From 0c2ff7e07eebee2c0840a451bb065348392a92ba Mon Sep 17 00:00:00 2001
From: Shulhi Sapli <913103+shulhi@users.noreply.github.com>
Date: Mon, 12 Jan 2026 21:28:11 +0800
Subject: [PATCH 1/9] New ast to represent index access

---
 compiler/ml/ast_helper.ml    |  1 +
 compiler/ml/ast_helper.mli   |  7 +++++++
 compiler/ml/ast_iterator.ml  |  4 ++++
 compiler/ml/ast_mapper.ml    |  3 +++
 compiler/ml/parsetree.ml     |  3 +++
 compiler/ml/pprintast.ml     |  5 +++++
 compiler/ml/printast.ml      | 10 +++++++++
 compiler/ml/tast_iterator.ml |  4 ++++
 compiler/ml/tast_mapper.ml   |  2 ++
 compiler/ml/translcore.ml    | 12 +++++++++++
 compiler/ml/typecore.ml      | 40 ++++++++++++++++++++++++++++++++++++
 compiler/ml/typedtree.ml     |  1 +
 compiler/ml/typedtree.mli    |  1 +
 13 files changed, 93 insertions(+)

diff --git a/compiler/ml/ast_helper.ml b/compiler/ml/ast_helper.ml
index 2fae640eb0..98e08031a4 100644
--- a/compiler/ml/ast_helper.ml
+++ b/compiler/ml/ast_helper.ml
@@ -174,6 +174,7 @@ module Exp = struct
   let record ?loc ?attrs a b = mk ?loc ?attrs (Pexp_record (a, b))
   let field ?loc ?attrs a b = mk ?loc ?attrs (Pexp_field (a, b))
   let setfield ?loc ?attrs a b c = mk ?loc ?attrs (Pexp_setfield (a, b, c))
+  let index ?loc ?attrs a b c = mk ?loc ?attrs (Pexp_index (a, b, c))
   let array ?loc ?attrs a = mk ?loc ?attrs (Pexp_array a)
   let ifthenelse ?loc ?attrs a b c = mk ?loc ?attrs (Pexp_ifthenelse (a, b, c))
   let sequence ?loc ?attrs a b = mk ?loc ?attrs (Pexp_sequence (a, b))
diff --git a/compiler/ml/ast_helper.mli b/compiler/ml/ast_helper.mli
index 11227b903a..a161433f95 100644
--- a/compiler/ml/ast_helper.mli
+++ b/compiler/ml/ast_helper.mli
@@ -163,6 +163,13 @@ module Exp : sig
   val field : ?loc:loc -> ?attrs:attrs -> expression -> lid -> expression
   val setfield :
     ?loc:loc -> ?attrs:attrs -> expression -> lid -> expression -> expression
+  val index :
+    ?loc:loc ->
+    ?attrs:attrs ->
+    expression ->
+    expression ->
+    expression option ->
+    expression
   val array : ?loc:loc -> ?attrs:attrs -> expression list -> expression
   val ifthenelse :
     ?loc:loc ->
diff --git a/compiler/ml/ast_iterator.ml b/compiler/ml/ast_iterator.ml
index a430bb0b7b..77320bd5ce 100644
--- a/compiler/ml/ast_iterator.ml
+++ b/compiler/ml/ast_iterator.ml
@@ -321,6 +321,10 @@ module E = struct
       sub.expr sub e1;
       iter_loc sub lid;
       sub.expr sub e2
+    | Pexp_index (e1, e2, e3) ->
+      sub.expr sub e1;
+      sub.expr sub e2;
+      iter_opt (sub.expr sub) e3
     | Pexp_array el -> List.iter (sub.expr sub) el
     | Pexp_ifthenelse (e1, e2, e3) ->
       sub.expr sub e1;
diff --git a/compiler/ml/ast_mapper.ml b/compiler/ml/ast_mapper.ml
index 673465477b..6f59d04aec 100644
--- a/compiler/ml/ast_mapper.ml
+++ b/compiler/ml/ast_mapper.ml
@@ -313,6 +313,9 @@ module E = struct
       field ~loc ~attrs (sub.expr sub e) (map_loc sub lid)
     | Pexp_setfield (e1, lid, e2) ->
       setfield ~loc ~attrs (sub.expr sub e1) (map_loc sub lid) (sub.expr sub e2)
+    | Pexp_index (e1, e2, e3) ->
+      index ~loc ~attrs (sub.expr sub e1) (sub.expr sub e2)
+        (map_opt (sub.expr sub) e3)
     | Pexp_array el -> array ~loc ~attrs (List.map (sub.expr sub) el)
     | Pexp_ifthenelse (e1, e2, e3) ->
       ifthenelse ~loc ~attrs (sub.expr sub e1) (sub.expr sub e2)
diff --git a/compiler/ml/parsetree.ml b/compiler/ml/parsetree.ml
index 78c9899f74..3d0870f503 100644
--- a/compiler/ml/parsetree.ml
+++ b/compiler/ml/parsetree.ml
@@ -279,6 +279,9 @@ and expression_desc =
     *)
   | Pexp_field of expression * Longident.t loc (* E.l *)
   | Pexp_setfield of expression * Longident.t loc * expression (* E1.l <- E2 *)
+  | Pexp_index of expression * expression * expression option
+    (* E1[E2]        (None)     - read access
+       E1[E2] = E3   (Some E3)  - write access *)
   | Pexp_array of expression list (* [| E1; ...; En |] *)
   | Pexp_ifthenelse of expression * expression * expression option
     (* if E1 then E2 else E3 *)
diff --git a/compiler/ml/pprintast.ml b/compiler/ml/pprintast.ml
index 585ac64b81..9ea76c16d9 100644
--- a/compiler/ml/pprintast.ml
+++ b/compiler/ml/pprintast.ml
@@ -691,6 +691,11 @@ and expression ctxt f x =
     | Pexp_setfield (e1, li, e2) ->
       pp f "@[<2>%a.%a@ <-@ %a@]" (simple_expr ctxt) e1 longident_loc li
         (simple_expr ctxt) e2
+    | Pexp_index (e1, e2, None) ->
+      pp f "%a.(%a)" (expression ctxt) e1 (expression ctxt) e2
+    | Pexp_index (e1, e2, Some e3) ->
+      pp f "%a.(%a)@ <-@ %a" (expression ctxt) e1 (expression ctxt) e2
+        (expression ctxt) e3
     | Pexp_ifthenelse (e1, e2, eo) ->
       (* @;@[<2>else@ %a@]@] *)
       let fmt : (_, _, _) format =
diff --git a/compiler/ml/printast.ml b/compiler/ml/printast.ml
index 44d699eb38..ed0e68c2ef 100644
--- a/compiler/ml/printast.ml
+++ b/compiler/ml/printast.ml
@@ -294,6 +294,16 @@ and expression i ppf x =
     expression i ppf e1;
     longident_loc i ppf li;
     expression i ppf e2
+  | Pexp_index (e1, e2, e3) -> (
+    line i ppf "Pexp_index\n";
+    expression i ppf e1;
+    line i ppf "index:\n";
+    expression i ppf e2;
+    match e3 with
+    | None -> line i ppf "read access\n"
+    | Some e ->
+      line i ppf "write access:\n";
+      expression i ppf e)
   | Pexp_array l ->
     line i ppf "Pexp_array\n";
     list i expression ppf l
diff --git a/compiler/ml/tast_iterator.ml b/compiler/ml/tast_iterator.ml
index 5c12d3da4d..2641e7d77a 100644
--- a/compiler/ml/tast_iterator.ml
+++ b/compiler/ml/tast_iterator.ml
@@ -176,6 +176,10 @@ let expr sub {exp_extra; exp_desc; exp_env; _} =
   | Texp_setfield (exp1, _, _, exp2) ->
     sub.expr sub exp1;
     sub.expr sub exp2
+  | Texp_index (exp1, exp2, expo) ->
+    sub.expr sub exp1;
+    sub.expr sub exp2;
+    Option.iter (sub.expr sub) expo
   | Texp_array list -> List.iter (sub.expr sub) list
   | Texp_ifthenelse (exp1, exp2, expo) ->
     sub.expr sub exp1;
diff --git a/compiler/ml/tast_mapper.ml b/compiler/ml/tast_mapper.ml
index 54eba02869..935c39d9c4 100644
--- a/compiler/ml/tast_mapper.ml
+++ b/compiler/ml/tast_mapper.ml
@@ -232,6 +232,8 @@ let expr sub x =
     | Texp_field (exp, lid, ld) -> Texp_field (sub.expr sub exp, lid, ld)
     | Texp_setfield (exp1, lid, ld, exp2) ->
       Texp_setfield (sub.expr sub exp1, lid, ld, sub.expr sub exp2)
+    | Texp_index (exp1, exp2, expo) ->
+      Texp_index (sub.expr sub exp1, sub.expr sub exp2, opt (sub.expr sub) expo)
     | Texp_array list -> Texp_array (List.map (sub.expr sub) list)
     | Texp_ifthenelse (exp1, exp2, expo) ->
       Texp_ifthenelse
diff --git a/compiler/ml/translcore.ml b/compiler/ml/translcore.ml
index 078cbf133a..be9556c4fe 100644
--- a/compiler/ml/translcore.ml
+++ b/compiler/ml/translcore.ml
@@ -891,6 +891,18 @@ and transl_exp0 (e : Typedtree.expression) : Lambda.lambda =
         Psetfield (lbl.lbl_pos + 1, Lambda.fld_record_extension_set lbl)
     in
     Lprim (access, [transl_exp arg; transl_exp newval], e.exp_loc)
+  | Texp_index (container, index, value_opt) -> (
+    let container_lambda = transl_exp container in
+    let index_lambda = transl_exp index in
+    match value_opt with
+    | None ->
+      (* Read: translate to Parrayrefu primitive (unsafe array get) *)
+      Lprim (Parrayrefu, [container_lambda; index_lambda], e.exp_loc)
+    | Some value ->
+      (* Write: translate to Parraysetu primitive (unsafe array set) *)
+      let value_lambda = transl_exp value in
+      Lprim
+        (Parraysetu, [container_lambda; index_lambda; value_lambda], e.exp_loc))
   | Texp_array expr_list ->
     let ll = transl_list expr_list in
     Lprim (Pmakearray Mutable, ll, e.exp_loc)
diff --git a/compiler/ml/typecore.ml b/compiler/ml/typecore.ml
index ee304dff7d..457c5e84be 100644
--- a/compiler/ml/typecore.ml
+++ b/compiler/ml/typecore.ml
@@ -182,6 +182,10 @@ let iter_expression f e =
       ->
       expr e1;
       expr e2
+    | Pexp_index (e1, e2, eo) ->
+      expr e1;
+      expr e2;
+      may expr eo
     | Pexp_ifthenelse (e1, e2, eo) ->
       expr e1;
       expr e2;
@@ -2834,6 +2838,42 @@ and type_expect_ ?deprecated_context ~context ?in_function ?(recarg = Rejected)
         exp_attributes = sexp.pexp_attributes;
         exp_env = env;
       }
+  | Pexp_index (scontainer, sindex, svalue_opt) -> (
+    (* Type check as array access (same as current Array.get/set behavior) *)
+    let container = type_exp ~context:None env scontainer in
+    let index =
+      type_expect ~context:None env sindex (instance_def Predef.type_int)
+    in
+    match svalue_opt with
+    | None ->
+      (* Read access: arr[i] -> array<'a> -> int -> 'a *)
+      let element_type = newgenvar () in
+      let array_type = instance_def (Predef.type_array element_type) in
+      unify_exp ~context:None env container array_type;
+      rue
+        {
+          exp_desc = Texp_index (container, index, None);
+          exp_loc = loc;
+          exp_extra = [];
+          exp_type = instance env element_type;
+          exp_attributes = sexp.pexp_attributes;
+          exp_env = env;
+        }
+    | Some svalue ->
+      (* Write access: arr[i] = v -> array<'a> -> int -> 'a -> unit *)
+      let element_type = newgenvar () in
+      let array_type = instance_def (Predef.type_array element_type) in
+      unify_exp ~context:None env container array_type;
+      let value = type_expect ~context:None env svalue element_type in
+      rue
+        {
+          exp_desc = Texp_index (container, index, Some value);
+          exp_loc = loc;
+          exp_extra = [];
+          exp_type = instance_def Predef.type_unit;
+          exp_attributes = sexp.pexp_attributes;
+          exp_env = env;
+        })
   | Pexp_array sargl ->
     let ty = newgenvar () in
     let to_unify = Predef.type_array ty in
diff --git a/compiler/ml/typedtree.ml b/compiler/ml/typedtree.ml
index e7274f1245..6dccdb2599 100644
--- a/compiler/ml/typedtree.ml
+++ b/compiler/ml/typedtree.ml
@@ -106,6 +106,7 @@ and expression_desc =
   | Texp_field of expression * Longident.t loc * label_description
   | Texp_setfield of
       expression * Longident.t loc * label_description * expression
+  | Texp_index of expression * expression * expression option
   | Texp_array of expression list
   | Texp_ifthenelse of expression * expression * expression option
   | Texp_sequence of expression * expression
diff --git a/compiler/ml/typedtree.mli b/compiler/ml/typedtree.mli
index b1e7083fc7..416577153b 100644
--- a/compiler/ml/typedtree.mli
+++ b/compiler/ml/typedtree.mli
@@ -207,6 +207,7 @@ and expression_desc =
   | Texp_field of expression * Longident.t loc * label_description
   | Texp_setfield of
       expression * Longident.t loc * label_description * expression
+  | Texp_index of expression * expression * expression option
   | Texp_array of expression list
   | Texp_ifthenelse of expression * expression * expression option
   | Texp_sequence of expression * expression

