Slack has deprecated the RTM API, and it won't work with new apps (error msg: {:error, :no_scheme})

[JediLuke]

If you make a new Slack app and try to use Hedwig, it won't work, you'll just get this very confusing error:

[error] GenServer #PID<0.592.0> terminating
** (MatchError) no match of right hand side value: {:error, :no_scheme}
    (hedwig_slack 1.0.0) lib/hedwig_slack/connection.ex:16: HedwigSlack.Connection.start/1
    (hedwig_slack 1.0.0) lib/hedwig_slack/adapter.ex:48: Hedwig.Adapters.Slack.handle_info/2
    (stdlib 3.15.2) gen_server.erl:695: :gen_server.try_dispatch/4
    (stdlib 3.15.2) gen_server.erl:771: :gen_server.handle_msg/6
    (stdlib 3.15.2) proc_lib.erl:226: :proc_lib.init_p_do_apply/3
Last message: {:EXIT, #PID<0.590.0>, {{:badmatch, {:error, :no_scheme}}, [{HedwigSlack.Connection, :start, 1, [file: 'lib/hedwig_slack/connection.ex', line: 16]}, {Hedwig.Adapters.Slack, :handle_info, 2, [file: 'lib/hedwig_slack/adapter.ex', line: 48]}, {:gen_server, :try_dispatch, 4, [file: 'gen_server.erl', line: 695]}, {:gen_server, :handle_msg, 6, [file: 'gen_server.erl', line: 771]}, {:proc_lib, :init_p_do_apply, 3, [file: 'proc_lib.erl', line: 226]}]}}
State: {:state, {#PID<0.592.0>, Supervisor.Default}, :simple_one_for_one, {[Hedwig.Responder], %{Hedwig.Responder => {:child, :undefined, Hedwig.Responder, {Hedwig.Responder, :start_link, []}, :transient, false, 5000, :worker, [Hedwig.Responder]}}}, {:maps, %{#PID<0.593.0> => [Hedwig.Responders.Help, {nil, "DubzCat", [], #PID<0.590.0>}], #PID<0.594.0> => [Hedwig.Responders.Ping, {nil, "DubzCat", [], #PID<0.590.0>}], #PID<0.595.0> => [DubzCat.Responders.Meow, {nil, "DubzCat", [], #PID<0.590.0>}]}}, 3, 5, [], 0, :never, Supervisor.Default, {:ok, {%{intensity: 3, period: 5, strategy: :simple_one_for_one}, [{Hedwig.Responder, {Hedwig.Responder, :start_link, []}, :transient, 5000, :worker, [Hedwig.Responder]}]}}}

The issue is that Slack has deprecated the RTM API, in favour of the Events API - see https://api.slack.com/authentication/basics

If you use IO.inspect on the data you get back here https://github.com/hedwig-im/hedwig_slack/blob/master/lib/hedwig_slack/adapter.ex#L45

you'll see %{"error" => "invalid_auth", "ok" => false}

I think the only way to fix this for new apps (my old app seems to still work) will be to upgrade this adapter to not use the RTM API