Approach the rebar3 alias implementation

This is a quick and dirty solution pushed for discussion purposes

apps/rebar/src/rebar_prv_alias.erl

@@ -6,25 +6,63 @@
 %%% years of stability. Only some error checks were added
 -module(rebar_prv_alias).
 
--export([init/1]).
+-behaviour(provider).
+
+-export([init/1, do/1, format_error/1]).
 -include("rebar.hrl").
 
+-define(PROVIDER, alias).
+-define(IMPL_PREFIX, "rebar_prv_alias_").
+-define(DESC_PREFIX, "Equivalent to running: rebar3 do ").
+
 %% ===================================================================
 %% Public API
 %% ===================================================================
 -spec init(rebar_state:t()) -> {ok, rebar_state:t()}.
 init(State) ->
     Aliases = rebar_state:get(State, alias, []),
-    lists:foldl(fun({Alias, Cmds}, {ok, StateAcc}) ->
-                    case validate_provider(Alias, Cmds, State) of
-                        true -> init_alias(Alias, Cmds, StateAcc);
-                        false -> {ok, State}
-                    end
-                end, {ok, State}, Aliases).
+    {ok, State1}
+        = lists:foldl(fun({Alias, Cmds}, {ok, StateAcc}) ->
+                          case validate_provider(Alias, Cmds, State) of
+                              true -> init_alias(Alias, Cmds, StateAcc);
+                              false -> {ok, State}
+                          end
+                      end, {ok, State}, Aliases),
+    State2 = rebar_state:add_provider(State1, providers:create([{name, ?PROVIDER},
+                                                                {module, ?MODULE},
+                                                                {bare, true},
+                                                                {deps, []},
+                                                                {example, "rebar3 alias"},
+                                                                {short_desc, "List defined aliases."},
+                                                                {desc, "List defined aliases."},
+                                                                {opts, []}])),
+    {ok, State2}.
+
+-spec do(rebar_state:t()) -> {ok, rebar_state:t()} | {error, string()}.
+do(State) ->
+    ImplPrefixLen = length(?IMPL_PREFIX),
+    lists:foreach(
+        fun (Provider) ->
+            MaybePrvAlias = atom_to_list(providers:module(Provider)),
+            case string:sub_string(MaybePrvAlias, 1, ImplPrefixLen) =:= ?IMPL_PREFIX of
+                true ->
+                    Alias = string:sub_string(MaybePrvAlias, ImplPrefixLen + 1),
+                    DescSuf = string:sub_string(providers:desc(Provider), length(?DESC_PREFIX) + 1),
+                    ?CONSOLE("~ts=~ts", [Alias, DescSuf]);
+                false -> ok
+            end
+        end,
+        rebar_state:providers(State)
+    ),
+    {ok, State}.
+
+-spec format_error(any()) -> iolist().
+format_error(Reason) ->
+    io_lib:format("~p", [Reason]).
 
 -dialyzer([{no_opaque, init_alias/3}, {no_return, init_alias/3}]). % warnings relate to use of opaque structures in :forms
 init_alias(Alias, Cmds, State) ->
-    Module = list_to_atom("rebar_prv_alias_" ++ atom_to_list(Alias)),
+    Module = list_to_atom(?IMPL_PREFIX ++ atom_to_list(Alias)),
 
     MF = module(Module),
     EF = exports(),
@@ -72,8 +110,7 @@ example(Alias) ->
 
 -dialyzer({no_unused, desc/1}). % required since we suppress warnings for init_alias/3
 desc(Cmds) ->
-    "Equivalent to running: rebar3 do "
-        ++ rebar_string:join(lists:map(fun to_desc/1, Cmds), ",").
+    ?DESC_PREFIX ++ rebar_string:join(lists:map(fun to_desc/1, Cmds), ",").
 
 to_desc({Cmd, Args}) when is_list(Args) ->
     atom_to_list(Cmd) ++ " " ++ Args;