ocaml: phase 2 mutable record fields r.f <- v (+4 tests, 451 total)

<- added to op-table at level 1 (same as :=). Eval short-circuits on
<- to mutate the lhs's field via host SX dict-set!. The lhs must be a
:field expression; otherwise raises.

Tested:
  let r = { x = 1; y = 2 } in r.x <- 5; r.x       (5)
  let r = { x = 0 } in for i = 1 to 5 do r.x <- r.x + i done; r.x  (15)
  let r = { name = ...; age = 30 } in r.name <- "Alice"; r.name

The 'mutable' keyword in record type decls is parsed-and-discarded;
runtime semantics: every field is mutable. Phase 2 closes this gap
without changing the dict-based record representation.

lib/ocaml/eval.sx

@@ -442,6 +442,18 @@
                 (let ((cell (ocaml-eval (nth ast 2) env))
                       (new-val (ocaml-eval (nth ast 3) env)))
                   (begin (set-nth! cell 0 new-val) nil)))
+              ;; <- mutates a record field. The lhs must be a (:field ...).
+              ((= op "<-")
+                (let ((lhs-ast (nth ast 2)) (new-val (ocaml-eval (nth ast 3) env)))
+                  (cond
+                    ((= (ocaml-tag-of lhs-ast) "field")
+                      (let ((target (ocaml-eval (nth lhs-ast 1) env))
+                            (fname (nth lhs-ast 2)))
+                        (begin (dict-set! target fname new-val) nil)))
+                    (else
+                      (error
+                        (str "ocaml-eval: <- expects a field-access lhs, got "
+                             (ocaml-tag-of lhs-ast)))))))
               (else
                 (ocaml-eval-op op
                   (ocaml-eval (nth ast 2) env)

lib/ocaml/parser.sx

@@ -49,6 +49,7 @@
   ocaml-op-table
   (list
     (list ":=" 1 :right)
+    (list "<-" 1 :right)
     (list "||" 2 :right)
     (list "or" 2 :right)
     (list "&&" 3 :right)

lib/ocaml/test.sh

@@ -1106,6 +1106,16 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 4002)
 (eval "(ocaml-run-program \"type point = { x : int; y : int };; let p = { x = 3; y = 4 };; p.x + p.y\")")
 
+;; ── Mutable record fields (r.f <- v) ──────────────────────────
+(epoch 4100)
+(eval "(ocaml-run \"let r = { x = 1; y = 2 } in r.x <- 5; r.x\")")
+(epoch 4101)
+(eval "(ocaml-run \"let r = { x = 0 } in for i = 1 to 5 do r.x <- r.x + i done; r.x\")")
+(epoch 4102)
+(eval "(ocaml-run \"let r = { name = \\\"Bob\\\"; age = 30 } in r.name <- \\\"Alice\\\"; r.name\")")
+(epoch 4103)
+(eval "(ocaml-run \"let r = { x = 1; y = 2 } in r.x <- r.y * 10; r.x\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 180 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1751,6 +1761,12 @@ check 4000 "record type decl"             '("type-def-record" "point" () (("x")
 check 4001 "mutable field decl"           '("mutable" "x")'
 check 4002 "record decl + use"            '7'
 
+# ── Mutable record fields ──────────────────────────────────────
+check 4100 "r.x <- 5; r.x"                '5'
+check 4101 "for-loop accum r.x"           '15'
+check 4102 "r.name <- str"                '"Alice"'
+check 4103 "r.x <- r.y * 10"              '20'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"

plans/ocaml-on-sx.md

@@ -157,6 +157,10 @@ SX CEK evaluator (both JS and OCaml hosts)
 - [x] Arithmetic, comparison, boolean ops, string `^`, `mod`.
 - [x] Unit `()` value; `ignore`.
 - [x] References: `ref`, `!`, `:=`.
+- [x] Mutable record fields via `r.f <- v` — uses host SX `dict-set!`
+      to mutate the underlying record dict in place. All record fields
+      are de-facto mutable (the `mutable` keyword in type-decls is
+      currently parsed-and-discarded).
 - [ ] Mutable record fields.
 - [x] `for i = lo to hi do ... done` loop; `while cond do ... done` (incl.
       `downto` direction).
@@ -403,6 +407,13 @@ _Newest first._
   binary search tree (`type 'a tree = Leaf | Node of 'a * 'a tree *
   'a tree`) with insert + in-order traversal. Tests parametric ADT,
   recursive match, List.append, List.fold_left.
+- 2026-05-08 Phase 2 — mutable record fields `r.f <- v` (+4 tests, 451
+  total). `<-` added to op-table at level 1 (same as `:=`). Eval
+  short-circuits on `<-` to mutate the lhs's field via host SX
+  `dict-set!`. Tested with for-loop accumulator (`for i = 1 to 5 do
+  r.x <- r.x + i done`) and string-field reassignment. The `mutable`
+  keyword in record-type decls is parsed-and-discarded; runtime
+  semantics: every field is mutable.
 - 2026-05-08 Phase 1+3 — record type declarations `type r = { x : int;
   mutable y : string }` (+3 tests, 447 total). Parser dispatches on
   `{` after `=` to parse field list (`mutable` keyword tracked).