10. Concurrency & Threads

10.1 Spawning

// Default thread type
let handle = spawn worker_fn(data);

// Explicit thread type
let handle = spawn worker_fn(data) with GreenThread;

// Fire-and-forget
let _ = spawn background_task();

// #wont_remain -- promise the thread dies before the scope ends
#wont_remain let _ = spawn quickTask(sharedData);

10.2 Compile-Time Safety Enforcement

If a thread accesses shared data, the compiler requires one of:

await handle before the shared data goes out of scope, or
the #wont_remain attribute on the spawn.

Missing both is a compile error. Violating the #wont_remain promise is not caught by the compiler — the shared data may become a dangling reference. An alternative to both is using global: share T or Rc.

Keyword / Method	Semantics
`await handle`	Waits for the thread to finish.
`await globalVar`	Blocks until the global variable has been initialized.
`handle.send(msg)`	Sends a message to the running thread.
`#wont_remain`	Attribute on `spawn`. Promises the thread dies before the enclosing scope ends.

let handle = spawn worker(sharedData);
await handle;

global cfg: Config;
await cfg;
@println(cfg.host);

Zith Language Specification — Draft v0.9