From 5b3a4b11d18d5c8ba9b16e582e412120bdf3fddb Mon Sep 17 00:00:00 2001
From: Shulhi Sapli <913103+shulhi@users.noreply.github.com>
Date: Mon, 12 Jan 2026 21:36:02 +0800
Subject: [PATCH 2/9] Handle printer

---
 compiler/syntax/src/res_ast_debugger.ml   |  7 +++++++
 compiler/syntax/src/res_comments_table.ml |  7 +++++++
 compiler/syntax/src/res_printer.ml        | 25 +++++++++++++++++++++++
 3 files changed, 39 insertions(+)

diff --git a/compiler/syntax/src/res_ast_debugger.ml b/compiler/syntax/src/res_ast_debugger.ml
index 52a57b89c5..cdfe67b6ad 100644
--- a/compiler/syntax/src/res_ast_debugger.ml
+++ b/compiler/syntax/src/res_ast_debugger.ml
@@ -641,6 +641,13 @@ module SexpAst = struct
             longident longident_loc.Asttypes.txt;
             expression expr2;
           ]
+      | Pexp_index (e1, e2, e3) ->
+        Sexp.list
+          ([Sexp.atom "Pexp_index"; expression e1; expression e2]
+          @
+          match e3 with
+          | None -> []
+          | Some e -> [expression e])
       | Pexp_array exprs ->
         Sexp.list
           [Sexp.atom "Pexp_array"; Sexp.list (map_empty ~f:expression exprs)]
diff --git a/compiler/syntax/src/res_comments_table.ml b/compiler/syntax/src/res_comments_table.ml
index 6774b2bc2b..4d414bfb79 100644
--- a/compiler/syntax/src/res_comments_table.ml
+++ b/compiler/syntax/src/res_comments_table.ml
@@ -476,6 +476,7 @@ let rec is_block_expr expr =
   | Pexp_constraint (expr, _) when is_block_expr expr -> true
   | Pexp_field (expr, _) when is_block_expr expr -> true
   | Pexp_setfield (expr, _, _) when is_block_expr expr -> true
+  | Pexp_index (expr, _, _) when is_block_expr expr -> true
   | _ -> false
 
 let is_if_then_else_expr expr =
@@ -1313,6 +1314,12 @@ and walk_expression expr t comments =
       attach t.leading expr2.pexp_loc leading;
       walk_expression expr2 t inside;
       attach t.trailing expr2.pexp_loc trailing
+  | Pexp_index (container, index, value_opt) -> (
+    walk_expression container t comments;
+    walk_expression index t comments;
+    match value_opt with
+    | None -> ()
+    | Some value -> walk_expression value t comments)
   | Pexp_ifthenelse (if_expr, then_expr, else_expr) -> (
     let leading, rest = partition_leading_trailing comments expr.pexp_loc in
     attach t.leading expr.pexp_loc leading;
diff --git a/compiler/syntax/src/res_printer.ml b/compiler/syntax/src/res_printer.ml
index 2010d23f6d..b157466d8e 100644
--- a/compiler/syntax/src/res_printer.ml
+++ b/compiler/syntax/src/res_printer.ml
@@ -3374,6 +3374,31 @@ and print_expression ~state (e : Parsetree.expression) cmt_tbl =
     | Pexp_setfield (expr1, longident_loc, expr2) ->
       print_set_field_expr ~state e.pexp_attributes expr1 longident_loc expr2
         e.pexp_loc cmt_tbl
+    | Pexp_index (container, index, value_opt) -> (
+      let container_doc =
+        let doc = print_expression_with_comments ~state container cmt_tbl in
+        match Parens.field_expr container with
+        | Parens.Parenthesized -> add_parens doc
+        | Braced braces -> print_braces doc container braces
+        | Nothing -> doc
+      in
+      let index_doc = print_expression_with_comments ~state index cmt_tbl in
+      match value_opt with
+      | None ->
+        (* Read: container[index] *)
+        Doc.concat [container_doc; Doc.lbracket; index_doc; Doc.rbracket]
+      | Some value ->
+        (* Write: container[index] = value *)
+        let value_doc = print_expression_with_comments ~state value cmt_tbl in
+        Doc.concat
+          [
+            container_doc;
+            Doc.lbracket;
+            index_doc;
+            Doc.rbracket;
+            Doc.text " = ";
+            value_doc;
+          ])
     | Pexp_ifthenelse (_ifExpr, _thenExpr, _elseExpr)
       when ParsetreeViewer.is_ternary_expr e ->
       let parts, alternate = ParsetreeViewer.collect_ternary_parts e in

From fe433c7dffe2774cae01c3161902f3924ce06b4e Mon Sep 17 00:00:00 2001
From: Shulhi Sapli <913103+shulhi@users.noreply.github.com>
Date: Mon, 12 Jan 2026 21:36:28 +0800
Subject: [PATCH 3/9] Fix build errors

