rescript-lang · cknitt · Sep 3, 2026 · Sep 3, 2026 · Sep 3, 2026 · Sep 3, 2026
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -14,6 +14,7 @@
 
 #### :boom: Breaking Change
 
+- Distinguish multiple constructor arguments from a tuple passed as a single argument. Constructors with one tuple payload must now use nested parentheses, for example `Some((x, y))`; `Some(x, y)` now reports an arity mismatch. This makes constructor arity explicit in the parsetree and removes the separate parser modes for printing and type checking. https://github.com/rescript-lang/rescript/pull/8610
 - Reject malformed UTF-8 in documentation comments and invalid string or template literal escapes that were previously accepted, including empty or out-of-range braced Unicode escapes (`\u{}`, `\u{110000}`) and legacy decimal or octal escapes in templates (`\1`, `\01`, `\8`). These inputs now produce syntax diagnostics instead of compiling to invalid or inconsistent JavaScript. https://github.com/rescript-lang/rescript/pull/8606
 - Reject tagged template literals in patterns. Patterns cannot invoke their tag; previously their raw payload was compiled as a plain string comparison. https://github.com/rescript-lang/rescript/pull/8606
 - Remove runtime APIs that were deprecated for removal in ReScript 13, including the `Char` module, unsafe `Obj` operations, legacy `Pervasives` helpers, and `Array.unsafe_get`. https://github.com/rescript-lang/rescript/pull/8564

diff --git a/analysis/reanalyze/src/annotation.ml b/analysis/reanalyze/src/annotation.ml
@@ -30,10 +30,11 @@ let rec get_attribute_payload check_text (attributes : Typedtree.attributes) =
        _;
       } ->
         Some (BoolPayload (s = "true"))
-      | {pexp_desc = Pexp_construct ({txt = Longident.Lident "[]"}, None)} ->
-        None
-      | {pexp_desc = Pexp_construct ({txt = Longident.Lident "::"}, Some e)} ->
-        from_expr e
+      | {pexp_desc = Pexp_construct ({txt = Longident.Lident "[]"}, [])} -> None
+      | {
+       pexp_desc = Pexp_construct ({txt = Longident.Lident "::"}, [head; tail]);
+      } ->
+        from_expr {expr with pexp_desc = Pexp_tuple [head; tail]}
       | {pexp_desc = Pexp_construct ({txt}, _); _} ->
         Some (ConstructPayload (txt |> Longident.flatten |> String.concat "."))
       | {pexp_desc = Pexp_tuple exprs | Pexp_array exprs} ->

diff --git a/analysis/src/codemod.ml b/analysis/src/codemod.ml
@@ -13,8 +13,8 @@ let transform_opt ~source ~pos ~debug ~typ ~hint =
     | AddMissingCases -> (
       let source = "let " ^ hint ^ " = ()" in
       let {Res_driver.parsetree = hint_structure} =
-        Res_driver.parse_implementation_from_source ~for_printer:false
-          ~display_filename:"<none>" ~source
+        Res_driver.parse_implementation_from_source ~display_filename:"<none>"
+          ~source
       in
       match hint_structure with
       | [{pstr_desc = Pstr_value (_, [{pvb_pat = pattern}])}] -> (

diff --git a/analysis/src/commands.ml b/analysis/src/commands.ml
@@ -304,8 +304,7 @@ let format ~source ~kind_file =
     match kind_file with
     | Files.Res -> (
       let {Res_driver.parsetree = structure; comments; diagnostics} =
-        Res_driver.parsing_engine.parse_implementation_from_source
-          ~for_printer:true ~source
+        Res_driver.parsing_engine.parse_implementation_from_source ~source
       in
       match List.length diagnostics > 0 with
       | true -> Error "Document has syntax errors"
@@ -314,8 +313,7 @@ let format ~source ~kind_file =
       )
     | Resi -> (
       let {Res_driver.parsetree = signature; comments; diagnostics} =
-        Res_driver.parsing_engine.parse_interface_from_source ~for_printer:true
-          ~source
+        Res_driver.parsing_engine.parse_interface_from_source ~source
       in
       match List.length diagnostics > 0 with
       | true -> Error "Document has syntax errors"

