Erlang. Correct way to stop process

Question

Good day, i have following setup for my little service:

-module(mrtask_net).

-export([start/0, stop/0, listen/1]).

-define(SERVER, mrtask_net).

start() ->
    Pid = spawn_link(fun() -> ?MODULE:listen(4488) end),
    register(?SERVER, Pid),
    Pid.

stop() ->
    exit(?SERVER, ok).

....

And here is the repl excerpt:

(emacs@rover)83> mrtask_net:start().
<0.445.0>
(emacs@rover)84> mrtask_net:stop().
** exception error: bad argument
     in function  exit/2
        called as exit(mrtask_net,ok)
     in call from mrtask_net:stop/0
(emacs@rover)85> 

As you see, stopping process produces error, process is stopping though. What does this error mean and how to make thing clean ?

Answer 1

Not being an Erlang programmer and just from the documentation of exit (here), I'd say, that exit requires a process id as first argument whereas you are passing an atom (?SERVER) to it.

Try

exit(whereis(?SERVER), ok).

instead (whereis returns the process id associated with a name, see here)

Answer 2

You need to change the call to exit/2 as @MartinStettner has pointed out. The reason the process stops anyway is that you have started it with spawn_link. Your process is then linked to the shell process. When you called mrtask_net:stop() the error caused the shell process to crash which then caused your process to crash as they were linked. A new shell process is then automatically started so you can keep working with the shell. You generally do want to start your servers with spawn_link but it can cause confusion when your are testing them from the shell and they just "happen" to die.