---
 analysis/src/Utils.ml              |  1 +
 compiler/frontend/bs_ast_mapper.ml |  3 +++
 compiler/ml/ast_mapper_to0.ml      | 21 +++++++++++++++++++++
 compiler/ml/depend.ml              |  4 ++++
 compiler/ml/printtyped.ml          | 10 ++++++++++
 compiler/ml/rec_check.ml           |  9 +++++++--
 compiler/ml/typedtreeIter.ml       |  6 ++++++
 7 files changed, 52 insertions(+), 2 deletions(-)

diff --git a/analysis/src/Utils.ml b/analysis/src/Utils.ml
index 863598dc56..d930b7edc2 100644
--- a/analysis/src/Utils.ml
+++ b/analysis/src/Utils.ml
@@ -95,6 +95,7 @@ let identifyPexp pexp =
   | Pexp_record _ -> "Pexp_record"
   | Pexp_field _ -> "Pexp_field"
   | Pexp_setfield _ -> "Pexp_setfield"
+  | Pexp_index _ -> "Pexp_index"
   | Pexp_array _ -> "Pexp_array"
   | Pexp_ifthenelse _ -> "Pexp_ifthenelse"
   | Pexp_sequence _ -> "Pexp_sequence"
diff --git a/compiler/frontend/bs_ast_mapper.ml b/compiler/frontend/bs_ast_mapper.ml
index 292e199b5a..1ce6f542bc 100644
--- a/compiler/frontend/bs_ast_mapper.ml
+++ b/compiler/frontend/bs_ast_mapper.ml
@@ -350,6 +350,9 @@ module E = struct
       field ~loc ~attrs (sub.expr sub e) (map_loc sub lid)
     | Pexp_setfield (e1, lid, e2) ->
       setfield ~loc ~attrs (sub.expr sub e1) (map_loc sub lid) (sub.expr sub e2)
+    | Pexp_index (e1, e2, e3) ->
+      index ~loc ~attrs (sub.expr sub e1) (sub.expr sub e2)
+        (map_opt (sub.expr sub) e3)
     | Pexp_array el -> array ~loc ~attrs (List.map (sub.expr sub) el)
     | Pexp_ifthenelse (e1, e2, e3) ->
       ifthenelse ~loc ~attrs (sub.expr sub e1) (sub.expr sub e2)
diff --git a/compiler/ml/ast_mapper_to0.ml b/compiler/ml/ast_mapper_to0.ml
index d0ac43d737..23bc01cd72 100644
--- a/compiler/ml/ast_mapper_to0.ml
+++ b/compiler/ml/ast_mapper_to0.ml
@@ -439,6 +439,27 @@ module E = struct
       field ~loc ~attrs (sub.expr sub e) (map_loc sub lid)
     | Pexp_setfield (e1, lid, e2) ->
       setfield ~loc ~attrs (sub.expr sub e1) (map_loc sub lid) (sub.expr sub e2)
+    | Pexp_index (e1, e2, e3) -> (
+      (* Map back to Array.get/Array.set for parsetree0 compatibility *)
+      let container = sub.expr sub e1 in
+      let index = sub.expr sub e2 in
+      match e3 with
+      | None ->
+        (* Read: Array.get(container, index) *)
+        let array_get =
+          ident ~loc
+            (mknoloc (Longident.Ldot (Longident.Lident "Array", "get")))
+        in
+        apply ~loc ~attrs array_get [(Nolabel, container); (Nolabel, index)]
+      | Some value ->
+        (* Write: Array.set(container, index, value) *)
+        let array_set =
+          ident ~loc
+            (mknoloc (Longident.Ldot (Longident.Lident "Array", "set")))
+        in
+        let value_expr = sub.expr sub value in
+        apply ~loc ~attrs array_set
+          [(Nolabel, container); (Nolabel, index); (Nolabel, value_expr)])
     | Pexp_array el -> array ~loc ~attrs (List.map (sub.expr sub) el)
     | Pexp_ifthenelse (e1, e2, e3) ->
       ifthenelse ~loc ~attrs (sub.expr sub e1) (sub.expr sub e2)
diff --git a/compiler/ml/depend.ml b/compiler/ml/depend.ml
index e5e39eb4b5..011b556ab5 100644
--- a/compiler/ml/depend.ml
+++ b/compiler/ml/depend.ml
@@ -248,6 +248,10 @@ let rec add_expr bv exp =
     add_expr bv e1;
     add bv fld;
     add_expr bv e2
+  | Pexp_index (e1, e2, e3) ->
+    add_expr bv e1;
+    add_expr bv e2;
+    add_opt add_expr bv e3
   | Pexp_array el -> List.iter (add_expr bv) el
   | Pexp_ifthenelse (e1, e2, opte3) ->
     add_expr bv e1;
diff --git a/compiler/ml/printtyped.ml b/compiler/ml/printtyped.ml
index 6e36b4276c..a74e1d823c 100644
--- a/compiler/ml/printtyped.ml
+++ b/compiler/ml/printtyped.ml
@@ -329,6 +329,16 @@ and expression i ppf x =
     expression i ppf e1;
     longident i ppf li;
     expression i ppf e2
+  | Texp_index (e1, e2, e3) -> (
+    line i ppf "Texp_index\n";
+    expression i ppf e1;
+    line i ppf "index:\n";
+    expression i ppf e2;
+    match e3 with
+    | None -> line i ppf "read access\n"
+    | Some e ->
+      line i ppf "write access:\n";
+      expression i ppf e)
   | Texp_array l ->
     line i ppf "Texp_array\n";
     list i expression ppf l
diff --git a/compiler/ml/rec_check.ml b/compiler/ml/rec_check.ml
index 3d016a7438..6c18a20a08 100644
--- a/compiler/ml/rec_check.ml
+++ b/compiler/ml/rec_check.ml
@@ -196,8 +196,8 @@ let rec classify_expression : Typedtree.expression -> sd =
     classify_expression e
   | Texp_ident _ | Texp_for _ | Texp_constant _ | Texp_tuple _ | Texp_array _
   | Texp_construct _ | Texp_variant _ | Texp_record _ | Texp_setfield _
-  | Texp_while _ | Texp_pack _ | Texp_function _ | Texp_extension_constructor _
-    ->
+  | Texp_index _ | Texp_while _ | Texp_pack _ | Texp_function _
+  | Texp_extension_constructor _ ->
     Static
   | Texp_apply {funct = {exp_desc = Texp_ident (_, _, vd)}} when is_ref vd ->
     Static
@@ -273,6 +273,11 @@ let rec expression : Env.env -> Typedtree.expression -> Use.t =
         (join (expression env ifso) (option expression env ifnot)))
   | Texp_setfield (e1, _, _, e2) ->
     Use.(join (inspect (expression env e1)) (inspect (expression env e2)))
+  | Texp_index (e1, e2, e3) ->
+    Use.(
+      join
+        (join (inspect (expression env e1)) (inspect (expression env e2)))
+        (inspect (option expression env e3)))
   | Texp_sequence (e1, e2) ->
     Use.(join (discard (expression env e1)) (expression env e2))
   | Texp_while (e1, e2) ->
diff --git a/compiler/ml/typedtreeIter.ml b/compiler/ml/typedtreeIter.ml
index 9a31b9b5b9..ea864004eb 100644
--- a/compiler/ml/typedtreeIter.ml
+++ b/compiler/ml/typedtreeIter.ml
@@ -261,6 +261,12 @@ end = struct
     | Texp_setfield (exp1, _, _label, exp2) ->
       iter_expression exp1;
       iter_expression exp2
+    | Texp_index (exp1, exp2, expo) -> (
+      iter_expression exp1;
+      iter_expression exp2;
+      match expo with
+      | None -> ()
+      | Some exp -> iter_expression exp)
     | Texp_array list -> List.iter iter_expression list
     | Texp_ifthenelse (exp1, exp2, expo) -> (
       iter_expression exp1;

From ce51bf024e4e2d66803f01df229411f8f8bd0d67 Mon Sep 17 00:00:00 2001
From: Shulhi Sapli <913103+shulhi@users.noreply.github.com>
Date: Mon, 12 Jan 2026 23:25:13 +0800
Subject: [PATCH 4/9] Split to two separate nodes