diff --git a/analysis/src/completion_expressions.ml b/analysis/src/completion_expressions.ml
@@ -24,9 +24,9 @@ let rec traverse_expr (exp : Parsetree.expression) ~expr_path ~pos
       (txt, [Completable.NRecordBody {seen_fields = []}] @ expr_path)
   | Pexp_ident {txt = Lident txt} -> some_if_has_cursor (txt, expr_path)
   | Pexp_construct ({txt = Lident "()"}, _) -> some_if_has_cursor ("", expr_path)
-  | Pexp_construct ({txt = Lident txt}, None) ->
+  | Pexp_construct ({txt = Lident txt}, []) ->
     some_if_has_cursor (txt, expr_path)
-  | Pexp_variant (label, None) -> some_if_has_cursor ("#" ^ label, expr_path)
+  | Pexp_variant (label, []) -> some_if_has_cursor ("#" ^ label, expr_path)
   | Pexp_array array_patterns -> (
     let next_expr_path = [Completable.NArray] @ expr_path in
     (* No fields but still has cursor = empty completion *)
@@ -121,8 +121,7 @@ let rec traverse_expr (exp : Parsetree.expression) ~expr_path ~pos
           ("", [Completable.NRecordBody {seen_fields}] @ expr_path)
       | _ -> None))
   | Pexp_construct
-      ( {txt},
-        Some {pexp_loc; pexp_desc = Pexp_construct ({txt = Lident "()"}, _)} )
+      ({txt}, [{pexp_loc; pexp_desc = Pexp_construct ({txt = Lident "()"}, _)}])
     when loc_has_cursor pexp_loc ->
     (* Empty payload with cursor, like: Test(<com>) *)
     Some
@@ -132,21 +131,24 @@ let rec traverse_expr (exp : Parsetree.expression) ~expr_path ~pos
             {constructor_name = Utils.get_unqualified_name txt; item_num = 0};
         ]
         @ expr_path )
-  | Pexp_construct ({txt}, Some e)
-    when pos >= (e.pexp_loc |> Loc.end_)
+  | Pexp_construct ({txt}, args)
+    when args <> []
+         && pos >= ((Ext_list.last args).pexp_loc |> Loc.end_)
          && first_char_before_cursor_no_white = Some ','
-         && is_expr_tuple e = false ->
+         && is_expr_tuple (Ext_list.last args) = false ->
     (* Empty payload with trailing ',', like: Test(true, <com>) *)
     Some
       ( "",
         [
           Completable.NVariantPayload
-            {constructor_name = Utils.get_unqualified_name txt; item_num = 1};
+            {
+              constructor_name = Utils.get_unqualified_name txt;
+              item_num = List.length args;
+            };
         ]
         @ expr_path )
-  | Pexp_construct ({txt}, Some {pexp_loc; pexp_desc = Pexp_tuple tuple_items})
-    when loc_has_cursor pexp_loc ->
-    tuple_items
+  | Pexp_construct ({txt}, args) when loc_has_cursor exp.pexp_loc ->
+    args
     |> traverse_expr_tuple_items ~first_char_before_cursor_no_white ~pos
          ~next_expr_path:(fun item_num ->
            [
@@ -163,38 +165,16 @@ let rec traverse_expr (exp : Parsetree.expression) ~expr_path ~pos
                };
            ]
            @ expr_path)
-  | Pexp_construct ({txt}, Some p) when loc_has_cursor exp.pexp_loc ->
-    p
-    |> traverse_expr ~first_char_before_cursor_no_white ~pos
-         ~expr_path:
-           ([
-              Completable.NVariantPayload
-                {
-                  constructor_name = Utils.get_unqualified_name txt;
-                  item_num = 0;
-                };
-            ]
-           @ expr_path)
   | Pexp_variant
-      (txt, Some {pexp_loc; pexp_desc = Pexp_construct ({txt = Lident "()"}, _)})
+      (txt, [{pexp_loc; pexp_desc = Pexp_construct ({txt = Lident "()"}, _)}])
     when loc_has_cursor pexp_loc ->
     (* Empty payload with cursor, like: #test(<com>) *)
     Some
       ( "",
         [Completable.NPolyvariantPayload {constructor_name = txt; item_num = 0}]
         @ expr_path )
-  | Pexp_variant (txt, Some e)
-    when pos >= (e.pexp_loc |> Loc.end_)
-         && first_char_before_cursor_no_white = Some ','
-         && is_expr_tuple e = false ->
-    (* Empty payload with trailing ',', like: #test(true, <com>) *)
-    Some
-      ( "",
-        [Completable.NPolyvariantPayload {constructor_name = txt; item_num = 1}]
-        @ expr_path )
-  | Pexp_variant (txt, Some {pexp_loc; pexp_desc = Pexp_tuple tuple_items})
-    when loc_has_cursor pexp_loc ->
-    tuple_items
+  | Pexp_variant (txt, args) when loc_has_cursor exp.pexp_loc ->
+    args
     |> traverse_expr_tuple_items ~first_char_before_cursor_no_white ~pos
          ~next_expr_path:(fun item_num ->
            [Completable.NPolyvariantPayload {constructor_name = txt; item_num}]
@@ -205,15 +185,6 @@ let rec traverse_expr (exp : Parsetree.expression) ~expr_path ~pos
                {constructor_name = txt; item_num = item_num + 1};
            ]
            @ expr_path)
-  | Pexp_variant (txt, Some p) when loc_has_cursor exp.pexp_loc ->
-    p
-    |> traverse_expr ~first_char_before_cursor_no_white ~pos
-         ~expr_path:
-           ([
-              Completable.NPolyvariantPayload
-                {constructor_name = txt; item_num = 0};
-            ]
-           @ expr_path)
   | _ -> None
 
