Programming Erlang, 2 e., Chapter 2, Exercise 4, put_file()

[Yao Bao]

Underscore (_) or variable (Any) works, but in this case the receiver needs to know the sender’s address to send a reply.

We have to include the address of the sender in the message or register it with a name.

Cheers,
Yao

> 在 2022年1月1日，13:36，Nalin Ranjan <ranjanified@REDACTED> 写道：
> 
> 
> How about using an underscore(_) inside the receive ??
> 
> नमस्ते।
> नलिन रंजन
> 
>> On Sat, Jan 1, 2022, 8:24 AM Yao Bao <free7by@REDACTED> wrote:
>> Hello,
>> 
>> > Now that we have figured out the bug -- the shell sent a command to the server for which there was no matching receive pattern -- it begs the question: how do I detect when a process is sent a message for which there is no matching receive pattern?
>> 
>> Perhaps we cannot put any guarantee on this.
>> 
>> >    receive
>> >       {Client, list_dir} ->
>> >           Client ! {self(), file:list_dir(Dir)};
>> >       {Client, {get_file, File}} ->
>> >           Full = filename:join(Dir, File),
>> >           Client ! {self(), file:read_file(Full)};
>> >       {Client, {put_file, File, Bytes}} ->
>> >           Full = filename:join(Dir, File),
>> >           Client ! {self(), file:write_file(Full, Bytes)};
>> >       {Client, U} ->
>> >           Client ! {self(), {error, "Unknown command", U}};
>> >       {Client, U, A} ->
>> >           Client ! {self(), {error, "Unknown command", U, A}};
>> >       {Client, U, A, B} ->
>> >           Client ! {self(), {error, "Unknown command", U, A, B}}
>> >    end,
>> 
>> We cannot enumerate all kinds of patterns in the limited receive expression,
>> but we can solve this problem via properly designed message pattern.
>> 
>> For the receiver, we can say, it can receive one kinds of message,
>> which is: {Client, Command}, then the sender should send messages matching
>> this pattern, or, the server has no responsibility for replying anything.
>> 
>> With this design, the receive expression can be written as:
>> 
>> >    receive
>> >       {Client, list_dir} ->
>> >           Client ! {self(), file:list_dir(Dir)};
>> >       {Client, {get_file, File}} ->
>> >           Full = filename:join(Dir, File),
>> >           Client ! {self(), file:read_file(Full)};
>> >       {Client, {put_file, File, Bytes}} ->
>> >           Full = filename:join(Dir, File),
>> >           Client ! {self(), file:write_file(Full, Bytes)};
>> >       {Client, Command} ->
>> >           Client ! {self(), {error, "Unknown command", Command}}
>> >    end,
>> 
>> The sender can send such messages:
>> 
>> > Server ! {self(), foo}
>> 
>> > Server ! {self(), {foo, bar}}.
>> > Server ! {self(), {foo, bar, baz}}.
>> 
>> After sending, the sender say:
>> 
>>     receive C -> C end
>> 
>> And pattern C can match those messages happily.
>> 
>> By the way, in my understanding, in the worst case, message passing
>> is reliable but no guarantee, so the receiver might consider that worst case and
>> handle it properly (add timeout mechanism is an example).
>> 
>> Cheers,
>> Yao
>> 
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