---
 compiler/frontend/bs_ast_mapper.ml |  5 +-
 compiler/ml/ast_helper.ml          |  3 +-
 compiler/ml/ast_helper.mli         |  5 +-
 compiler/ml/ast_iterator.ml        |  7 ++-
 compiler/ml/ast_mapper.ml          |  5 +-
 compiler/ml/ast_mapper_to0.ml      | 35 +++++++-------
 compiler/ml/depend.ml              |  7 ++-
 compiler/ml/parsetree.ml           |  5 +-
 compiler/ml/pprintast.ml           |  4 +-
 compiler/ml/printast.ml            | 14 +++---
 compiler/ml/printtyped.ml          | 14 +++---
 compiler/ml/rec_check.ml           | 10 ++--
 compiler/ml/tast_iterator.ml       |  7 ++-
 compiler/ml/tast_mapper.ml         |  6 ++-
 compiler/ml/translcore.ml          | 19 ++++----
 compiler/ml/typecore.ml            | 75 ++++++++++++++++--------------
 compiler/ml/typedtree.ml           |  3 +-
 compiler/ml/typedtree.mli          |  3 +-
 compiler/ml/typedtreeIter.ml       |  9 ++--
 19 files changed, 129 insertions(+), 107 deletions(-)

diff --git a/compiler/frontend/bs_ast_mapper.ml b/compiler/frontend/bs_ast_mapper.ml
index 1ce6f542bc..afa7a7559f 100644
--- a/compiler/frontend/bs_ast_mapper.ml
+++ b/compiler/frontend/bs_ast_mapper.ml
@@ -350,9 +350,10 @@ module E = struct
       field ~loc ~attrs (sub.expr sub e) (map_loc sub lid)
     | Pexp_setfield (e1, lid, e2) ->
       setfield ~loc ~attrs (sub.expr sub e1) (map_loc sub lid) (sub.expr sub e2)
-    | Pexp_index (e1, e2, e3) ->
+    | Pexp_index (e1, e2) ->
       index ~loc ~attrs (sub.expr sub e1) (sub.expr sub e2)
-        (map_opt (sub.expr sub) e3)
+    | Pexp_setindex (e1, e2, e3) ->
+      setindex ~loc ~attrs (sub.expr sub e1) (sub.expr sub e2) (sub.expr sub e3)
     | Pexp_array el -> array ~loc ~attrs (List.map (sub.expr sub) el)
     | Pexp_ifthenelse (e1, e2, e3) ->
       ifthenelse ~loc ~attrs (sub.expr sub e1) (sub.expr sub e2)
diff --git a/compiler/ml/ast_helper.ml b/compiler/ml/ast_helper.ml
index 98e08031a4..cd55b743ca 100644
--- a/compiler/ml/ast_helper.ml
+++ b/compiler/ml/ast_helper.ml
@@ -174,7 +174,8 @@ module Exp = struct
   let record ?loc ?attrs a b = mk ?loc ?attrs (Pexp_record (a, b))
   let field ?loc ?attrs a b = mk ?loc ?attrs (Pexp_field (a, b))
   let setfield ?loc ?attrs a b c = mk ?loc ?attrs (Pexp_setfield (a, b, c))
-  let index ?loc ?attrs a b c = mk ?loc ?attrs (Pexp_index (a, b, c))
+  let index ?loc ?attrs a b = mk ?loc ?attrs (Pexp_index (a, b))
+  let setindex ?loc ?attrs a b c = mk ?loc ?attrs (Pexp_setindex (a, b, c))
   let array ?loc ?attrs a = mk ?loc ?attrs (Pexp_array a)
   let ifthenelse ?loc ?attrs a b c = mk ?loc ?attrs (Pexp_ifthenelse (a, b, c))
   let sequence ?loc ?attrs a b = mk ?loc ?attrs (Pexp_sequence (a, b))
diff --git a/compiler/ml/ast_helper.mli b/compiler/ml/ast_helper.mli
index a161433f95..ba3830aac0 100644
--- a/compiler/ml/ast_helper.mli
+++ b/compiler/ml/ast_helper.mli
@@ -163,12 +163,13 @@ module Exp : sig
   val field : ?loc:loc -> ?attrs:attrs -> expression -> lid -> expression
   val setfield :
     ?loc:loc -> ?attrs:attrs -> expression -> lid -> expression -> expression
-  val index :
+  val index : ?loc:loc -> ?attrs:attrs -> expression -> expression -> expression
+  val setindex :
     ?loc:loc ->
     ?attrs:attrs ->
     expression ->
     expression ->
-    expression option ->
+    expression ->
     expression
   val array : ?loc:loc -> ?attrs:attrs -> expression list -> expression
   val ifthenelse :
diff --git a/compiler/ml/ast_iterator.ml b/compiler/ml/ast_iterator.ml
index 77320bd5ce..ad61a99377 100644
--- a/compiler/ml/ast_iterator.ml
+++ b/compiler/ml/ast_iterator.ml
@@ -321,10 +321,13 @@ module E = struct
       sub.expr sub e1;
       iter_loc sub lid;
       sub.expr sub e2
-    | Pexp_index (e1, e2, e3) ->
+    | Pexp_index (e1, e2) ->
+      sub.expr sub e1;
+      sub.expr sub e2
+    | Pexp_setindex (e1, e2, e3) ->
       sub.expr sub e1;
       sub.expr sub e2;
-      iter_opt (sub.expr sub) e3
+      sub.expr sub e3
     | Pexp_array el -> List.iter (sub.expr sub) el
     | Pexp_ifthenelse (e1, e2, e3) ->
       sub.expr sub e1;
diff --git a/compiler/ml/ast_mapper.ml b/compiler/ml/ast_mapper.ml
index 6f59d04aec..20683bfa98 100644
--- a/compiler/ml/ast_mapper.ml
+++ b/compiler/ml/ast_mapper.ml
@@ -313,9 +313,10 @@ module E = struct
       field ~loc ~attrs (sub.expr sub e) (map_loc sub lid)
     | Pexp_setfield (e1, lid, e2) ->
       setfield ~loc ~attrs (sub.expr sub e1) (map_loc sub lid) (sub.expr sub e2)
-    | Pexp_index (e1, e2, e3) ->
+    | Pexp_index (e1, e2) ->
       index ~loc ~attrs (sub.expr sub e1) (sub.expr sub e2)
-        (map_opt (sub.expr sub) e3)
+    | Pexp_setindex (e1, e2, e3) ->
+      setindex ~loc ~attrs (sub.expr sub e1) (sub.expr sub e2) (sub.expr sub e3)
     | Pexp_array el -> array ~loc ~attrs (List.map (sub.expr sub) el)
     | Pexp_ifthenelse (e1, e2, e3) ->
       ifthenelse ~loc ~attrs (sub.expr sub e1) (sub.expr sub e2)
diff --git a/compiler/ml/ast_mapper_to0.ml b/compiler/ml/ast_mapper_to0.ml
index 23bc01cd72..3f0b10345f 100644
--- a/compiler/ml/ast_mapper_to0.ml
+++ b/compiler/ml/ast_mapper_to0.ml
@@ -439,27 +439,24 @@ module E = struct
       field ~loc ~attrs (sub.expr sub e) (map_loc sub lid)
     | Pexp_setfield (e1, lid, e2) ->
       setfield ~loc ~attrs (sub.expr sub e1) (map_loc sub lid) (sub.expr sub e2)
-    | Pexp_index (e1, e2, e3) -> (
-      (* Map back to Array.get/Array.set for parsetree0 compatibility *)
+    | Pexp_index (e1, e2) ->
+      (* Map back to Array.get for parsetree0 compatibility *)
       let container = sub.expr sub e1 in
       let index = sub.expr sub e2 in
-      match e3 with
-      | None ->
-        (* Read: Array.get(container, index) *)
-        let array_get =
-          ident ~loc
-            (mknoloc (Longident.Ldot (Longident.Lident "Array", "get")))
-        in
-        apply ~loc ~attrs array_get [(Nolabel, container); (Nolabel, index)]
-      | Some value ->
-        (* Write: Array.set(container, index, value) *)
-        let array_set =
-          ident ~loc
-            (mknoloc (Longident.Ldot (Longident.Lident "Array", "set")))
-        in
-        let value_expr = sub.expr sub value in
-        apply ~loc ~attrs array_set
-          [(Nolabel, container); (Nolabel, index); (Nolabel, value_expr)])
+      let array_get =
+        ident ~loc (mknoloc (Longident.Ldot (Longident.Lident "Array", "get")))
+      in
+      apply ~loc ~attrs array_get [(Nolabel, container); (Nolabel, index)]
+    | Pexp_setindex (e1, e2, e3) ->
+      (* Map back to Array.set for parsetree0 compatibility *)
+      let container = sub.expr sub e1 in
+      let index = sub.expr sub e2 in
+      let value_expr = sub.expr sub e3 in
+      let array_set =
+        ident ~loc (mknoloc (Longident.Ldot (Longident.Lident "Array", "set")))
+      in
+      apply ~loc ~attrs array_set
+        [(Nolabel, container); (Nolabel, index); (Nolabel, value_expr)]
     | Pexp_array el -> array ~loc ~attrs (List.map (sub.expr sub) el)
     | Pexp_ifthenelse (e1, e2, e3) ->
       ifthenelse ~loc ~attrs (sub.expr sub e1) (sub.expr sub e2)
diff --git a/compiler/ml/depend.ml b/compiler/ml/depend.ml
index 011b556ab5..6b396bb618 100644
--- a/compiler/ml/depend.ml
+++ b/compiler/ml/depend.ml
@@ -248,10 +248,13 @@ let rec add_expr bv exp =
     add_expr bv e1;
     add bv fld;
     add_expr bv e2
-  | Pexp_index (e1, e2, e3) ->
+  | Pexp_index (e1, e2) ->
+    add_expr bv e1;
+    add_expr bv e2
+  | Pexp_setindex (e1, e2, e3) ->
     add_expr bv e1;
     add_expr bv e2;
-    add_opt add_expr bv e3
+    add_expr bv e3
   | Pexp_array el -> List.iter (add_expr bv) el
   | Pexp_ifthenelse (e1, e2, opte3) ->
     add_expr bv e1;
diff --git a/compiler/ml/parsetree.ml b/compiler/ml/parsetree.ml
index 3d0870f503..1d35143d99 100644
--- a/compiler/ml/parsetree.ml
+++ b/compiler/ml/parsetree.ml
@@ -279,9 +279,8 @@ and expression_desc =
     *)
   | Pexp_field of expression * Longident.t loc (* E.l *)
   | Pexp_setfield of expression * Longident.t loc * expression (* E1.l <- E2 *)
-  | Pexp_index of expression * expression * expression option
-    (* E1[E2]        (None)     - read access
-       E1[E2] = E3   (Some E3)  - write access *)
+  | Pexp_index of expression * expression (* E1[E2] *)
+  | Pexp_setindex of expression * expression * expression (* E1[E2] = E3 *)
   | Pexp_array of expression list (* [| E1; ...; En |] *)
   | Pexp_ifthenelse of expression * expression * expression option
     (* if E1 then E2 else E3 *)
diff --git a/compiler/ml/pprintast.ml b/compiler/ml/pprintast.ml
index 9ea76c16d9..743b1dfba9 100644
--- a/compiler/ml/pprintast.ml
+++ b/compiler/ml/pprintast.ml
@@ -691,9 +691,9 @@ and expression ctxt f x =
     | Pexp_setfield (e1, li, e2) ->
       pp f "@[<2>%a.%a@ <-@ %a@]" (simple_expr ctxt) e1 longident_loc li
         (simple_expr ctxt) e2
-    | Pexp_index (e1, e2, None) ->
+    | Pexp_index (e1, e2) ->
       pp f "%a.(%a)" (expression ctxt) e1 (expression ctxt) e2
-    | Pexp_index (e1, e2, Some e3) ->
+    | Pexp_setindex (e1, e2, e3) ->
       pp f "%a.(%a)@ <-@ %a" (expression ctxt) e1 (expression ctxt) e2
         (expression ctxt) e3
     | Pexp_ifthenelse (e1, e2, eo) ->
diff --git a/compiler/ml/printast.ml b/compiler/ml/printast.ml
index ed0e68c2ef..22d7aaa8a3 100644
--- a/compiler/ml/printast.ml
+++ b/compiler/ml/printast.ml
@@ -294,16 +294,18 @@ and expression i ppf x =
     expression i ppf e1;
     longident_loc i ppf li;
     expression i ppf e2
-  | Pexp_index (e1, e2, e3) -> (
+  | Pexp_index (e1, e2) ->
     line i ppf "Pexp_index\n";
     expression i ppf e1;
     line i ppf "index:\n";
+    expression i ppf e2
+  | Pexp_setindex (e1, e2, e3) ->
+    line i ppf "Pexp_setindex\n";
+    expression i ppf e1;
+    line i ppf "index:\n";
     expression i ppf e2;
-    match e3 with
-    | None -> line i ppf "read access\n"
-    | Some e ->
-      line i ppf "write access:\n";
-      expression i ppf e)
+    line i ppf "value:\n";
+    expression i ppf e3
   | Pexp_array l ->
     line i ppf "Pexp_array\n";
     list i expression ppf l
diff --git a/compiler/ml/printtyped.ml b/compiler/ml/printtyped.ml
index a74e1d823c..a334dd07ab 100644
--- a/compiler/ml/printtyped.ml
+++ b/compiler/ml/printtyped.ml
@@ -329,16 +329,18 @@ and expression i ppf x =
     expression i ppf e1;
     longident i ppf li;
     expression i ppf e2
-  | Texp_index (e1, e2, e3) -> (
+  | Texp_index (e1, e2) ->
     line i ppf "Texp_index\n";
     expression i ppf e1;
     line i ppf "index:\n";
+    expression i ppf e2
+  | Texp_setindex (e1, e2, e3) ->
+    line i ppf "Texp_setindex\n";
+    expression i ppf e1;
+    line i ppf "index:\n";
     expression i ppf e2;
-    match e3 with
-    | None -> line i ppf "read access\n"
-    | Some e ->
-      line i ppf "write access:\n";
