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

[Athanasios Valsamakis]

I think the timer should also be cleared in an earlier step.

On Fri, Mar 16, 2018 at 7:37 PM, Athanasios Valsamakis <
athanasios.valsamakis@REDACTED> wrote:

> 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  timer is cleared |     |                   |
|     | 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, timer is cleared |     +-------------+          |
|      +-------------------+    +---------------+     +-----------------------------------+                              |
|            |No                                                                                                         |
|            |                                                                                                           |
|      +-----v-------------+    +---------------+     +-----------------------------------+     +-------------+          |
|      | match the message +---->remove message +----->if any messages have been put into +----->evaluate     +---------->
|      | against pattern2  |Yes |from mailbox   |     |the sa^e queue, are reentered      |     |expressions2 |          |
|      +-------------------+    +---------------+     |into the mailbox  timer is cleared |     +-------------+          |
|            |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  timer is cleared |     +-------------+          |
|            |No                                      +-----------------------------------+                              |
|            |                                                                                                           |
|      +-----v------------------------------+                                                                            |
|      | message is removed from the mailbox|                                                                            |
+------+ and put into a "save queue"        |                                                                            |
       |                                    |                                                                            |
       +------------------------------------+                                                                            |
                                                                                                                         |
        +--------------+                                                                                                 |
        | Exit receive <-------------------------------------------------------------------------------------------------+
        +--------------+