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Joined 2026-09-21 Message
Incoming links: Guarded alternation
Communicating Scala Objects 2026-10-31
A Scala library for constructing communicating concurrent processes with typed message-passing channels, parallel composition and guarded alternation.[ref]
Store the accepted but undelivered values in an ordinary FIFO queue, owned exclusively by the Communicating Scala Objects process. An input event is enabled precisely when the FIFO queue is not full, and an output event precisely when it is not empty. An output event's expression is evaluated only after that event is selected; its epilogue runs after the output completes. The following uses no buffered communication message-passing channels.
def strictBuffer[T](capacity: Int, in: ?[T], out: ![T]): PROC = proc {
require(capacity > 0)
val q = scala.collection.mutable.Queue[T]()
val poll = 1000000L // nanoseconds
try {
while (out.canOutput && (in.canInput || q.nonEmpty)) {
alt(
(q.size < capacity && in) =?=> { x => q.enqueue(x) }
| (q.nonEmpty && out) =!=> { q.front } ==> { q.dequeue(); () }
| after(poll) ==> { () }
)
}
} catch { case _: io.threadcso.process.Stopped => () }
finally { in.closeIn; out.closeOut }
}Here
canInput and canOutput mean that the port has not become permanently unable to communicate; they are not instantaneous readiness tests. A closed input event is disabled by the Communicating Scala Objects guarded alternation. The timer periodically rechecks closure even when the affected port's data event is disabled, for example when the FIFO queue is empty and the downstream port closes. It neither transfers data nor imposes a deadline on a healthy producer. This avoids silently assuming that closure of a disabled event wakes an alternation. The relevant output-event and closure conventions are documented in [ref].Write and for the input and output sequences. The loop invariant is that is followed by the contents of the FIFO queue. An input appends one value to its rear; an output removes exactly its front. Consequently,The two data events remain offered whenever their respective size conditions hold. Under a fair choice policy, an endlessly ready event is eventually selected. The timer requires the usual assumption that expiry cannot starve a ready communication forever.
When the input closes, no more values are accepted, but all remaining values stay eligible for output. Once the FIFO queue empties, the process closes its output and terminates. If the output closes first, there is no recipient for those remaining values: the process abandons them, closes the input to release the producer, and terminates. Draining after upstream closure requires a consumer that continues receiving. The positive-capacity precondition is deliberate: two separate synchronous message passing events cannot provide a capacity-zero buffer with zero discrepancy after every event; that would require a simultaneous three-party transfer.
Use a return control channel to carry both cancellation and one-slot flow control. The latter is what prevents a silent consumer from obstructing other streams. Keep the preceding
MUX type and addsealed trait CTRL
final case class Grant(stream: Int) extends CTRL
final case class Cancel(stream: Int) extends CTRLHere is a precise event-loop specification of the modified generators.
choose means a fair Communicating Scala Objects guarded alternation over the listed input events, output events and a short closure-poll timer; every selected transition below is finite and contains no further blocking communication. An output transition commits its state change only after its selected send succeeds. The notation is operational pseudocode, with the requested signatures; it does not introduce an assumed nonblocking send primitive.def iMux[T](ins: Seq[?[T]], ctrl: ?[CTRL], mid: ![MUX[T]]): PROC
for each stream i:
credit[i] = true; inputOpen[i] = true; terminal[i] = false
pending[i] = None // Option[MUX[T]]
until every terminal[i]:
poll closures:
if !ins[i].canInput: inputOpen[i] = false
if !terminal[i] && !inputOpen[i] && pending[i].isEmpty:
pending[i] = Some(MUX(i, None)) // end follows earlier data
choose:
ctrl ? Grant(i):
if !terminal[i]: credit[i] = true
ctrl ? Cancel(i):
terminal[i] = true; pending[i] = None; ins[i].closeIn
ins[i] ? x, enabled when inputOpen[i] && !terminal[i]
&& credit[i] && pending[i].isEmpty:
credit[i] = false; pending[i] = Some(MUX(i, Some(x)))
mid ! pending[i].get, enabled when pending[i].nonEmpty:
if pending[i].get.item.isEmpty: terminal[i] = true
pending[i] = None
poll timer: no action
finally: close every ins[i] input; close mid output and ctrl input
def iDemux[T](ctrl: ![CTRL], mid: ?[MUX[T]], outs: Seq[![T]]): PROC
for each stream i:
slot[i] = None; ended[i] = false; cancelled[i] = false
replies = empty FIFO queue of CTRL
until every (cancelled[i] || (ended[i] && slot[i].isEmpty)):
poll closures:
if !cancelled[i] && !ended[i] && !outs[i].canOutput:
cancelled[i] = true; slot[i] = None
delete any queued Grant(i); append Cancel(i) to replies
if !cancelled[i] && ended[i] && slot[i].nonEmpty
&& !outs[i].canOutput:
cancelled[i] = true; slot[i] = None
if ended[i] && slot[i].isEmpty: outs[i].closeOut
choose:
mid ? MUX(i, Some(x)):
if !cancelled[i]: assert slot[i].isEmpty; slot[i] = Some(x)
mid ? MUX(i, None):
ended[i] = true
outs[i] ! slot[i].get, enabled when !cancelled[i] && slot[i].nonEmpty:
slot[i] = None
if !ended[i]: append Grant(i) to replies
ctrl ! replies.front, enabled when replies.nonEmpty:
remove replies.front
poll timer: no action
finally: close mid input, every output, and ctrl outputThere is one receive event on
mid, with the two packet forms dispatched in its body, and one send event on mid, offering a fairly chosen pending stream. There is likewise one send event on ctrl. If mid becomes exhausted, the channel demultiplexer marks any remaining streams ended, discards queued replies, and drains or cancels their slots; the sender has then finished and needs no more grants. If the control input closes, the channel multiplexer enters cleanup. If the receiver finds its control output closed, it discards queued replies and disables further control sends, but continues receiving end markers and draining its slots; sender completion must not discard already accepted values. A channel closing during an individual guarded operation is handled locally: a failed outs(i) send cancels that stream and queues Cancel(i) unless it had already ended; exhaustion of a shared port uses the cleanup rules just stated. Ports that closed while their data events were disabled are found by the poll. These rules specify closure handling as well as normal transitions.The credit starts at one and is consumed when an input is accepted. It is returned only after the corresponding output finishes. Thus the one permitted outstanding value for stream is either pending at the sender or stored at the receiver, never both. An end marker needs no credit and follows that stream's last data packet on the ordered shared message-passing channel. It can arrive while that value is still waiting for its consumer; the receiver then delays closing the output until its slot empties. The invariant isAs in a strict buffer, cancellation can discard the last outstanding value. A cancelled receiver ignores a data packet already in flight; a later grant cannot resurrect a cancelled sender. No selected data delivery blocks the event loop, and control replies are queued rather than sent inside a receive handler. In particular, there is no cycle in which the sender blocks sending data while the receiver blocks sending cancellation. Closing the control output at final receiver termination also releases the sender if a last cancellation was still queued.
Under fair event selection and eventual scheduling, an individual cancellation reaches its producer without waiting for any unrelated consumer. Producer closure similarly delivers its own end marker without waiting for another stream to drain. This gives finite flow control equivalent in ordering and termination to capacity-one strict buffers, with finite protocol delays. A physical shared message-passing channel necessarily serializes transfers, so instantaneous readiness and absolute real-time delay bounds are not identical to physically separate buffers.