Simple ping/pong Erlang program

gistfile1.txt

-module(ping_module).
-export([start/1, ping/2, pong/1, ending_timer/0]).

ping(0, Pong_PID) ->
    Pong_PID ! finished,
    io:format("ping finished~n", []);

ping(N, Pong_PID) ->
    io:format("Sending ping~n", []),
    Pong_PID ! {ping, self()},
    receive
        pong ->
            io:format("Received pong~n", [])
    end,
    ping(N - 1, Pong_PID).

pong(Timer_PID) ->
    receive
        finished ->
            io:format("Pong finished~n", []),
            Timer_PID ! finished;
        {ping, Ping_PID} ->
            io:format("Received ping. Sending pong~n", []),
            Ping_PID ! pong,
            pong(Timer_PID)
    end.

ending_timer() ->
    Start_time = now(),
    receive
        finished ->
            Diff_time = timer:now_diff(now(), Start_time),
            io:format("Ending timer finished~n", []),
            io:format("~p~n", [Diff_time])
    end.

start(N) ->
    Timer_PID = spawn(ping_module, ending_timer, []),
    Pong_PID = spawn(ping_module, pong, [Timer_PID]),
    spawn(ping_module, ping, [N, Pong_PID]).

Example shell session:

Eshell V5.8.5 (abort with ^G)
1> c(ping_module).
{ok,ping_module}
2> ping_module:start(1000).
Sending ping
<0.41.0>
Received ping. Sending pong
Received pong
Sending ping
...(redacted)... circa 1000 ping/pongs later...
ping finished
Pong finished
Ending timer finished
107355