Process chaining with and without a receive block

linkmon.erl

% The cause of the second output is (where the 
% receive block is commented out) is that because
% the process does not expect any messages and it
% ends naturally.
%
% On the other hand, the version with receive
% expects messages (spits out any message that
% arrives) and it keeps on running until it is
% told to stop.

chain(N) ->¬                                                                                                                                                                          
    spawn_link(?MODULE,chain,[N,self()]).¬                                                                                                                                            
¬                                                                                                                                                                                     
chain(0,_) ->¬                                                                                                                                                                        
    receive¬                                                                                                                                                                          
    after¬                                                                                                                                                                            
        7000 -> exit("chain ends")¬                                                                                                                                                   
    end;¬                                                                                                                                                                             
chain(N,Shell) ->¬                                                                                                                                                                    
    Pid = spawn_link(fun() -> chain(N-1,Shell) end),¬                                                                                                                                 
    Shell ! Pid,¬                                                                                                                                                                     
    io:format("~p started (~p)~n",[N-1, Pid]),                                                                                                                                      
    receive¬                                                                                                                                                                        
        _ -> ok¬                                                                                                                                                                    
    end.¬                                                                                                                                                                           
¬                                                                                                                                                                                     
collect() ->¬                                                                                                                                                                         
    collect([]).¬                                                                                                                                                                     
¬                                                                                                                                                                                     
collect(List) ->¬                                                                                                                                                                     
    receive¬                                                                                                                                                                          
        Pid -> [Pid|collect(List)]¬                                                                                                                                                   
    after¬                                                                                                                                                                            
        0 -> List¬                                                                                                                                                                    
    end.

7> linkmon:chain(7).                       
6 started (<0.70.0>)
5 started (<0.71.0>)
4 started (<0.72.0>)
3 started (<0.73.0>)
2 started (<0.74.0>)
1 started (<0.75.0>)
0 started (<0.76.0>)
<0.69.0>
8> f(L),L = linkmon:collect().             
[<0.70.0>,<0.71.0>,<0.72.0>,<0.73.0>,<0.74.0>,<0.75.0>,
 <0.76.0>]
9> [erlang:is_process_alive(P) || P <- L ].
[true,true,true,true,true,true,true]
** exception error: "chain ends"            
10> [erlang:is_process_alive(P) || P <- L ].
[false,false,false,false,false,false,false]
11> 
11> 
11> 

chain(0,_) ->¬                                                                                                                                                                        
    receive¬                                                                                                                                                                          
    after¬                                                                                                                                                                            
        7000 -> exit("chain ends")¬                                                                                                                                                   
    end;¬                                                                                                                                                                             
chain(N,Shell) ->¬                                                                                                                                                                    
    Pid = spawn_link(fun() -> chain(N-1,Shell) end),¬                                                                                                                                 
    Shell ! Pid,¬                                                                                                                                                                     
    io:format("~p started (~p)~n",[N-1, Pid]).¬                                                                                                                                       
    % receive¬                                                                                                                                                                        
    %     _ -> ok¬                                                                                                                                                                    
    % end.¬  
11> c(linkmon).                             
{ok,linkmon}
12> linkmon:chain(7).                       
6 started (<0.88.0>)
5 started (<0.89.0>)
4 started (<0.90.0>)
3 started (<0.91.0>)
2 started (<0.92.0>)
1 started (<0.93.0>)
0 started (<0.94.0>)
<0.87.0>
13> f(L),L = linkmon:collect().             
[<0.88.0>,<0.89.0>,<0.90.0>,<0.91.0>,<0.92.0>,<0.93.0>,
 <0.94.0>]
14> [erlang:is_process_alive(P) || P <- L ].
[false,false,false,false,false,false,true]
15> [erlang:is_process_alive(P) || P <- L ].
[false,false,false,false,false,false,false]