2.15 Supervision https://www.futurelearn.com/courses/concurrent-programming-erlang/3/steps/488360/

super.erl

-module(super).
-export([super/0]).

%% Comments on killing processes

%% 1. Without a supervisor:

%% evaluating ~exit(Pid,kill)~ kills Pid but the listener keeps looping.

%% If we were to kill the listener instead with ~exit(whereis(echo), kill)~ then
%% Pid would terminate as well with an error because it asumes a registered
%% process named 'echo' on talk.erl line 8 “echo!Msg”.


%% 2. When using the supervisor in =super.erl=

%% We must call ~observer:star()~ first in order to be able to kill processes spawned by =super/0=.

%% If we kill =talk=, =super= will trap its exit and respawn it.

%% If we kill =echo=, =super= will respawn it, and if we add the
%% ~timer:sleep(1000)~ instruction, =talk= will exit with error when it tries to
%% send a message to =echo= and it will be respawned by =super= as well.


super() ->
  process_flag(trap_exit, true),
  E = spawn_link(echo,listener,[]),
  register(echo,E),
  io:format("echo spawned.~n"),
  T = spawn_link(talk,worker,[]),
  register(talk,T),
  io:format("worked spawned as Pid ~w.~n",[whereis(talk)]),
  loop(E, T).

loop(E,T) ->
  receive
    {'EXIT', T, _} ->
      NewT = spawn_link(talk,worker,[]),
      register(talk,NewT),
      io:format("worker re-spawned as Pid ~w.~n",[whereis(talk)]),
      loop(E,NewT);
    {'EXIT', E, _} ->
      timer:sleep(1000),
      NewE = spawn_link(echo,listener,[]),
      register(echo,NewE),
      io:format("echo re-spawned.~n"),
      loop(NewE,T)
  end.