Adding clients to a list in Erlang - functional-programming

Question

so I am writing a basic server that clients can connect to. I'm testing it with telnet, but my issue is how I store my clients in a list, so I could then print out who is connected to the server. I've tried using the ++ operator on a list that stores clients, but I keep getting a process error.
Here is my code so far:
cell_process()->
io:fwrite("In cell~n"),
{ok,Listening_socket} = gen_tcp:listen(21,
[binary,
{backlog,5},
{active,false},
{packet,line}]),
loop(Listening_socket).
loop(Listening_socket)->
case gen_tcp:accept(Listening_socket) of
{ok,Client_socket} ->
gen_tcp:send(Client_socket, "Hello, what is your name?"),
{_,Name} = gen_tcp:recv(Client_socket,0),
gen_tcp:send(Client_socket, "Hello, "),
gen_tcp:send(Client_socket, Name),
io:fwrite([Name]),
spawn(fun()-> client_loop(Client_socket) end),
loop(Listening_socket);
{error,Why}->io:fwrite("Error: "),
io:fwrite([Why])
end.
client_loop(Client_socket)->
case gen_tcp:recv(Client_socket,0) of
{ok,Message}-> gen_tcp:send(Client_socket,Message),
%% code not yet implemented here
client_loop(Client_socket);
{error,Why}-> io:fwrite("Error: ~s~n",[Why]),
gen_tcp:close(Client_socket)
end.
Any help would be much appreciated, thanks for reading.

Answer 1

To keep a trace of connected clients you have to add a variable to the loop function, and store any relevant information in a Clients list as you suggest.
cell_process()->
io:fwrite("In cell~n"),
{ok,Listening_socket} = gen_tcp:listen(21,
[binary,
{backlog,5},
{active,false},
{packet,line}]),
loop(Listening_socket,[]). %% initial Clients list is empty
loop(Listening_socket,Clients)->
case gen_tcp:accept(Listening_socket) of
{ok,Client_socket} ->
gen_tcp:send(Client_socket, "Hello, what is your name?"),
{_,Name} = gen_tcp:recv(Client_socket,0),
gen_tcp:send(Client_socket, "Hello, "),
gen_tcp:send(Client_socket, Name),
io:fwrite([Name]),
Pid = spawn(fun()-> client_loop(Client_socket) end),
loop(Listening_socket,[{Pid,Client_socket,Name}|Clients]);
%% add new Client to the lists But see my comment
{error,Why}->io:fwrite("Error: "),
io:fwrite([Why])
end.
client_loop(Client_socket)->
case gen_tcp:recv(Client_socket,0) of
{ok,Message}-> gen_tcp:send(Client_socket,Message),
%% code not yet implemented here
client_loop(Client_socket);
{error,Why}-> io:fwrite("Error: ~s~n",[Why]),
gen_tcp:close(Client_socket)
end.
Comment: It is not the right place to store the Client list, because you will have to manage the client who are dying, either with links or monitor, so you need a receive block, incompatible with the socket listening. So I think that you should add a server to manage this list, which role is to listen the messages sent each time a new Client is launched, monitor the Clients, answer to the admin when he want to know the Client list ...

Answer 2

Here's a suggestion:
loop(Listening_socket, ClientList)->
case gen_tcp:accept(Listening_socket) of
{ok,Client_socket} ->
gen_tcp:send(Client_socket, "Hello, what is your name?"),
{_,Name} = gen_tcp:recv(Client_socket,0),
gen_tcp:send(Client_socket, "Hello, "),
gen_tcp:send(Client_socket, Name),
io:fwrite([Name]),
spawn(fun()-> client_loop(Client_socket) end),
loop(Listening_socket, [Client_socket | ClientList]);
{error,Why}->io:fwrite("Error: "),
io:fwrite([Why])
end.
A better solution would be to have a state record where you store all the vars you need (in this case, Listening_socket and ClientList) and loop with that state.