[erlang-questions] Supervision Tree

[Richard O'Keefe]

(1) When I paste your code into a file and compile it with erlc,
    there are no complaints.

(2) Your start_link()
    calls supervisor:start_link(ch_sup, [])
    which calls ch_sup:init([])
                ^^arg1      ^^arg2

    If you called supervisor:start_link(robots, [gort,r2d2])
    it would call back to robots:init([gort,r2d2]).

    This is explained in the documentation for the supervisor: module
    http://www.erlang.org/doc/man/supervisor.html#start_link-2
    which says

	Module is the name of the callback module.

	Args is an arbitrary term which is passed as the argument to
	Module:init/1.

For a non-trivial example, I looked in the OTP sources
and found snmpm_supervisor.erl, which has

start_link(Type, Opts) ->
    ?d("start_link -> entry with"
       "~n   Opts: ~p", [Opts]),
    SupName = {local, ?MODULE},
    supervisor:start_link(SupName, ?MODULE, [Type, Opts]).

...

init([Opts]) when is_list(Opts) ->    %% OTP-5963: Due to the addition
    init([normal, Opts]);             %% OTP-5963: of server_sup
init([Type, Opts]) ->
    ?d("init -> entry with"
       "~n   Type: ~p"
       "~n   Opts: ~p", [Type, Opts]),
    Restart   = get_restart(Opts),
    Flags     = {one_for_all, 0, 3600},
    Config    = worker_spec(snmpm_config, [Opts], Restart, [gen_server]),
    MiscSup   = sup_spec(snmpm_misc_sup, [], Restart),
    ServerSup = sup_spec(snmpm_server_sup, [Type, Opts], Restart),
    Sups      = [Config, MiscSup, ServerSup],
    {ok, {Flags, Sups}}.