fed-sx-m1: Step 7b — gen_server-per-projection (start_link/3 + async_fold + query) + 11 tests

next/kernel/projection.erl

@@ -1,6 +1,9 @@
 -module(projection).
+-behaviour(gen_server).
 -export([new/2, new/3, fold_activity/2, replay/2,
          name/1, state/1, fold_fn/1]).
+-export([start_link/3, async_fold/2, query/1, stop/1]).
+-export([init/1, handle_call/3, handle_cast/2, handle_info/2]).
 
 %% Pure-functional projection driver per design §10.
 %%
@@ -52,3 +55,43 @@ field(_, []) -> erlang:error(badkey).
 set_field(K, V, [{K, _} | Rest]) -> [{K, V} | Rest];
 set_field(K, V, [P | Rest]) -> [P | set_field(K, V, Rest)];
 set_field(K, V, []) -> [{K, V}].
+
+%% ── Step 7b: gen_server wrapper ─────────────────────────────────
+%%
+%% Each projection runs in its own gen_server, registered under the
+%% projection's Name atom. `async_fold/2` casts an activity into the
+%% process; `query/1` synchronously fetches the current state.
+%%
+%% Port notes (mirroring Step 5b on the registry): `gen_server:start_link`
+%% returns the raw Pid; `?MODULE` macro is unsupported; spawned
+%% processes don't survive across separate `erlang-eval-ast` calls
+%% so tests must inline start_link with their operations.
+
+start_link(Name, InitialState, FoldFn) ->
+    Pid = gen_server:start_link(projection, [Name, InitialState, FoldFn]),
+    erlang:register(Name, Pid),
+    Pid.
+
+async_fold(Name, Activity) ->
+    gen_server:cast(Name, {fold, Activity}).
+
+query(Name) ->
+    gen_server:call(Name, get_state).
+
+stop(Name) ->
+    R = gen_server:call(Name, '$gen_stop'),
+    erlang:unregister(Name),
+    R.
+
+%% gen_server callbacks
+
+init([Name, InitialState, FoldFn]) ->
+    {ok, new(Name, InitialState, FoldFn)}.
+
+handle_call(get_state, _From, Proj) ->
+    {reply, state(Proj), Proj}.
+
+handle_cast({fold, Activity}, Proj) ->
+    {noreply, fold_activity(Proj, Activity)}.
+
+handle_info(_, Proj) -> {noreply, Proj}.