-      expression i ppf e)
+    line i ppf "value:\n";
+    expression i ppf e3
   | Texp_array l ->
     line i ppf "Texp_array\n";
     list i expression ppf l
diff --git a/compiler/ml/rec_check.ml b/compiler/ml/rec_check.ml
index 6c18a20a08..95217d1318 100644
--- a/compiler/ml/rec_check.ml
+++ b/compiler/ml/rec_check.ml
@@ -196,8 +196,8 @@ let rec classify_expression : Typedtree.expression -> sd =
     classify_expression e
   | Texp_ident _ | Texp_for _ | Texp_constant _ | Texp_tuple _ | Texp_array _
   | Texp_construct _ | Texp_variant _ | Texp_record _ | Texp_setfield _
-  | Texp_index _ | Texp_while _ | Texp_pack _ | Texp_function _
-  | Texp_extension_constructor _ ->
+  | Texp_index _ | Texp_setindex _ | Texp_while _ | Texp_pack _
+  | Texp_function _ | Texp_extension_constructor _ ->
     Static
   | Texp_apply {funct = {exp_desc = Texp_ident (_, _, vd)}} when is_ref vd ->
     Static
@@ -273,11 +273,13 @@ let rec expression : Env.env -> Typedtree.expression -> Use.t =
         (join (expression env ifso) (option expression env ifnot)))
   | Texp_setfield (e1, _, _, e2) ->
     Use.(join (inspect (expression env e1)) (inspect (expression env e2)))
-  | Texp_index (e1, e2, e3) ->
+  | Texp_index (e1, e2) ->
+    Use.(join (inspect (expression env e1)) (inspect (expression env e2)))
+  | Texp_setindex (e1, e2, e3) ->
     Use.(
       join
         (join (inspect (expression env e1)) (inspect (expression env e2)))
-        (inspect (option expression env e3)))
+        (inspect (expression env e3)))
   | Texp_sequence (e1, e2) ->
     Use.(join (discard (expression env e1)) (expression env e2))
   | Texp_while (e1, e2) ->
diff --git a/compiler/ml/tast_iterator.ml b/compiler/ml/tast_iterator.ml
index 2641e7d77a..56658826f1 100644
--- a/compiler/ml/tast_iterator.ml
+++ b/compiler/ml/tast_iterator.ml
@@ -176,10 +176,13 @@ let expr sub {exp_extra; exp_desc; exp_env; _} =
   | Texp_setfield (exp1, _, _, exp2) ->
     sub.expr sub exp1;
     sub.expr sub exp2
-  | Texp_index (exp1, exp2, expo) ->
+  | Texp_index (exp1, exp2) ->
+    sub.expr sub exp1;
+    sub.expr sub exp2
+  | Texp_setindex (exp1, exp2, exp3) ->
     sub.expr sub exp1;
     sub.expr sub exp2;
-    Option.iter (sub.expr sub) expo
+    sub.expr sub exp3
   | Texp_array list -> List.iter (sub.expr sub) list
   | Texp_ifthenelse (exp1, exp2, expo) ->
     sub.expr sub exp1;
diff --git a/compiler/ml/tast_mapper.ml b/compiler/ml/tast_mapper.ml
index 935c39d9c4..8547012c97 100644
--- a/compiler/ml/tast_mapper.ml
+++ b/compiler/ml/tast_mapper.ml
@@ -232,8 +232,10 @@ let expr sub x =
     | Texp_field (exp, lid, ld) -> Texp_field (sub.expr sub exp, lid, ld)
     | Texp_setfield (exp1, lid, ld, exp2) ->
       Texp_setfield (sub.expr sub exp1, lid, ld, sub.expr sub exp2)
-    | Texp_index (exp1, exp2, expo) ->
-      Texp_index (sub.expr sub exp1, sub.expr sub exp2, opt (sub.expr sub) expo)
+    | Texp_index (exp1, exp2) ->
+      Texp_index (sub.expr sub exp1, sub.expr sub exp2)
+    | Texp_setindex (exp1, exp2, exp3) ->
+      Texp_setindex (sub.expr sub exp1, sub.expr sub exp2, sub.expr sub exp3)
     | Texp_array list -> Texp_array (List.map (sub.expr sub) list)
     | Texp_ifthenelse (exp1, exp2, expo) ->
       Texp_ifthenelse
diff --git a/compiler/ml/translcore.ml b/compiler/ml/translcore.ml
index be9556c4fe..4f31957aa1 100644
--- a/compiler/ml/translcore.ml
+++ b/compiler/ml/translcore.ml
@@ -891,18 +891,17 @@ and transl_exp0 (e : Typedtree.expression) : Lambda.lambda =
         Psetfield (lbl.lbl_pos + 1, Lambda.fld_record_extension_set lbl)
     in
     Lprim (access, [transl_exp arg; transl_exp newval], e.exp_loc)
-  | Texp_index (container, index, value_opt) -> (
+  | Texp_index (container, index) ->
+    (* Read: translate to Parrayrefu primitive (unsafe array get) *)
     let container_lambda = transl_exp container in
     let index_lambda = transl_exp index in
-    match value_opt with
-    | None ->
-      (* Read: translate to Parrayrefu primitive (unsafe array get) *)
-      Lprim (Parrayrefu, [container_lambda; index_lambda], e.exp_loc)
-    | Some value ->
-      (* Write: translate to Parraysetu primitive (unsafe array set) *)
-      let value_lambda = transl_exp value in
-      Lprim
-        (Parraysetu, [container_lambda; index_lambda; value_lambda], e.exp_loc))
+    Lprim (Parrayrefu, [container_lambda; index_lambda], e.exp_loc)
+  | Texp_setindex (container, index, value) ->
+    (* Write: translate to Parraysetu primitive (unsafe array set) *)
+    let container_lambda = transl_exp container in
+    let index_lambda = transl_exp index in
+    let value_lambda = transl_exp value in
+    Lprim (Parraysetu, [container_lambda; index_lambda; value_lambda], e.exp_loc)
   | Texp_array expr_list ->
     let ll = transl_list expr_list in
     Lprim (Pmakearray Mutable, ll, e.exp_loc)
diff --git a/compiler/ml/typecore.ml b/compiler/ml/typecore.ml
index 457c5e84be..6934d9f80b 100644
--- a/compiler/ml/typecore.ml
+++ b/compiler/ml/typecore.ml
@@ -178,14 +178,16 @@ let iter_expression f e =
     | Pexp_letexception (_, e)
     | Pexp_field (e, _) ->
       expr e
-    | Pexp_while (e1, e2) | Pexp_sequence (e1, e2) | Pexp_setfield (e1, _, e2)
-      ->
+    | Pexp_while (e1, e2)
+    | Pexp_sequence (e1, e2)
+    | Pexp_setfield (e1, _, e2)
+    | Pexp_index (e1, e2) ->
       expr e1;
       expr e2
-    | Pexp_index (e1, e2, eo) ->
+    | Pexp_setindex (e1, e2, e3) ->
       expr e1;
       expr e2;
-      may expr eo
+      expr e3
     | Pexp_ifthenelse (e1, e2, eo) ->
       expr e1;
       expr e2;
@@ -2838,42 +2840,43 @@ and type_expect_ ?deprecated_context ~context ?in_function ?(recarg = Rejected)
         exp_attributes = sexp.pexp_attributes;
         exp_env = env;
       }
-  | Pexp_index (scontainer, sindex, svalue_opt) -> (
-    (* Type check as array access (same as current Array.get/set behavior) *)
+  | Pexp_index (scontainer, sindex) ->
+    (* Read access: arr[i] -> array<'a> -> int -> 'a *)
     let container = type_exp ~context:None env scontainer in
     let index =
       type_expect ~context:None env sindex (instance_def Predef.type_int)
     in
-    match svalue_opt with
-    | None ->
-      (* Read access: arr[i] -> array<'a> -> int -> 'a *)
-      let element_type = newgenvar () in
-      let array_type = instance_def (Predef.type_array element_type) in
-      unify_exp ~context:None env container array_type;
-      rue
-        {
-          exp_desc = Texp_index (container, index, None);
-          exp_loc = loc;
-          exp_extra = [];
-          exp_type = instance env element_type;
-          exp_attributes = sexp.pexp_attributes;
-          exp_env = env;
-        }
-    | Some svalue ->
-      (* Write access: arr[i] = v -> array<'a> -> int -> 'a -> unit *)
-      let element_type = newgenvar () in
-      let array_type = instance_def (Predef.type_array element_type) in
-      unify_exp ~context:None env container array_type;
-      let value = type_expect ~context:None env svalue element_type in
-      rue
-        {
-          exp_desc = Texp_index (container, index, Some value);
-          exp_loc = loc;
-          exp_extra = [];
-          exp_type = instance_def Predef.type_unit;
-          exp_attributes = sexp.pexp_attributes;
-          exp_env = env;
-        })
+    let element_type = newgenvar () in
+    let array_type = instance_def (Predef.type_array element_type) in
+    unify_exp ~context:None env container array_type;
+    rue
+      {
+        exp_desc = Texp_index (container, index);
+        exp_loc = loc;
+        exp_extra = [];
+        exp_type = instance env element_type;
+        exp_attributes = sexp.pexp_attributes;
+        exp_env = env;
+      }
+  | Pexp_setindex (scontainer, sindex, svalue) ->
+    (* Write access: arr[i] = v -> array<'a> -> int -> 'a -> unit *)
+    let container = type_exp ~context:None env scontainer in
+    let index =
+      type_expect ~context:None env sindex (instance_def Predef.type_int)
+    in
+    let element_type = newgenvar () in
+    let array_type = instance_def (Predef.type_array element_type) in
+    unify_exp ~context:None env container array_type;
+    let value = type_expect ~context:None env svalue element_type in
+    rue
+      {
+        exp_desc = Texp_setindex (container, index, value);
+        exp_loc = loc;
+        exp_extra = [];
+        exp_type = instance_def Predef.type_unit;
+        exp_attributes = sexp.pexp_attributes;
+        exp_env = env;
+      }
   | Pexp_array sargl ->
     let ty = newgenvar () in
     let to_unify = Predef.type_array ty in
diff --git a/compiler/ml/typedtree.ml b/compiler/ml/typedtree.ml
index 6dccdb2599..2c9c8b5433 100644
--- a/compiler/ml/typedtree.ml
+++ b/compiler/ml/typedtree.ml
@@ -106,7 +106,8 @@ and expression_desc =
   | Texp_field of expression * Longident.t loc * label_description
   | Texp_setfield of
       expression * Longident.t loc * label_description * expression
-  | Texp_index of expression * expression * expression option
+  | Texp_index of expression * expression
+  | Texp_setindex of expression * expression * expression
   | Texp_array of expression list
   | Texp_ifthenelse of expression * expression * expression option
   | Texp_sequence of expression * expression
diff --git a/compiler/ml/typedtree.mli b/compiler/ml/typedtree.mli
index 416577153b..13f5fcaba3 100644
--- a/compiler/ml/typedtree.mli
+++ b/compiler/ml/typedtree.mli
@@ -207,7 +207,8 @@ and expression_desc =
   | Texp_field of expression * Longident.t loc * label_description
   | Texp_setfield of
       expression * Longident.t loc * label_description * expression
-  | Texp_index of expression * expression * expression option
+  | Texp_index of expression * expression
+  | Texp_setindex of expression * expression * expression
   | Texp_array of expression list
   | Texp_ifthenelse of expression * expression * expression option
   | Texp_sequence of expression * expression
diff --git a/compiler/ml/typedtreeIter.ml b/compiler/ml/typedtreeIter.ml
index ea864004eb..5596b4dda3 100644
--- a/compiler/ml/typedtreeIter.ml
+++ b/compiler/ml/typedtreeIter.ml
@@ -261,12 +261,13 @@ end = struct
     | Texp_setfield (exp1, _, _label, exp2) ->
       iter_expression exp1;
       iter_expression exp2
-    | Texp_index (exp1, exp2, expo) -> (
+    | Texp_index (exp1, exp2) ->
+      iter_expression exp1;
+      iter_expression exp2
+    | Texp_setindex (exp1, exp2, exp3) ->
       iter_expression exp1;
       iter_expression exp2;
-      match expo with
-      | None -> ()
-      | Some exp -> iter_expression exp)
+      iter_expression exp3
     | Texp_array list -> List.iter iter_expression list
     | Texp_ifthenelse (exp1, exp2, expo) -> (
       iter_expression exp1;

