Question

startTrains() ->
    TotalDist = 100,
    Trains = [trainA,trainB ],
    PID = spawn(fun() -> 
            train(1,length(Trains)) end),
    [ PID ! {self(),TrainData,TotalDist} || TrainData <- Trains],
    receive
    {_From, Mesg} ->
        error_logger:info_msg("~n Mesg ~p ~n",[Mesg])
            after 10500 ->
        refresh
    end.

so, I created Two Processes named trainA, trainB. I want to increment these process by 5 till it gets 100. I made different processes to make each of the train (process) increments its position parallely. But I was surprised to get the output sequentially i.e process trainA ends then process trainB starts. But I want to increment themselves at simultaneously.
I want to run processes like this

---

trainA 10 trainB 0
trainA 15 trainB 5
....
trainA 100 trainB 100 

---

but I m getting

---

trainA 0
....
trainA 90
trainA 95
trainA 100
trainA ends

trainB 0
trainB 5
trainB 10
.....
trainB 100

---

How to make the processes run parallel/simultaneously? Hope you get my Q’s. Please help me.

Answer 1

You spawn only one process initialized by function train/2. Your presented code is incomplete so I can only guess but I think your code is wrong because you have only one train process. For inspiration:

-module(trains).

-export([startTrains/0]).

startTrains() ->
  TotalDist = 100,
  Names = [trainA,trainB ],
  Self = self(),
  Trains = [spawn_link(fun() -> 
          train(Name, Self) end) || Name <- Names],
  [ Train ! {start, Self, 0, TotalDist} || Train <- Trains],
  ok = collectResults(Names).

collectResults([]) -> ok;
collectResults(Trains) ->
  receive
    {stop, Name, Pos, Length} ->
      io:format("~p stops at ~p (~p)~n", [Name, Pos, Length]),
      collectResults(Trains -- [Name]);
    Msg ->
      io:format("Supervisor received unexpected message ~p~n", [Msg]),
      collectResults(Trains)
  after 10500 -> timeout
  end.

train(Name, Sup) ->
  receive
    {start, Sup, Pos, Length} -> run_train(Name, Sup, Pos, Length);
    Msg ->
      io:format("~p received unexpected message ~p~n", [Name, Msg]),
      train(Name, Sup)
  end.

run_train(Name, Sup, Pos, Length)
  when Pos < Length ->
    receive after 500 ->
        NewPos = Pos + 5,
        io:format("~p ~p~n", [Name, Pos]),
        run_train(Name, Sup, NewPos, Length)
    end;
run_train(Name, Sup, Pos, Length) ->
  Sup ! {stop, Name, Pos, Length}.

But if I would think it seriously I should look to gen_fsm and OTP principles. But in your current stage keep play with erlang primitives to take better feeling first.