MetaScript's base concurrency primitives — async and spawn — are
language-level constructs that compose with actors.
async
Language-level async/await. Functions declared async return
Promise<T>; await suspends the current coroutine without blocking
the scheduler thread. On the C backend this lowers to state machines;
on JS it maps to native promises.
spawn
Fork-join lightweight tasks. Start a unit of work on a separate executor thread and either await the result or detach.
Composition with actors
Actors are built on top of these primitives — an actor method runs on
the scheduler, await spawn(...) inside an actor suspends the actor
cooperatively. See the Actors page for the full
actor programming model: isolated state, message passing, and
supervision trees.