Re: confusion about message passing between the tasks

[Robert A Duff <bobduff@shell01.TheWorld.com>]

"J-P. Rosen" <rosen@adalog.fr> writes:

> Le 26/10/2014 00:23, Robert A Duff a écrit :
>> I prefer protected objects.  Less coupling: two tasks can communicate
>> with each other with neither one knowing the identity of the other --
>> they both just know about the protected object.
>> 
>> If I were redesigning Ada from scratch, I'd leave out rendezvous.
>> Everything you can do with rendezvous you can do with (something like)
>> protected objects.
>
> ... and everything you can do with structured statements you can do with
> if and goto.

OK, then I'll make a stronger claim: Everything you can do with
rendezvous you can do with (something like) protected objects,
and do it just as cleanly, and sometimes even better.

Now you're goto analogy doesn't work.  ;-)  I like case statements
and while loops.

> I strongly disagree with this position. Rendezvous is a nice model that
> implements directly a client-server model. Of course, in this model, the
> client has to know the server. So what?
>
> I view protected objects as a much lower level abstraction than
> rendezvous; they are nice for simple synchronisation and simple needs,
> but they don't scale up well for higher level abstactions.

I agree rendezvous is higher level than PO's, but "higher level"
is not always synonymous with "better".  I find PO's to be at
the *right* level.  Ada 83 tasking has some baroque bells and
whistles that, though they could reasonably be considered
"high level", the just get in the way.

In Ada 83, I found that in between every pair of communicating tasks,
I wanted to add a passive "buffer" task or some such.  I'm happy
to use a PO there.

>...Try to
> Ravenscarize (!) existing software that uses rendezvous, and you'll see
> what I mean (I did that recently).

That's not a fair comparison, because Ravenscar is far more restricted
than just "no rendezvous".  For example, I find the queue-length-1
restriction pretty onerous.

> And don't forget that with PO, you lose a very nice and poweful feature
> of rendezvous: automatic termination.

That's why I used the weasel words "(something like)".  I would extend
PO's to support everything that rendezvous can do, and eliminate
rendezvous.  It's tricky to figure out how terminate alternatives
should work with PO's, but I think I've got it.  Another thing
is that you'd want multi-way call.

I don't mean to say rendezvous is a Bad Thing.  It's just not necessary
when you have (something like) PO's, and it adds a lot of complexity
to the language.  I'm pretty sure that if Ada 83 had PO's, Ada 95
would NOT have added rendezvous!

- Bob