erlang: echo.erl minimal server (+7 tests)

lib/erlang/tests/programs/echo.sx

@@ -0,0 +1,140 @@
+;; Echo server — minimal classic Erlang server. Receives {From, Msg}
+;; and sends Msg back to From, then loops. `stop` ends the server.
+
+(define er-echo-test-count 0)
+(define er-echo-test-pass 0)
+(define er-echo-test-fails (list))
+
+(define
+  er-echo-test
+  (fn
+    (name actual expected)
+    (set! er-echo-test-count (+ er-echo-test-count 1))
+    (if
+      (= actual expected)
+      (set! er-echo-test-pass (+ er-echo-test-pass 1))
+      (append! er-echo-test-fails {:actual actual :expected expected :name name}))))
+
+(define echo-ev erlang-eval-ast)
+
+(define
+  er-echo-server-src
+  "EchoSrv = fun () ->
+     Loop = fun () ->
+       receive
+         {From, Msg} -> From ! Msg, Loop();
+         stop -> ok
+       end
+     end,
+     Loop()
+   end")
+
+;; Single round-trip with an atom.
+(er-echo-test
+  "atom round-trip"
+  (get
+    (echo-ev
+      (str
+        er-echo-server-src
+        ", Me = self(),
+         Echo = spawn(EchoSrv),
+         Echo ! {Me, hello},
+         receive R -> Echo ! stop, R end"))
+    :name)
+  "hello")
+
+;; Number round-trip.
+(er-echo-test
+  "number round-trip"
+  (echo-ev
+    (str
+      er-echo-server-src
+      ", Me = self(),
+       Echo = spawn(EchoSrv),
+       Echo ! {Me, 42},
+       receive R -> Echo ! stop, R end"))
+  42)
+
+;; Tuple round-trip — pattern-match the reply to extract V.
+(er-echo-test
+  "tuple round-trip"
+  (echo-ev
+    (str
+      er-echo-server-src
+      ", Me = self(),
+       Echo = spawn(EchoSrv),
+       Echo ! {Me, {ok, 7}},
+       receive {ok, V} -> Echo ! stop, V end"))
+  7)
+
+;; List round-trip.
+(er-echo-test
+  "list round-trip"
+  (echo-ev
+    (str
+      er-echo-server-src
+      ", Me = self(),
+       Echo = spawn(EchoSrv),
+       Echo ! {Me, [1, 2, 3]},
+       receive [H | _] -> Echo ! stop, H end"))
+  1)
+
+;; Multiple sequential round-trips.
+(er-echo-test
+  "three round-trips"
+  (echo-ev
+    (str
+      er-echo-server-src
+      ", Me = self(),
+       Echo = spawn(EchoSrv),
+       Echo ! {Me, 10}, A = receive Ra -> Ra end,
+       Echo ! {Me, 20}, B = receive Rb -> Rb end,
+       Echo ! {Me, 30}, C = receive Rc -> Rc end,
+       Echo ! stop,
+       A + B + C"))
+  60)
+
+;; Two clients sharing one echo server. Each gets its own reply.
+(er-echo-test
+  "two clients"
+  (get
+    (echo-ev
+      (str
+        er-echo-server-src
+        ", Me = self(),
+         Echo = spawn(EchoSrv),
+         Client = fun (Tag) ->
+           spawn(fun () ->
+             Echo ! {self(), Tag},
+             receive R -> Me ! {got, R} end
+           end)
+         end,
+         Client(a),
+         Client(b),
+         receive {got, _} -> ok end,
+         receive {got, _} -> ok end,
+         Echo ! stop,
+         finished"))
+    :name)
+  "finished")
+
+;; Echo via io trace — verify each message round-trips through.
+(er-echo-test
+  "trace 4 messages"
+  (do
+    (er-io-flush!)
+    (echo-ev
+      (str
+        er-echo-server-src
+        ", Me = self(),
+         Echo = spawn(EchoSrv),
+         Send = fun (V) -> Echo ! {Me, V}, receive R -> io:format(\"~p \", [R]) end end,
+         Send(1), Send(2), Send(3), Send(4),
+         Echo ! stop,
+         done"))
+    (er-io-buffer-content))
+  "1 2 3 4 ")
+
+(define
+  er-echo-test-summary
+  (str "echo " er-echo-test-pass "/" er-echo-test-count))

