Import concurrency explicitly

The concurrency operations in these lessons also have explicit standard-library imports. Module aliases and item aliases name the same facilities and types as the familiar prelude spellings. Existing programs continue to work.

import std.thread as threads
from std.channel import channel as bounded, Receiver as Inbox
from std.concurrent import ThreadPoolExecutor as Pool
from std.sync import CancellationToken as Stop

def receive(inbox: Inbox[str]) -> Result[str, RecvError]:
    inbox.recv()

def stopped(stop: Stop) -> bool:
    stop.wait()
    stop.is_cancelled()

type Errors = AllocError | IoError | SendError[str] | RecvError | SpawnError[()] | PoolError | FutureError | SubmitError[()]

def main() -> Result[(), Errors]:
    sender, receiver = bounded[str](1)?
    sender.send("hello")?
    drop(sender)
    with threads.spawn(receive, receiver).drop_job()? as task:
        print(task.join()?)?
    stop = Stop()?
    with Pool(1, 1)? as pool:
        future = pool.submit(stopped, stop.share()).drop_job()?
        stop.cancel()
        print(future.result()?)?
    Ok(())

hello
True

Dropping the sender before joining permits a receiving worker to finish even when it loops until disconnection. Imported names retain the same loan, worker-effect, queue-bound and hang diagnostics as prelude names. Error types can also be imported: from std.concurrent import FutureError, for example. Importing an existing builtin under its own name is allowed only when it names that exact builtin; unrelated imports still cannot replace it.

The module references list the complete surfaces: std.thread, std.channel, std.concurrent, and std.sync. Imports ship inside the compiler and need no network or separately installed Plenty sources.

A failed submission returns its unstarted job inside SubmitError; drop_job() drops it and leaves SubmitError[()], as the executor lesson explains. A spawn of a named function with owned arguments works the same way: it returns them inside SpawnError, and drop_job() leaves SpawnError[()].