  1. Send messages to other actors it knows about.
  2. Create new actors (its children).
  3. Designate the behavior for the next message it receives.

That's the entire model. There is no shared memory between actors, no synchronization primitives, no mutex. Concurrency emerges naturally: each actor processes one message at a time from its mailbox, and multiple actors run concurrently on the asyncio event loop.

A message is any value sent to an actor. In Casty, messages are frozen dataclasses — immutable by construction. Related messages are grouped using PEP 695 type aliases, which enables exhaustive pattern matching via match statements:

@dataclass(frozen=True)
class Deposit:
    amount: int

@dataclass(frozen=True)
class Withdraw:
    amount: int
    reply_to: ActorRef[str]

@dataclass(frozen=True)
class GetBalance:
    reply_to: ActorRef[int]

type AccountMsg = Deposit | Withdraw | GetBalance

The type alias AccountMsg is a union of all messages the actor accepts. When handling messages with match, the type checker verifies that all variants are covered. If a new message type is added to the union but not handled, Pyright reports an error at development time.

Next: Behaviors as Values