[erlang-questions] Ideas for a new Erlang

Richard A. O'Keefe ok@REDACTED
Wed Jul 2 04:36:24 CEST 2008

On 1 Jul 2008, at 6:51 am, Sven-Olof Nystr|m wrote:
> But an Erlang process is always willing to accept messages. Unlike an
> Ada process (say) there is no way an Erlang process can prevent
> another process from sending it a message. The process can choose not
> to look at it, but that's another matter.

I would say that
  - a MAILBOX is always willing to accept any message
  - a PROCESS is only willing to accept a message when
    it is executing a 'receive' construct that will
    match it.

Back when I first invented abstract patterns, I pointed out that
the fact that they are limited in the same way that all matches
and guards are limited means that it would be safe and useful to
let a process install an abstract pattern as a filter, so that
any message that didn't match the abstract pattern would never
go into the mailbox.  (In particular, the abstract pattern can
be executed by the *sender* if it's on the same node; an
abstract pattern cannot crash or affect the process dictionary
or send other messages &c.  So local rejected messages would
never even be copied, let alone added to a mailbox.)

I think that the distinction between what the mailbox does and
what the process does is too important to blur by (mis)calling
"addition to a mailbox" "acceptance by a process".

By the way, the bounded buffer example can obviously be
"rescued" thus:

	producer(Buffer) ->
	    Buffer ! {put,self(),Item},
	    receive {ok,Buffer} -> ok end,

	consumer(Buffer) ->
	    Buffer ! {get,self()},
	    receive {ok,Buffer,Item} -> ok end,

	buffer(Pool, Left) ->
		{put,Producer,Item} when Left > 0 ->
		    Producer ! {ok,Buffer},
		    buffer(Pool ++ [Item], Left - 1)
	      ; {get,Consumer} when Pool =/= [] ->
		    Consumer ! {ok,Buffer,hd(Pool)},
		    buffer(tl(Pool), Left + 1)

Assuming that this is part of a system where only the
producer and consumer can refer to Buffer, this is
the traditional bounded buffer.  The argument that the
mailbox (but NOT the process) is always willing to
accept more messages has no force between the senders
won't *send* more messages; the buffer's mailbox will
have at most one 'put' and and most one 'get' at any
one time.  The substantive point, that the *process*
(not the mailbox) is willing to accept different
messages at different times remains.

This is a nice illustration of why mailbox length is
often not a problem:  the need for a response means
that the mailbox is bounded in size by the number of
processes that might send messages.  In effect, the
responses are a form of flow control.
> So your argument is: Concurrent ML tried something similar.
> It did not work out.

It worked out in the sense that the design was implementable,
was implemented, was used, and is still available in SML/NJ
and Mlton.  It also worked out in the sense that *simple*
problems don't look too bad.  The point is that things which
would still be pretty straightforward in Erlang with its
'receive' quickly become very complex in CML.

Interestingly, the Limbo programming language has CML-like
channels, but without all the extra stuff in CML.  (For
example, CML has both channels and mailboxes.)

	chan of <data-type>		is a type.
	ch <-= msg			is a send.
	<- ch				is a receive
	<- ch_array			is multi-receive
For example,
	msg = <- ch
receives a message from one channel.
	(ix, msg) = <- ch_array
receives an (index, message) pair where the index says
which element of the channel array the message came from.
	ch <-= msg
blocks if no process is reading from ch.
	alt {... msg = <- ch => stmt;
	     ... ch <- = msg => stmt;
is like an Occam ALT.

Let me quote section 12.4 in full.

