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

How about using an underscore(_) inside the receive ??

नमस्ते।
नलिन रंजन

On Sat, Jan 1, 2022, 8:24 AM Yao Bao <[email protected]> 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
>
>