plans/erlang-on-sx.md

@@ -73,7 +73,7 @@ Core mapping:
   - [x] `ring.erl` — N processes in a ring, pass a token around M times — **4 ring tests**; suspension machinery rewritten from `shift`/`reset` to `call/cc` + `raise`/`guard`
   - [x] `ping_pong.erl` — two processes exchanging messages — **4 ping-pong tests**
   - [x] `bank.erl` — account server (deposit/withdraw/balance) — **8 bank tests**
-  - [ ] `echo.erl` — minimal server
+  - [x] `echo.erl` — minimal server — **7 echo tests**
   - [ ] `fib_server.erl` — compute fib on request
 - [ ] `lib/erlang/conformance.sh` + runner, `scoreboard.json` + `scoreboard.md`
 - [ ] Target: 5/5 classic programs + 1M-process ring benchmark runs
@@ -99,6 +99,7 @@ Core mapping:
 
 _Newest first._
 
+- **2026-04-25 echo.erl green** — `lib/erlang/tests/programs/echo.sx` with 7 tests. Server: `receive {From, Msg} -> From ! Msg, Loop(); stop -> ok end`. Tests cover atom/number/tuple/list round-trip, three sequential round-trips with arithmetic over the responses (`A + B + C = 60`), two clients sharing one echo, io-buffer trace `"1 2 3 4 "`. Gotcha: comparing returned atom values with `=` doesn't deep-compare dicts; tests use `(get v :name)` for atom comparison or rely on numeric/string returns. Total suite 350/350.
 - **2026-04-24 bank.erl green** — `lib/erlang/tests/programs/bank.sx` with 8 tests. Stateful server pattern: `Server = fun (Balance) -> receive ... Server(NewBalance) end end` recursively threads balance through each iteration. Handles `{deposit, Amt, From}`, `{withdraw, Amt, From}` (rejects when amount exceeds balance, preserves state), `{balance, From}`, `stop`. Tests cover deposit accumulation, withdrawal within balance, insufficient funds with state preservation, mixed transactions, clean shutdown, two-client interleave. Total suite 343/343.
 - **2026-04-24 ping_pong.erl green** — `lib/erlang/tests/programs/ping_pong.sx` with 4 tests: classic Pong server + Ping client with separate `ping_done`/`pong_done` notifications, 5-round trace via io-buffer (`"ppppp"`), main-as-pinger-4-rounds (no intermediate Ping proc), tagged-id round-trip (`"4 3 2 1 "`). All driven by `Ping = fun (Target, K) -> ... Ping(Target, K-1) ... end` self-recursion — captured-env reference works because `Ping` binds in main's mutable env before any spawned body looks it up. Total suite 335/335.
 - **2026-04-24 ring.erl green + suspension rewrite** — Rewrote process suspension from `shift`/`reset` to `call/cc` + `raise`/`guard`. **Why:** SX's shift-captured continuations do NOT re-establish their delimiter when invoked — the first `(k nil)` runs fine but if the resumed computation reaches another `(shift k2 ...)` it raises "shift without enclosing reset". Ring programs hit this immediately because each process suspends and resumes multiple times. `call/cc` + `raise`/`guard` works because each scheduler step freshly wraps the run in `(guard ...)`, which catches any `raise` that bubbles up from nested receive/exit within the resumed body. Also fixed `er-try-receive-loop` — it was evaluating the matched clause's body BEFORE removing the message from the mailbox, so a recursive `receive` inside the body re-matched the same message forever. Added `lib/erlang/tests/programs/ring.sx` with 4 tests (N=3 M=6, N=2 M=4, N=1 M=5 self-loop, N=3 M=9 hop-count via io-buffer). All process-communication eval tests still pass. Total suite 331/331.