What is an actor and which data race does it prevent?
The essence
An actor is a reference type whose mutable state is isolated: it can be touched only from inside the actor, and from outside only via await. The compiler checks isolation statically, and at runtime the actor has a serial executor — a queue of jobs executed one at a time without blocking threads: a task waiting for the actor is suspended, not parked on a mutex. This is how an actor rules out data races — simultaneous memory access with a write. It is not a mutex wrapped in syntax: a mutex blocks a thread, an actor suspends a task.
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