Handle channel backpressure

Use send_nowait when a full queue should return control to the caller. Its Full error retains your message, so you can retry it after making space.

type MessageError = RecvError | SendError[list[i64]] | AllocError

def main() -> Result[(), AllocError | IoError | MessageError | OverflowError[i64]]:
    sender, receiver = channel[list[i64]](1)?
    sender.send([10]?)?
    match sender.send_nowait([20, 30]?):
        case Ok(_):
            pass
        case Err(SendError.Full(message)):
            print(receiver.recv()?)?
            sender.send(message)?
        case Err(SendError.Disconnected(message)):
            print(message)?
    print(receiver.recv()?)?
    drop(sender)
    print(str.repr(receiver.recv())?)?
    Ok(())

[10]
[20, 30]
Result[list[i64], RecvError].Err(RecvError.Disconnected)

recv_nowait() similarly reports RecvError.Empty when the channel is connected but has no message ready. Neither nowait operation waits for the queue to change state, though both synchronize access to the queue.

If you need only part of a stream, the receiver guard handles shutdown:

def produce(sender: Sender[i64]) -> Result[(), AllocError | IoError | SendError[i64] | OverflowError[i64]]:
    for n in range(10, 100):
        sender.send(n * n)?
    Ok(())

def first_square() -> Result[i64, SpawnError[()] | AllocError | RecvError]:
    sender, receiver = channel[i64](1)?
    with spawn(produce, sender).drop_job()?, receiver as inbox:
        return Ok(inbox.recv()?)

def main() -> Result[(), SpawnError[()] | AllocError | RecvError | IoError | OverflowError[i64]]:
    print(first_square()?)?
    Ok(())

100

Returning drops the receiver first. The producer’s next send reports disconnection; its ? ends the worker. The task scope then joins it and discards its unclaimed result.

Channel capacity has type Positive[i64]. A positive literal converts at compile time; for configuration read at run time, pass Positive.try_from(capacity)?. Construction then reports only allocation failure. See integer domains.