From c68bc30a2d0fbb8d72a952ec78ca9038f021fad8 Mon Sep 17 00:00:00 2001
From: Shulhi Sapli <913103+shulhi@users.noreply.github.com>
Date: Mon, 12 Jan 2026 23:25:23 +0800
Subject: [PATCH 5/9] Handle printer

---
 analysis/src/Utils.ml                     |  1 +
 compiler/syntax/src/res_ast_debugger.ml   | 15 +++++----
 compiler/syntax/src/res_comments_table.ml | 12 ++++---
 compiler/syntax/src/res_printer.ml        | 40 +++++++++++++----------
 4 files changed, 40 insertions(+), 28 deletions(-)

diff --git a/analysis/src/Utils.ml b/analysis/src/Utils.ml
index d930b7edc2..33bee9ad16 100644
--- a/analysis/src/Utils.ml
+++ b/analysis/src/Utils.ml
@@ -96,6 +96,7 @@ let identifyPexp pexp =
   | Pexp_field _ -> "Pexp_field"
   | Pexp_setfield _ -> "Pexp_setfield"
   | Pexp_index _ -> "Pexp_index"
+  | Pexp_setindex _ -> "Pexp_setindex"
   | Pexp_array _ -> "Pexp_array"
   | Pexp_ifthenelse _ -> "Pexp_ifthenelse"
   | Pexp_sequence _ -> "Pexp_sequence"
diff --git a/compiler/syntax/src/res_ast_debugger.ml b/compiler/syntax/src/res_ast_debugger.ml
index cdfe67b6ad..8d283c9f7f 100644
--- a/compiler/syntax/src/res_ast_debugger.ml
+++ b/compiler/syntax/src/res_ast_debugger.ml
@@ -641,13 +641,16 @@ module SexpAst = struct
             longident longident_loc.Asttypes.txt;
             expression expr2;
           ]
-      | Pexp_index (e1, e2, e3) ->
+      | Pexp_index (e1, e2) ->
+        Sexp.list [Sexp.atom "Pexp_index"; expression e1; expression e2]
+      | Pexp_setindex (e1, e2, e3) ->
         Sexp.list
-          ([Sexp.atom "Pexp_index"; expression e1; expression e2]
-          @
-          match e3 with
-          | None -> []
-          | Some e -> [expression e])
+          [
+            Sexp.atom "Pexp_setindex";
+            expression e1;
+            expression e2;
+            expression e3;
+          ]
       | Pexp_array exprs ->
         Sexp.list
           [Sexp.atom "Pexp_array"; Sexp.list (map_empty ~f:expression exprs)]
diff --git a/compiler/syntax/src/res_comments_table.ml b/compiler/syntax/src/res_comments_table.ml
index 4d414bfb79..6338380827 100644
--- a/compiler/syntax/src/res_comments_table.ml
+++ b/compiler/syntax/src/res_comments_table.ml
@@ -476,7 +476,8 @@ let rec is_block_expr expr =
   | Pexp_constraint (expr, _) when is_block_expr expr -> true
   | Pexp_field (expr, _) when is_block_expr expr -> true
   | Pexp_setfield (expr, _, _) when is_block_expr expr -> true
-  | Pexp_index (expr, _, _) when is_block_expr expr -> true
+  | Pexp_index (expr, _) when is_block_expr expr -> true
+  | Pexp_setindex (expr, _, _) when is_block_expr expr -> true
   | _ -> false
 
