erlang: gen_server behaviour (+10 tests)

lib/erlang/runtime.sx

@@ -814,3 +814,58 @@
                 (er-mk-atom mod)
                 (er-mk-atom name)))))
         (er-apply-fun (get mod-env name) vs)))))
+
+;; ── gen_server (OTP-lite) ───────────────────────────────────────
+;; A minimal gen_server behaviour — `start_link/2`, `call/2`, `cast/2`,
+;; `stop/1`, plus the receive loop dispatching `Mod:handle_call/3`,
+;; `Mod:handle_cast/2`, `Mod:handle_info/2`. Loaded into the user
+;; module registry on demand via `(er-load-gen-server!)`.
+(define
+  er-gen-server-source
+  "-module(gen_server).
+   start_link(Mod, Args) ->
+     spawn(fun () ->
+       case Mod:init(Args) of
+         {ok, State} -> gen_server:loop(Mod, State);
+         {stop, Reason} -> exit(Reason)
+       end
+     end).
+   call(Pid, Req) ->
+     Ref = make_ref(),
+     Pid ! {'$gen_call', {self(), Ref}, Req},
+     receive {Ref, Reply} -> Reply end.
+   cast(Pid, Msg) ->
+     Pid ! {'$gen_cast', Msg},
+     ok.
+   stop(Pid) ->
+     gen_server:call(Pid, '$gen_stop').
+   loop(Mod, State) ->
+     receive
+       {'$gen_call', {From, Ref}, '$gen_stop'} ->
+         From ! {Ref, ok};
+       {'$gen_call', {From, Ref}, Req} ->
+         case Mod:handle_call(Req, From, State) of
+           {reply, Reply, NewState} ->
+             From ! {Ref, Reply},
+             gen_server:loop(Mod, NewState);
+           {noreply, NewState} ->
+             gen_server:loop(Mod, NewState);
+           {stop, Reason, Reply, NewState} ->
+             From ! {Ref, Reply},
+             exit(Reason)
+         end;
+       {'$gen_cast', Msg} ->
+         case Mod:handle_cast(Msg, State) of
+           {noreply, NewState} -> gen_server:loop(Mod, NewState);
+           {stop, Reason, NewState} -> exit(Reason)
+         end;
+       Other ->
+         case Mod:handle_info(Other, State) of
+           {noreply, NewState} -> gen_server:loop(Mod, NewState);
+           {stop, Reason, NewState} -> exit(Reason)
+         end
+     end.")
+
+(define
+  er-load-gen-server!
+  (fn () (erlang-load-module er-gen-server-source)))