Erlang receive loop - variable unbound issue - erlang

Question

So I need to write this function that receives a number from three different processes and saves them. I can't get it to work, I get the "variable unbound" error.
serverB2(Time, Client1, Client2, Client3) ->
receive
{From, TimeClient} ->
if
From == Client1 ->
TimeClient1 = TimeClient;
From == Client2 ->
TimeClient2 = TimeClient;
From == Client3 ->
TimeClient3 = TimeClient;
end,
serverB2(Time, Client1, Client2, Client3)
end,
List = [Time, TimeClient1, TimeClient2, TimeClient3],
io:format("~w \n", List).

Answer 1

You are getting variable unbound error because your code has three paths and in each path you will bound only one of variables TimeClient1, TimeClient2 and TimeClient3 so you have always two of them unbound. What's worse your code never stop. You will receive message, then evaluate if statement and then recursive call serverB2 code and again and again and again. There is not any code path to your statements after receive.
I would write this in this way
serverB2(Time, Client1, Client2, Client3) ->
List = [ Time
| [ receive {Client, TimeClient} -> TimeClient end
|| Client <- [Client1, Client2, Client3]]],
io:format("~w~n", [List]).
or in more conventional way
serverB2(Time, Client1, Client2, Client3) ->
List = [ Time | collect_times([Client1, Client2, Client3])],
io:format("~w~n", [List]).
collect_times([]) -> [];
collect_times([Client|Tail]) ->
TimeClient = receive {Client, Time} -> Time end,
[ TimeClient | collect_times(Tail) ].
Which is how list comprehension above would be translated by compiler.
The trick what I'm using here is that I receive messages in order what I want instead of order in which they are arrived to mailbox. I'm using selective receive there because variable Client is bound in receive pattern in both examples above. There is more complicated way how receive messages in order how they arrive which you should use only with very good reason (performance for example).