[erlang-questions] [LONG] How to make synchronous message passing

[Richard Carlsson]

ZeD wrote:
> if a process P want to make a synchronous call to Q, then it send a message 
> and waits for a reply.
> 
> What's appened when another process, R (just for make and example) send a
> message to P 'before' the Q reply?

Then, the message from R will be before the reply from Q in the mailbox
of P. But if you use selective matching, as in synchro2, then P can
ignore the message from R and accept only the message from Q.

> In particular, what if Q first send another message to P, then waits for the
> reply, and only after all this, send the reply to the original message to
> P?

If P is in synchro2, and Q sends another message to P (not the reply),
then either P will mistakenly think that it is the reply (if it can have
the same form as a reply), or P will just ignore the message (like it
ignored the message from R) and keep waiting for the reply. This is
the whole point: if the communication is _synchronous_, then P will not
do anything at all (per definition) until it gets the reply. Other
messages will be queued up in the mailbox.

> Oh, and just to know, let's say P was described by the function f(), which
> have a receive; now P want to talk to Q, then now the behavior of P is
> described by synchro2(), wich also have a receive. Let's say P finally had
> a reply, so the control came back to f(). What append to messages not
> matched by synchro2()? they became automatically "receveived" by f()? They
> get lost?

No. The process P has a mailbox (a queue), and when the receive-
expression in synchro2 finishes, it will only remove the matched
message. All other messages remain in the queue. Nothing is lost
unless you throw it away yourself.

    /Richard