 let is_if_then_else_expr expr =
@@ -1314,12 +1315,13 @@ and walk_expression expr t comments =
       attach t.leading expr2.pexp_loc leading;
       walk_expression expr2 t inside;
       attach t.trailing expr2.pexp_loc trailing
-  | Pexp_index (container, index, value_opt) -> (
+  | Pexp_index (container, index) ->
+    walk_expression container t comments;
+    walk_expression index t comments
+  | Pexp_setindex (container, index, value) ->
     walk_expression container t comments;
     walk_expression index t comments;
-    match value_opt with
-    | None -> ()
-    | Some value -> walk_expression value t comments)
+    walk_expression value t comments
   | Pexp_ifthenelse (if_expr, then_expr, else_expr) -> (
     let leading, rest = partition_leading_trailing comments expr.pexp_loc in
     attach t.leading expr.pexp_loc leading;
diff --git a/compiler/syntax/src/res_printer.ml b/compiler/syntax/src/res_printer.ml
index b157466d8e..fd4acb9cda 100644
--- a/compiler/syntax/src/res_printer.ml
+++ b/compiler/syntax/src/res_printer.ml
@@ -3374,7 +3374,8 @@ and print_expression ~state (e : Parsetree.expression) cmt_tbl =
     | Pexp_setfield (expr1, longident_loc, expr2) ->
       print_set_field_expr ~state e.pexp_attributes expr1 longident_loc expr2
         e.pexp_loc cmt_tbl
-    | Pexp_index (container, index, value_opt) -> (
+    | Pexp_index (container, index) ->
+      (* Read: container[index] *)
       let container_doc =
         let doc = print_expression_with_comments ~state container cmt_tbl in
         match Parens.field_expr container with
@@ -3383,22 +3384,27 @@ and print_expression ~state (e : Parsetree.expression) cmt_tbl =
         | Nothing -> doc
       in
       let index_doc = print_expression_with_comments ~state index cmt_tbl in
-      match value_opt with
-      | None ->
-        (* Read: container[index] *)
-        Doc.concat [container_doc; Doc.lbracket; index_doc; Doc.rbracket]
-      | Some value ->
-        (* Write: container[index] = value *)
-        let value_doc = print_expression_with_comments ~state value cmt_tbl in
-        Doc.concat
-          [
-            container_doc;
-            Doc.lbracket;
-            index_doc;
-            Doc.rbracket;
-            Doc.text " = ";
-            value_doc;
-          ])
+      Doc.concat [container_doc; Doc.lbracket; index_doc; Doc.rbracket]
+    | Pexp_setindex (container, index, value) ->
+      (* Write: container[index] = value *)
+      let container_doc =
+        let doc = print_expression_with_comments ~state container cmt_tbl in
+        match Parens.field_expr container with
+        | Parens.Parenthesized -> add_parens doc
+        | Braced braces -> print_braces doc container braces
+        | Nothing -> doc
+      in
+      let index_doc = print_expression_with_comments ~state index cmt_tbl in
+      let value_doc = print_expression_with_comments ~state value cmt_tbl in
+      Doc.concat
+        [
+          container_doc;
+          Doc.lbracket;
+          index_doc;
+          Doc.rbracket;
+          Doc.text " = ";
+          value_doc;
+        ]
     | Pexp_ifthenelse (_ifExpr, _thenExpr, _elseExpr)
       when ParsetreeViewer.is_ternary_expr e ->
       let parts, alternate = ParsetreeViewer.collect_ternary_parts e in

From 6905665c39dbffbedae9e74a2a535e30fc18ed78 Mon Sep 17 00:00:00 2001
From: Shulhi Sapli <913103+shulhi@users.noreply.github.com>
Date: Thu, 15 Jan 2026 21:52:49 +0800
Subject: [PATCH 6/9] Change parser

---
 compiler/syntax/src/res_core.ml | 27 +++++----------------------
 1 file changed, 5 insertions(+), 22 deletions(-)

diff --git a/compiler/syntax/src/res_core.ml b/compiler/syntax/src/res_core.ml
index aab15e930f..3ab19af28f 100644
--- a/compiler/syntax/src/res_core.ml
+++ b/compiler/syntax/src/res_core.ml
@@ -2156,7 +2156,6 @@ and parse_first_class_module_expr ~start_pos p =
 
 and parse_bracket_access p expr start_pos =
   Parser.leave_breadcrumb p Grammar.ExprArrayAccess;
-  let lbracket = p.start_pos in
   Parser.expect Lbracket p;
   let string_start = p.start_pos in
   match p.Parser.token with
@@ -2189,38 +2188,22 @@ and parse_bracket_access p expr start_pos =
     let access_expr = parse_constrained_or_coerced_expr p in
     Parser.expect Rbracket p;
     Parser.eat_breadcrumb p;
-    let rbracket = p.prev_end_pos in
-    let array_loc = mk_loc lbracket rbracket in
     match p.token with
     | Equal ->
       Parser.leave_breadcrumb p ExprArrayMutation;
       Parser.next p;
       let rhs_expr = parse_expr p in
-      (* FIXME: Do not implicitly rely on specific module name, even primitive one
-
-         This can be abused like
-           module Array = MyModule
-
-         Find better mechanism to support it
-      *)
-      let array_set =
-        Location.mkloc (Longident.Ldot (Lident "Array", "set")) array_loc
-      in
       let end_pos = p.prev_end_pos in
-      let array_set =
-        Ast_helper.Exp.apply ~loc:(mk_loc start_pos end_pos)
-          (Ast_helper.Exp.ident ~loc:array_loc array_set)
-          [(Nolabel, expr); (Nolabel, access_expr); (Nolabel, rhs_expr)]
+      let setindex_expr =
+        Ast_helper.Exp.setindex ~loc:(mk_loc start_pos end_pos) expr access_expr
+          rhs_expr
       in
       Parser.eat_breadcrumb p;
-      array_set
+      setindex_expr
     | _ ->
       let end_pos = p.prev_end_pos in
       let e =
-        Ast_helper.Exp.apply ~loc:(mk_loc start_pos end_pos)
-          (Ast_helper.Exp.ident ~loc:array_loc
-             (Location.mkloc (Longident.Ldot (Lident "Array", "get")) array_loc))
-          [(Nolabel, expr); (Nolabel, access_expr)]
+        Ast_helper.Exp.index ~loc:(mk_loc start_pos end_pos) expr access_expr
       in
       parse_primary_expr ~operand:e p)
 

From de2dc1865be8fe55c47cf5f07feb97ab20b90a63 Mon Sep 17 00:00:00 2001
From: Shulhi Sapli <913103+shulhi@users.noreply.github.com>
Date: Mon, 19 Jan 2026 11:39:06 +0800
Subject: [PATCH 7/9] Include existing arity information

---
 compiler/ml/typecore.ml | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/compiler/ml/typecore.ml b/compiler/ml/typecore.ml
index 6934d9f80b..96adf873ff 100644
--- a/compiler/ml/typecore.ml
+++ b/compiler/ml/typecore.ml
@@ -2864,9 +2864,17 @@ and type_expect_ ?deprecated_context ~context ?in_function ?(recarg = Rejected)
     let index =
       type_expect ~context:None env sindex (instance_def Predef.type_int)
     in
-    let element_type = newgenvar () in
-    let array_type = instance_def (Predef.type_array element_type) in
-    unify_exp ~context:None env container array_type;
+    (* Extract element type from container to preserve arity information *)
+    let element_type =
+      match (expand_head env container.exp_type).desc with
+      | Tconstr (Pident {name = "array"}, [element_ty], _) -> element_ty
+      | _ ->
+        (* Fallback: create fresh type variable and unify *)
+        let element_type = newgenvar () in
+        let array_type = instance_def (Predef.type_array element_type) in
+        unify_exp ~context:None env container array_type;
+        element_type
+    in
     let value = type_expect ~context:None env svalue element_type in
     rue
       {

From c4b741e18276b3f8810c4bc8d3cbe20a37844fc5 Mon Sep 17 00:00:00 2001
From: Shulhi Sapli <913103+shulhi@users.noreply.github.com>
Date: Wed, 28 Jan 2026 20:19:30 +0800
Subject: [PATCH 8/9] Handle comments attachment

