[erlang-questions] How exactly Erlang receive expression works?

[Athanasios Valsamakis]

Mr. Xavier, you are absolutely right!

Thank you for pointing my error.

Is the following flowchart closer to the truth?


      +-----------------+
      |  Enter receive. |               ** Timer may have already been
elapsed
      |  Start timer    |                when entering this state.
      +------+----------+
             |
       +-----v------+            +--------------------------------+
       | is mailbox | Yes        | suspend process until          |timer
elapsed
+------> empty?     +------------> a new message is put in
mailbox+-----------------+
|      |            |            | or  timer has elapsed**        |
         |
|      +------------+            +-----+--------------------------+
 +-------------v---------------------+     +-------------------+
|            |No                       |                              |if
any messages have been put into |     | evaluate          |
|     +------v----------------+        |new msg arrived               |the
save queue, are reentered      +-----> expressionsTimeout|
|     | take the head message |        |                              |into
the mailbox                   |     |                   |
|     | from the mailbox      <--------+
+-----------------------------------+     +--------+----------+
|     +------+----------------+
                                              |
|            |
+-----------------------------------+     +-------------+          |
|      +-----v-------------+    +---------------+     |if any messages have
been put into +----->evaluate     +---------->
|      | match the message +---->remove message +----->the save queue, are
reentered      |     |expressions1 |          |
|      | against pattern1  |Yes |from mailbox   |     |into the mailbox
               |     +-------------+          |
|      +-------------------+    +---------------+
 +-----------------------------------+                              |
|            |No
                                             |
|            |
                                             |
|      +-----v-------------+    +---------------+
 +-----------------------------------+     +-------------+          |
|      | match the message +---->remove message +----->if any messages have
been put into +----->evaluate     +---------->
|      | against pattern2  |Yes |from mailbox   |     |the save queue, are
reentered      |     |expressions2 |          |
|      +-------------------+    +---------------+     |into the mailbox
               |     +-------------+          |
|            |No
+-----------------------------------+                              |
|            |
                                             |
|      +-----v-------------+    +---------------+
 +-----------------------------------+     +-------------+          |
|      | match the message +---->remove message +----->if any messages have
been put into +----->evaluate     +---------->
|      | against pattern3  |Yes |from mailbox   |     |the save queue, are
reentered      |     |expressions3 |          |
|      +-------------------+    +---------------+     |into the mailbox
               |     +-------------+          |
|            |No
+-----------------------------------+                              |
|            |
                                             |
|      +-----v------------------------------+
                                              |
|      | message is removed from the mailbox|
                                              |
+------+ and put into a "save queue"        |
                                              |
       |                                    |
                                              |
       +------------------------------------+
                                              |

                                             |
        +------------------+
                                             |
        | Exit receive.
<---------------------------------------------------------------------------------------------+
        | Timer is cleared.|
        +------------------+






On Fri, Mar 16, 2018 at 4:57 PM, Xavier Noria <fxn@REDACTED> wrote:

> I think there is a detail that does not square.
>
> Take this example[*]:
>
> -module(foo).
> -export([start/0]).
>
> start() ->
> self() ! matched_by_b,
> self() ! matched_by_a,
> a(),
> halt().
>
> a() ->
> receive
> matched_by_a ->
> io:format("matched by a/0~n"),
> b()
> end.
>
> b() ->
> receive
> matched_by_b ->
> io:format("matched by b/0~n")
> end.
>
>
> According to your (very nice!) diagram, this program should hang, because
> when b/0 is invoked, the message `matched_by_b` has already been moved into
> the save queue, and b/0 is called *while* the expression that matches
> `matched_by_a` is evaluated. The diagram says `matched_by_b` is not in the
> mailbox at that point.
>
> But the program terminates, ergo reductio ad absurdum :).
>
> I guess that diagram comes from the algorithm outlined on page 194 of the
> book. I believe you need to factor in step (5).
>
> Xavier
>
> [*] erlc foo.erl && erl -noshell -run foo
>
>
>
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      +-----------------+
      |  Enter receive. |               ** Timer may have already been elapsed
      |  Start timer    |                when entering this state.
      +------+----------+
             |
       +-----v------+            +--------------------------------+
       | is mailbox | Yes        | suspend process until          |timer elapsed
+------> empty?     +------------> a new message is put in mailbox+-----------------+
|      |            |            | or  timer has elapsed**        |                 |
|      +------------+            +-----+--------------------------+   +-------------v---------------------+     +-------------------+
|            |No                       |                              |if any messages have been put into |     | evaluate          |
|     +------v----------------+        |new msg arrived               |the save queue, are reentered      +-----> expressionsTimeout|
|     | take the head message |        |                              |into the mailbox                   |     |                   |
|     | from the mailbox      <--------+                              +-----------------------------------+     +--------+----------+
|     +------+----------------+                                                                                          |
|            |                                        +-----------------------------------+     +-------------+          |
|      +-----v-------------+    +---------------+     |if any messages have been put into +----->evaluate     +---------->
|      | match the message +---->remove message +----->the save queue, are reentered      |     |expressions1 |          |
|      | against pattern1  |Yes |from mailbox   |     |into the mailbox                   |     +-------------+          |
|      +-------------------+    +---------------+     +-----------------------------------+                              |
|            |No                                                                                                         |
|            |                                                                                                           |
|      +-----v-------------+    +---------------+     +-----------------------------------+     +-------------+          |
|      | match the message +---->remove message +----->if any messages have been put into +----->evaluate     +---------->
|      | against pattern2  |Yes |from mailbox   |     |the save queue, are reentered      |     |expressions2 |          |
|      +-------------------+    +---------------+     |into the mailbox                   |     +-------------+          |
|            |No                                      +-----------------------------------+                              |
|            |                                                                                                           |
|      +-----v-------------+    +---------------+     +-----------------------------------+     +-------------+          |
|      | match the message +---->remove message +----->if any messages have been put into +----->evaluate     +---------->
|      | against pattern3  |Yes |from mailbox   |     |the save queue, are reentered      |     |expressions3 |          |
|      +-------------------+    +---------------+     |into the mailbox                   |     +-------------+          |
|            |No                                      +-----------------------------------+                              |
|            |                                                                                                           |
|      +-----v------------------------------+                                                                            |
|      | message is removed from the mailbox|                                                                            |
+------+ and put into a "save queue"        |                                                                            |
       |                                    |                                                                            |
       +------------------------------------+                                                                            |
                                                                                                                         |
        +------------------+                                                                                             |
        | Exit receive.    <---------------------------------------------------------------------------------------------+
        | Timer is cleared.|
        +------------------+