ocaml: phase 4 module type S = sig … end parser (+3 tests, 389 total)

module type S = sig DECLS end is parsed-and-discarded — sig..end
balanced skipping in parse-decl-module-type. AST (:module-type-def
NAME). Runtime no-op (signatures are type-level only).

Allows real OCaml programs with module type decls to parse and run
without stripping the sig blocks.

lib/ocaml/eval.sx

@@ -704,6 +704,7 @@
                           (set! result (merge result (dict mname mod-val))))))))
                 ((= tag "type-def") nil)
                 ((= tag "exception-def") nil)
+                ((= tag "module-type-def") nil)
                 ((= tag "open")
                   (let ((mod-val (ocaml-resolve-module-path (nth decl 1) env)))
                     (cond
@@ -884,6 +885,9 @@
                   ;; exception E [of T] — purely declarative; raise+match
                   ;; already work on tagged ctor values.
                   nil)
+                ((= tag "module-type-def")
+                  ;; module type S = sig … end — no-op at runtime.
+                  nil)
                 ((or (= tag "open") (= tag "include"))
                   ;; open M / include M — bring M's bindings into scope.
                   (let ((mod-val (ocaml-resolve-module-path (nth decl 1) env)))

lib/ocaml/parser.sx

@@ -1282,10 +1282,53 @@
                           (else (begin (advance-tok!) (skip))))))
                     (skip)))))))
 
+        ;; module type S = sig ... end
+        ;; Parsed-and-discarded (signatures are type-level only). Returns
+        ;; a (:module-type-def NAME) marker for the eval loop to ignore.
+        (define
+          parse-decl-module-type
+          (fn ()
+            (advance-tok!) ;; "type"
+            (let ((name (ocaml-tok-value (consume! "ctor" nil))))
+              (begin
+                (consume! "op" "=")
+                (cond
+                  ((at-kw? "sig")
+                    (begin
+                      (advance-tok!)
+                      (let ((depth 1))
+                        (begin
+                          (define skip
+                            (fn ()
+                              (cond
+                                ((>= idx tok-len) nil)
+                                ((= (ocaml-tok-type (peek-tok)) "eof") nil)
+                                ((or (at-kw? "sig") (at-kw? "struct") (at-kw? "begin"))
+                                  (begin (set! depth (+ depth 1)) (advance-tok!) (skip)))
+                                ((at-kw? "end")
+                                  (cond
+                                    ((= depth 1) nil)
+                                    (else
+                                      (begin (set! depth (- depth 1)) (advance-tok!) (skip)))))
+                                (else (begin (advance-tok!) (skip))))))
+                          (skip)
+                          (consume! "keyword" "end")))))
+                  (else
+                    ;; module type S = AnotherSig — skip-to-boundary.
+                    (skip-to-boundary!)))
+                (list :module-type-def name)))))
+
         (define
           parse-decl-module
           (fn ()
             (advance-tok!)
+            (cond
+              ((at-kw? "type") (parse-decl-module-type))
+              (else (parse-decl-module-rest)))))
+
+        (define
+          parse-decl-module-rest
+          (fn ()
             (let ((name (ocaml-tok-value (consume! "ctor" nil)))
                   (params (list)))
               (begin

lib/ocaml/test.sh

@@ -956,6 +956,14 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 2603)
 (eval "(ocaml-run \"match { x = 5; y = 2 } with | { x = 1; y = y } -> y | _ -> 0\")")
 
+;; ── module type S = sig … end ─────────────────────────────────
+(epoch 2700)
+(eval "(ocaml-parse-program \"module type S = sig val x : int val f : int -> int end\")")
+(epoch 2701)
+(eval "(ocaml-run-program \"module type S = sig val x : int end ;; module M = struct let x = 42 end ;; M.x\")")
+(epoch 2702)
+(eval "(ocaml-parse-program \"module type EMPTY = sig end\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 180 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1512,6 +1520,11 @@ check 2601 "match record name+age"        '30'
 check 2602 "match record literal x=1"     '2'
 check 2603 "match record literal fail"    '0'
 
+# ── module type S = sig … end ──────────────────────────────────
+check 2700 "module type S parses"         '("module-type-def" "S")'
+check 2701 "module type then module"      '42'
+check 2702 "module type EMPTY"            '("module-type-def" "EMPTY")'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"

plans/ocaml-on-sx.md

@@ -191,9 +191,10 @@ SX CEK evaluator (both JS and OCaml hosts)
 
 - [x] `module M = struct let x = 1 let f y = x + y end` → SX dict
       `{"x" 1 "f" <fn>}`.
-- [~] `module type S = sig val x : int val f : int -> int end` — signature
-      annotations are parsed-and-skipped (`skip-optional-sig`); typed
-      checking deferred to Phase 5.
+- [x] `module type S = sig val x : int val f : int -> int end` parses
+      via `parse-decl-module-type`. Signature contents are skipped
+      (sig..end nesting tracked) — runtime no-op since types are
+      structural. AST: `(:module-type-def NAME)`.
 - [x] `module M : S = struct ... end` — coercive sealing (signature ignored).
 - [x] `functor (M : S) -> struct ... end` via shorthand `module F (M) = …`.
 - [x] `module F = Functor(Base)` — functor application; multi-param via
@@ -376,6 +377,11 @@ the "mother tongue" closure: OCaml → SX → OCaml. This means:
 
 _Newest first._
 
+- 2026-05-08 Phase 4 — `module type S = sig … end` parser (+3 tests,
+  389 total). Signatures are parsed-and-discarded — sig..end balanced
+  skipping. AST: `(:module-type-def NAME)`. Runtime no-op (signatures
+  are type-level). Allows real OCaml code with module type decls to
+  parse and run without removing the sig blocks.
 - 2026-05-08 Phase 1+3 — record patterns `{ f = pat; … }` (+4 tests,
   386 total). Parser adds `(:precord (FIELD PAT) …)` alongside
   the existing record-literal `{` handling. Eval matches against