     12.4 Buffered channels

     Limbo channels are unbuffered; a sender blocks until there is a
     receiver.  This example shows a way to make a buffered channel of
     strings from an unbuffered channel.  It is written as a module  
     bufchan function takes a chan of string and a size as argument, and
     returns a new channel; it creates an asynchronous task that accepts
     input from the argument channel and saves up to size strings,
     meanwhile trying to send them to its user.

     implement Bufchan;

     Bufchan: module {
         bufchan: fn(c: chan of string, size: int): chan of string;

     xfer(oldchan, newchan: chan of string, size: int) {
         temp := array[size] of string;
         fp := 0;    # first string in buffer
         n := 0;     # number of strings in buffer
         dummy := chan of string;
         sendch, recvch: chan of string;
         s: string;

         for (;;) {
             sendch = recvch = dummy;
             if (n > 0)    sendch = newchan;
             if (n < size) recvch = oldchan;
             alt {
                 s = <-recvch =>
                     temp[(fp+n)%size] = s;

                 sendch <- = temp[fp] =>
                     temp[fp++] = nil;
                     if (fp>=size) fp -= size;

     bufchan(oldchan: chan of string, size: int): chan of string {
         newchan := chan of string;
         spawn xfer(oldchan, newchan, size);
         return newchan;

     The module is somewhat specialized, but it illustrates useful
     programming techniques.  The most interesting occurs in xfer, which
     does the work.  The problem xfer faces is that it doesn't want to
     receive input when its buffer is full, nor to try to send when it
     has nothing to transmit.  The solution here is to use a dummy
     channel on which nothing is ever sent or received; in the alt
     statement, that channel substitutes for the real input channel when
     the buffer is full, and for the output channel when the buffer is


This is "real" bounded buffer.  Note the trick: you need three
channels, one of them a dummy that will never be used.  If you
don't want a particular alternative to really be there, make
that one wait for a communication that can never happen.  To me
this is much harder to understand than a guarded 'select' in
Ada or 'alt' in Occam, because what's _really_ going on is hidden
under a layer of what I can only call obfuscation.

> So we agree that it's *not* an implementation of bounded buffers????

Why do you harp on this one note?
Idunno, you strip things to the bone to make a point concisely,
and someone makes a federal case out of your kindness.

 From now on, please do not refer to the stripped down example,
but only to the full producer/buffer/consumer example, which contains
material not relevant to the state-dependent-reception POINT.

> Since I came up with the channel concept, I've been looking for some
> convincing example of an Erlang program that could be implemented
> using selective receive, but was not possible to implement using
> channels (or where the solution with channels was more complex).

Try looking in the Erlang sources.  There are plenty of simple cases
there, true.  But there are also plenty of cases where many channels
would be needed, making the solution with channels look a lot more
complex to me.

It may be that we have different ideas of what counts as a 'more
complex' solution.  If this thread is not to degenerate into something
out of Dr Suess (was it the West-going something and the East-going
something else or was it North and South?) we really need to see
some concrete cases FROM THE PROPOSER.

The proposal in the document I was responding to does not contain
any multi-receive, and it's not clear that one can be programmed from
the operations that are in the document.  I don't know any message-
based language that doesn't have some analogue of an Occam 'ALT'/
Limbo 'alt'/Ada 'select', and that document doesn't include one,
so it's clearly incomplete that way as well.

So we need to see a functionally complete proposal (in order to
judge how complex the proposed replacement for 'receive' is in
itself) *AND* some examples of rewritten Erlang code to judge
how complex the replacement is in use.

For the statement
> I've been looking for some
> convincing example of an Erlang program that could be implemented
> using selective receive, but was not possible to implement using
> channels (or where the solution with channels was more complex).
to have any force, there must already BE a number of examples which
have been so rewritten.  So let's see them.

Once again, I am no fan of Erlang syntax and I certainly do not
advocate it staying put forever.  Who wouldn't be happy about the
bit syntax?  Who wouldn't really like to have something like
Joe Armstrong's "proper structs" (or my very slight variant of
his idea)?  It's just that I'll take a lot of persuading that the
feature of Erlang which suddenly made concurrent programming look
really easy should be replaced by something I've seen before and
been disappointed by.

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