ocaml: phase 4 's.[i]' string indexing syntax (+3 tests, 484 total)

parse-atom-postfix now dispatches three cases after consuming '.':

  .field  -> existing field/module access
  .(EXPR) -> existing local-open
  .[EXPR] -> new string-get syntax  (this commit)

Eval reduces (:string-get S I) to host (nth S I), which already returns
a one-character string for OCaml's char model.

Lets us write idiomatic OCaml string traversal:

  let s = "hi" in
  let n = ref 0 in
  for i = 0 to String.length s - 1 do
    n := !n + Char.code s.[i]
  done;
  !n  (* = 209 *)

lib/ocaml/eval.sx

@@ -557,6 +557,11 @@
                 ((dict? target) (get target fname))
                 (else (error
                   (str "ocaml-eval: not a record/module on .field: " target)))))))
+        ((= tag "string-get")
+          ;; (:string-get S I) — evaluate s.[i] as a char access.
+          (let ((s (ocaml-eval (nth ast 1) env))
+                (i (ocaml-eval (nth ast 2) env)))
+            (nth s i)))
         ((= tag "for")
           ;; (:for NAME LO HI DIR BODY) — DIR is "ascend" or "descend".
           (let ((name (nth ast 1))

lib/ocaml/parser.sx

@@ -571,6 +571,14 @@
                                   (consume! "op" ")")
                                   (set! head (list :let-open head inner))
                                   (loop)))))
+                          ((at-op? "[")
+                            (begin
+                              (advance-tok!)
+                              (let ((idx-expr (parse-expr)))
+                                (begin
+                                  (consume! "op" "]")
+                                  (set! head (list :string-get head idx-expr))
+                                  (loop)))))
                           (else
                             (let ((tok (peek-tok)))
                               (begin

lib/ocaml/test.sh

@@ -1194,6 +1194,14 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 4912)
 (eval "(ocaml-run \"Option.(map (fun x -> x * 10) (Some 4))\")")
 
+;; ── s.[i] string indexing ─────────────────────────────────────
+(epoch 4920)
+(eval "(ocaml-run \"let s = \\\"hello\\\" in s.[0]\")")
+(epoch 4921)
+(eval "(ocaml-run \"let s = \\\"abc\\\" in Char.code s.[2]\")")
+(epoch 4922)
+(eval "(ocaml-run \"let s = \\\"hi\\\" in let n = ref 0 in for i = 0 to String.length s - 1 do n := !n + Char.code s.[i] done; !n\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 360 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1893,6 +1901,11 @@ check 4910 "M.(expr) length"              '3'
 check 4911 "M.(expr) map"                 '(2 3 4)'
 check 4912 "M.(expr) Option map"          '("Some" 40)'
 
+# ── s.[i] string indexing ────────────────────────────────────────
+check 4920 "s.[0] hello"                  '"h"'
+check 4921 "Char.code s.[2] abc"          '99'
+check 4922 "for i s.[i] sum hi"           '209'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"

plans/ocaml-on-sx.md

@@ -407,6 +407,12 @@ _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 4 — `s.[i]` string indexing syntax (+3 tests, 484
+  total). parse-atom-postfix now handles `.[expr]` after `.`,
+  emitting `(:string-get S I)`; eval reduces to host `(nth s i)`.
+  Pairs with the existing `M.(expr)` and `.field` postfixes — all three
+  share one dot loop. `let s = "hi" in for i = 0 to String.length s -
+  1 do n := !n + Char.code s.[i] done; !n` returns 209 (h+i).
 - 2026-05-08 Phase 5.1 — roman.ml baseline (Roman numeral greedy
   encoding). Side-quest: top-level `let () = expr` was unsupported by
   ocaml-parse-program — now parse-decl-let recognises `()` as a unit