ocaml: phase 3 exception declarations (+4 tests, 304 total)

exception NAME [of TYPE] parses to (:exception-def NAME [ARG-SRC]).
Runtime is a no-op: raise/match already work on tagged ctor values, so
'exception E of int;; try raise (E 5) with | E n -> n' end-to-end with
zero new eval logic.

lib/ocaml/eval.sx

@@ -633,6 +633,7 @@
                           (set! env (ocaml-env-extend env mname mod-val))
                           (set! result (merge result (dict mname mod-val))))))))
                 ((= tag "type-def") nil)
+                ((= tag "exception-def") nil)
                 ((= tag "open")
                   (let ((mod-val (ocaml-resolve-module-path (nth decl 1) env)))
                     (cond
@@ -809,6 +810,10 @@
                   ;; are dispatched by tag at eval/match time. Phase 5
                   ;; HM extensions will register ctor types here.
                   nil)
+                ((= tag "exception-def")
+                  ;; exception E [of T] — purely declarative; raise+match
+                  ;; already work on tagged ctor values.
+                  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

@@ -909,6 +909,7 @@
               ((at-kw? "include") nil)
               ((at-kw? "and") nil)
               ((at-kw? "type") nil)
+              ((at-kw? "exception") nil)
               (else (begin (advance-tok!) (skip-to-boundary!))))))
         (define
           parse-decl-let
@@ -978,6 +979,35 @@
         ;; module M = struct DECLS end
         ;; Parsed by sub-tokenising the body source between `struct` and
         ;; the matching `end`. Nested modules / sigs increment depth.
+        ;; exception NAME [of TYPE [* TYPE]*]
+        (define
+          parse-decl-exception
+          (fn ()
+            (advance-tok!) ;; consume 'exception'
+            (let ((name (ocaml-tok-value (consume! "ctor" nil)))
+                  (arg-srcs (list)))
+              (begin
+                (when (at-kw? "of")
+                  (begin
+                    (advance-tok!)
+                    (let ((arg-start (cur-pos)))
+                      (begin
+                        (define skip-type
+                          (fn ()
+                            (cond
+                              ((>= idx tok-len) nil)
+                              ((= (ocaml-tok-type (peek-tok)) "eof") nil)
+                              ((at-op? ";;") nil)
+                              ((at-kw? "let") nil)
+                              ((at-kw? "type") nil)
+                              ((at-kw? "and") nil)
+                              ((at-kw? "module") nil)
+                              ((at-kw? "exception") nil)
+                              (else (begin (advance-tok!) (skip-type))))))
+                        (skip-type)
+                        (append! arg-srcs (slice src arg-start (cur-pos)))))))
+                (cons :exception-def (cons name arg-srcs))))))
+
         ;; type [PARAMS] NAME = | Ctor [of T1 [* T2]*] | …
         ;;
         ;; PARAMS is `'a` or `('a, 'b)` (single or paren-tuple of tyvars).
@@ -1217,6 +1247,8 @@
                     (begin (append! decls (parse-decl-open true)) (loop)))
                   ((at-kw? "type")
                     (begin (append! decls (parse-decl-type)) (loop)))
+                  ((at-kw? "exception")
+                    (begin (append! decls (parse-decl-exception)) (loop)))
                   (else (begin (append! decls (parse-decl-expr)) (loop))))))))
         (loop)
         (cons :program decls)))))

lib/ocaml/test.sh

@@ -754,6 +754,16 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 1304)
 (eval "(ocaml-run-program \"type shape = Circle of int | Square of int ;; match Circle 5 with | Circle r -> r | Square s -> s\")")
 
+;; ── exception declarations ────────────────────────────────────
+(epoch 1320)
+(eval "(ocaml-parse-program \"exception MyExn\")")
+(epoch 1321)
+(eval "(ocaml-parse-program \"exception MyExn of int\")")
+(epoch 1322)
+(eval "(ocaml-run-program \"exception E of int ;; try raise (E 5) with | E n -> n\")")
+(epoch 1323)
+(eval "(ocaml-run-program \"exception E of string ;; try raise (E \\\"oops\\\") with | E s -> s\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 180 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1194,6 +1204,12 @@ check 1302 "type-decl + match Red"        '1'
 check 1303 "type-decl + match Blue"       '3'
 check 1304 "type-decl + Circle r"         '5'
 
+# ── exception declarations ─────────────────────────────────────
+check 1320 "exception nullary"            '("exception-def" "MyExn")'
+check 1321 "exception arg"                '("exception-def" "MyExn"'
+check 1322 "raise+catch with arg"         '5'
+check 1323 "raise+catch string arg"       '"oops"'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"

plans/ocaml-on-sx.md

@@ -178,8 +178,10 @@ SX CEK evaluator (both JS and OCaml hosts)
 - [x] Exhaustiveness: runtime error on incomplete match (no compile-time check yet).
 - [ ] Built-in types: `option` (`None`/`Some`), `result` (`Ok`/`Error`),
       `list` (nil/cons), `bool`, `unit`, `exn`.
-- [ ] `exception` declarations; built-in: `Not_found`, `Invalid_argument`,
-      `Failure`, `Match_failure`.
+- [x] `exception` declarations: `exception NAME [of TYPE]`. Parser emits
+      `(:exception-def NAME [ARG-TYPE-SRC])`. Runtime no-op since
+      raise/match work on tagged ctor values. Built-ins:
+      `Failure`/`Invalid_argument` via `failwith`/`invalid_arg`.
 - [ ] Polymorphic variants (surface syntax `\`Tag value`; runtime same tagged list).
 - [ ] Tests in `lib/ocaml/tests/adt.sx` — 40+ tests: ADTs, match, option/result.
 
@@ -358,6 +360,11 @@ the "mother tongue" closure: OCaml → SX → OCaml. This means:
 
 _Newest first._
 
+- 2026-05-08 Phase 3 — `exception` declarations (+4 tests, 304 total).
+  `exception NAME [of TYPE]` parses to `(:exception-def NAME [ARG-SRC])`.
+  Runtime is a no-op: exception values are just tagged ctor values, so
+  the existing `raise`/`try`/`with` machinery works without any extra
+  wiring.
 - 2026-05-08 Phase 3 — `type` declarations (+5 tests, 300 total). Parser
   handles `type [PARAMS] NAME = | Ctor [of T1 [* T2]*] | ...`, with
   optional `'a` or `('a, 'b)` type parameters. Argument types are