ocaml: phase 6 Stack + Queue modules (+5 tests, 430 total)

Stack: ref-holding-list LIFO. push/pop/top/length/is_empty/clear.
Queue: two-list (front, back) amortised O(1) queue. push/pop/length/
is_empty/clear. Both in OCaml syntax in runtime.sx.

lib/ocaml/runtime.sx

@@ -366,6 +366,48 @@
      let printf fmt = print_string fmt
    end ;;
 
+   module Stack = struct
+     let create () = ref []
+     let push x s = s := x :: !s
+     let pop s =
+       match !s with
+       | [] -> failwith \"Stack.pop: empty\"
+       | h :: t -> s := t ; h
+     let top s =
+       match !s with
+       | [] -> failwith \"Stack.top: empty\"
+       | h :: _ -> h
+     let is_empty s = !s = []
+     let length s = List.length !s
+     let clear s = s := []
+   end ;;
+
+   module Queue = struct
+     (* Simple two-list amortized queue: (front, back). pop drains
+        front; refill from rev back when front is empty. *)
+     let create () = ref ([], [])
+     let push x q =
+       let p = !q in
+       q := (List.append [] (match p with (f, b) -> f), x :: (match p with (_, b) -> b))
+     let length q =
+       let p = !q in
+       let f = match p with (f, _) -> f in
+       let b = match p with (_, b) -> b in
+       List.length f + List.length b
+     let is_empty q = length q = 0
+     let pop q =
+       let p = !q in
+       let f = match p with (f, _) -> f in
+       let b = match p with (_, b) -> b in
+       match f with
+       | h :: t -> q := (t, b) ; h
+       | [] ->
+         (match List.rev b with
+          | [] -> failwith \"Queue.pop: empty\"
+          | h :: t -> q := (t, []) ; h)
+     let clear q = q := ([], [])
+   end ;;
+
    module Buffer = struct
      let create _ = ref []
      let add_string b s = b := s :: !b

lib/ocaml/test.sh

@@ -1052,6 +1052,18 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 3502)
 (eval "(ocaml-run-program \"let b = Buffer.create 16 ;; Buffer.add_string b \\\"x\\\" ;; Buffer.clear b ;; Buffer.contents b\")")
 
+;; ── Stack + Queue modules ─────────────────────────────────────
+(epoch 3600)
+(eval "(ocaml-run-program \"let s = Stack.create () ;; Stack.push 1 s ;; Stack.push 2 s ;; Stack.push 3 s ;; Stack.pop s\")")
+(epoch 3601)
+(eval "(ocaml-run-program \"let s = Stack.create () ;; Stack.push 1 s ;; Stack.push 2 s ;; Stack.length s\")")
+(epoch 3602)
+(eval "(ocaml-run-program \"let s = Stack.create () ;; Stack.push 1 s ;; Stack.top s\")")
+(epoch 3603)
+(eval "(ocaml-run-program \"let q = Queue.create () ;; Queue.push 1 q ;; Queue.push 2 q ;; Queue.push 3 q ;; Queue.pop q\")")
+(epoch 3604)
+(eval "(ocaml-run-program \"let q = Queue.create () ;; Queue.push 1 q ;; Queue.push 2 q ;; Queue.length q\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 180 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1665,6 +1677,13 @@ check 3500 "Buffer concat 'Hello, World'" '"Hello, World"'
 check 3501 "Buffer.length 3"              '3'
 check 3502 "Buffer.clear empties"         '""'
 
+# ── Stack + Queue ──────────────────────────────────────────────
+check 3600 "Stack.pop LIFO"               '3'
+check 3601 "Stack.length"                 '2'
+check 3602 "Stack.top"                    '1'
+check 3603 "Queue.pop FIFO"               '1'
+check 3604 "Queue.length"                 '2'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"

plans/ocaml-on-sx.md

@@ -267,6 +267,11 @@ SX CEK evaluator (both JS and OCaml hosts)
 - [~] `Buffer`: `create`, `add_string`, `add_char`, `contents`, `length`,
       `clear`, `reset`. Backed by a ref holding a list of strings; reverse +
       `String.concat` on `contents`. Mostly-OCaml impl.
+- [~] `Stack`: `create`, `push`, `pop`, `top`, `is_empty`, `length`,
+      `clear`. Backed by a ref-holding-list (LIFO).
+- [~] `Queue`: `create`, `push`, `pop`, `is_empty`, `length`, `clear`.
+      Backed by a `(front, back)` tuple-of-lists pair (amortised O(1)
+      enqueue/dequeue via list reversal).
 - [~] `Sys`: `os_type` (`"SX"`), `word_size`, `max_array_length`,
       `max_string_length`, `executable_name`, `big_endian`, `unix`,
       `win32`, `cygwin`. Constants only; `argv`/`getenv_opt`/`command`
@@ -394,6 +399,11 @@ _Newest first._
   recognise `!` as the prefix-deref of an application argument, so
   `String.concat "" (List.rev !b)` parses as `(... (deref b))`. Buffer
   uses a ref holding a string list; contents reverses and concats.
+- 2026-05-08 Phase 6 — `Stack` and `Queue` modules in OCaml (+5 tests,
+  430 total). Stack: ref-holding-list LIFO with push/pop/top/length/
+  is_empty/clear. Queue: amortised O(1) two-list `(front, back)` queue
+  with push/pop/length/is_empty/clear. Both written entirely in OCaml
+  via lib/ocaml/runtime.sx.
 - 2026-05-08 Phase 5.1+1+2 — calc.ml baseline (11/11 pass) — a
   recursive-descent calculator parsing `(1 + 2) * 3 + 4` to 13. Two
   parser bugs fixed along the way: parse-let now handles inline