---
 compiler/syntax/src/res_comments_table.ml | 72 +++++++++++++++++++++--
 1 file changed, 67 insertions(+), 5 deletions(-)

diff --git a/compiler/syntax/src/res_comments_table.ml b/compiler/syntax/src/res_comments_table.ml
index 6338380827..c14b6a1852 100644
--- a/compiler/syntax/src/res_comments_table.ml
+++ b/compiler/syntax/src/res_comments_table.ml
@@ -1316,12 +1316,74 @@ and walk_expression expr t comments =
       walk_expression expr2 t inside;
       attach t.trailing expr2.pexp_loc trailing
   | Pexp_index (container, index) ->
-    walk_expression container t comments;
-    walk_expression index t comments
+    let leading, inside, trailing =
+      partition_by_loc comments container.pexp_loc
+    in
+    let rest =
+      if is_block_expr container then (
+        let after_expr, rest =
+          partition_adjacent_trailing container.pexp_loc trailing
+        in
+        walk_expression container t (List.concat [leading; inside; after_expr]);
+        rest)
+      else
+        let after_expr, rest =
+          partition_adjacent_trailing container.pexp_loc trailing
+        in
+        attach t.leading container.pexp_loc leading;
+        walk_expression container t inside;
+        attach t.trailing container.pexp_loc after_expr;
+        rest
+    in
+    if is_block_expr index then walk_expression index t rest
+    else
+      let leading, inside, trailing = partition_by_loc rest index.pexp_loc in
+      attach t.leading index.pexp_loc leading;
+      walk_expression index t inside;
+      attach t.trailing index.pexp_loc trailing
   | Pexp_setindex (container, index, value) ->
-    walk_expression container t comments;
-    walk_expression index t comments;
-    walk_expression value t comments
+    let leading, inside, trailing =
+      partition_by_loc comments container.pexp_loc
+    in
+    let rest =
+      if is_block_expr container then (
+        let after_expr, rest =
+          partition_adjacent_trailing container.pexp_loc trailing
+        in
+        walk_expression container t (List.concat [leading; inside; after_expr]);
+        rest)
+      else
+        let after_expr, rest =
+          partition_adjacent_trailing container.pexp_loc trailing
+        in
+        attach t.leading container.pexp_loc leading;
+        walk_expression container t inside;
+        attach t.trailing container.pexp_loc after_expr;
+        rest
+    in
+    let leading, inside, trailing = partition_by_loc rest index.pexp_loc in
+    let rest =
+      if is_block_expr index then (
+        let after_expr, rest =
+          partition_adjacent_trailing index.pexp_loc trailing
+        in
+        walk_expression index t (List.concat [leading; inside; after_expr]);
+        rest)
+      else
+        let after_expr, rest =
+          partition_adjacent_trailing index.pexp_loc trailing
+        in
+        attach t.leading index.pexp_loc leading;
+        walk_expression index t inside;
+        attach t.trailing index.pexp_loc after_expr;
+        rest
+    in
+    if is_block_expr value then walk_expression value t rest
+    else
+      let leading, inside, trailing = partition_by_loc rest value.pexp_loc in
+      attach t.leading value.pexp_loc leading;
+      walk_expression value t inside;
+      attach t.trailing value.pexp_loc trailing
   | Pexp_ifthenelse (if_expr, then_expr, else_expr) -> (
     let leading, rest = partition_leading_trailing comments expr.pexp_loc in
     attach t.leading expr.pexp_loc leading;

From 9e884e1f24c460932028f050af3d6b18c3f9143a Mon Sep 17 00:00:00 2001
From: Shulhi Sapli <913103+shulhi@users.noreply.github.com>
Date: Wed, 28 Jan 2026 20:19:47 +0800
Subject: [PATCH 9/9] Handle printer

---
 compiler/ml/pprintast.ml                    |  6 +-
 compiler/syntax/src/res_parsetree_viewer.ml |  2 +
 compiler/syntax/src/res_printer.ml          | 71 ++++++++++++++++++---
 3 files changed, 66 insertions(+), 13 deletions(-)

diff --git a/compiler/ml/pprintast.ml b/compiler/ml/pprintast.ml
index 743b1dfba9..9688f606cd 100644
--- a/compiler/ml/pprintast.ml
+++ b/compiler/ml/pprintast.ml
@@ -692,10 +692,10 @@ and expression ctxt f x =
       pp f "@[<2>%a.%a@ <-@ %a@]" (simple_expr ctxt) e1 longident_loc li
         (simple_expr ctxt) e2
     | Pexp_index (e1, e2) ->
-      pp f "%a.(%a)" (expression ctxt) e1 (expression ctxt) e2
+      pp f "%a.(%a)" (simple_expr ctxt) e1 (expression ctxt) e2
     | Pexp_setindex (e1, e2, e3) ->
-      pp f "%a.(%a)@ <-@ %a" (expression ctxt) e1 (expression ctxt) e2
-        (expression ctxt) e3
+      pp f "%a.(%a)@ <-@ %a" (simple_expr ctxt) e1 (expression ctxt) e2
+        (simple_expr ctxt) e3
     | Pexp_ifthenelse (e1, e2, eo) ->
       (* @;@[<2>else@ %a@]@] *)
       let fmt : (_, _, _) format =
diff --git a/compiler/syntax/src/res_parsetree_viewer.ml b/compiler/syntax/src/res_parsetree_viewer.ml
index 391e51bede..1715bd0cf4 100644
--- a/compiler/syntax/src/res_parsetree_viewer.ml
+++ b/compiler/syntax/src/res_parsetree_viewer.ml
@@ -429,6 +429,8 @@ let has_attributes attrs =
 
 let is_array_access expr =
   match expr.pexp_desc with
+  | Pexp_index _ -> true
+  (* TODO: Now we have dedicated index access, we can clean up the below AST? *)
   | Pexp_apply
       {
         funct =
diff --git a/compiler/syntax/src/res_printer.ml b/compiler/syntax/src/res_printer.ml
index fd4acb9cda..5541031c71 100644
--- a/compiler/syntax/src/res_printer.ml
+++ b/compiler/syntax/src/res_printer.ml
@@ -3383,8 +3383,28 @@ and print_expression ~state (e : Parsetree.expression) cmt_tbl =
         | Braced braces -> print_braces doc container braces
         | Nothing -> doc
       in
-      let index_doc = print_expression_with_comments ~state index cmt_tbl in
-      Doc.concat [container_doc; Doc.lbracket; index_doc; Doc.rbracket]
+      let index_has_braces, index_doc =
+        let doc = print_expression_with_comments ~state index cmt_tbl in
+        match Parens.expr index with
+        | Parens.Parenthesized -> (false, add_parens doc)
+        | Braced braces -> (true, print_braces doc index braces)
+        | Nothing -> (false, doc)
+      in
+      if index_has_braces then
+        Doc.concat [container_doc; Doc.lbracket; index_doc; Doc.rbracket]
+      else
+        Doc.concat
+          [
+            container_doc;
+            Doc.group
+              (Doc.concat
+                 [
+                   Doc.lbracket;
+                   Doc.indent (Doc.concat [Doc.soft_line; index_doc]);
+                   Doc.soft_line;
+                   Doc.rbracket;
+                 ]);
+          ]
     | Pexp_setindex (container, index, value) ->
       (* Write: container[index] = value *)
       let container_doc =
@@ -3394,16 +3414,47 @@ and print_expression ~state (e : Parsetree.expression) cmt_tbl =
         | Braced braces -> print_braces doc container braces
         | Nothing -> doc
       in
-      let index_doc = print_expression_with_comments ~state index cmt_tbl in
-      let value_doc = print_expression_with_comments ~state value cmt_tbl in
+      let index_has_braces, index_doc =
+        let doc = print_expression_with_comments ~state index cmt_tbl in
+        match Parens.expr index with
+        | Parens.Parenthesized -> (false, add_parens doc)
+        | Braced braces -> (true, print_braces doc index braces)
+        | Nothing -> (false, doc)
+      in
+      let value_has_braces, value_doc =
+        let doc = print_expression_with_comments ~state value cmt_tbl in
+        match Parens.set_field_expr_rhs value with
+        | Parens.Parenthesized -> (false, add_parens doc)
+        | Braced braces -> (true, print_braces doc value braces)
+        | Nothing -> (false, doc)
+      in
+      let should_indent =
+        (not value_has_braces) && ParsetreeViewer.is_binary_expression value
+      in
+      let bracket_doc =
+        if index_has_braces then
+          Doc.concat [container_doc; Doc.lbracket; index_doc; Doc.rbracket]
+        else
+          Doc.concat
+            [
+              container_doc;
+              Doc.group
+                (Doc.concat
+                   [
+                     Doc.lbracket;
+                     Doc.indent (Doc.concat [Doc.soft_line; index_doc]);
+                     Doc.soft_line;
+                     Doc.rbracket;
+                   ]);
+            ]
+      in
       Doc.concat
         [
-          container_doc;
-          Doc.lbracket;
-          index_doc;
-          Doc.rbracket;
-          Doc.text " = ";
-          value_doc;
+          bracket_doc;
+          Doc.text " =";
+          (if should_indent then
+             Doc.group (Doc.indent (Doc.concat [Doc.line; value_doc]))
+           else Doc.concat [Doc.space; value_doc]);
         ]
     | Pexp_ifthenelse (_ifExpr, _thenExpr, _elseExpr)
       when ParsetreeViewer.is_ternary_expr e ->