 and traverse_expr_tuple_items tuple_items ~next_expr_path
@@ -280,35 +251,18 @@ let pretty_print_fn_template_arg_name ?current_index ~env ~state ~full
     | _ -> default_var_name)
 
 let complete_constructor_payload ~pos_before_cursor
-    ~first_char_before_cursor_no_white
+    ~first_char_before_cursor_no_white ~item_num
     (constructor_lid : Longident.t Location.loc) expr =
   match
     traverse_expr expr ~expr_path:[] ~pos:pos_before_cursor
       ~first_char_before_cursor_no_white
   with
   | None -> None
   | Some (prefix, nested) ->
-    (* The nested path must start with the constructor name found, plus
-       the target argument number for the constructor. We translate to
-       that here, because we need to account for multi arg constructors
-       being represented as tuples. *)
     let nested =
-      match List.rev nested with
-      | Completable.NTupleItem {item_num} :: rest ->
-        [
-          Completable.NVariantPayload
-            {constructor_name = Longident.last constructor_lid.txt; item_num};
-        ]
-        @ rest
-      | nested ->
-        [
-          Completable.NVariantPayload
-            {
-              constructor_name = Longident.last constructor_lid.txt;
-              item_num = 0;
-            };
-        ]
-        @ nested
+      Completable.NVariantPayload
+        {constructor_name = Longident.last constructor_lid.txt; item_num}
+      :: List.rev nested
     in
     let variant_ctx_path =
       Completable.CTypeAtPos

diff --git a/analysis/src/completion_front_end.ml b/analysis/src/completion_front_end.ml
@@ -222,7 +222,7 @@ let rec expr_to_context_path_inner ~(in_jsx_context : bool)
     | None -> None)
   | Pexp_constant (Pconst_integer _) -> Some CPInt
   | Pexp_constant (Pconst_float _) -> Some CPFloat
-  | Pexp_construct ({txt = Lident ("true" | "false")}, None) -> Some CPBool
+  | Pexp_construct ({txt = Lident ("true" | "false")}, []) -> Some CPBool
   | Pexp_array exprs ->
     Some
       (CPArray
@@ -492,9 +492,9 @@ let completion_with_parser1 ~debug ~offset ~pos_cursor ~kind_file
           scope_pattern p
             ~pattern_path:(NTupleItem {item_num = index} :: pattern_path)
             ?context_path)
-    | Ppat_construct (_, None) -> ()
-    | Ppat_construct ({txt}, Some {ppat_desc = Ppat_tuple pl}) ->
-      pl
+    | Ppat_construct (_, []) -> ()
+    | Ppat_construct ({txt}, patterns) ->
+      patterns
       |> List.iteri (fun index p ->
           scope_pattern p
             ~pattern_path:
@@ -505,28 +505,15 @@ let completion_with_parser1 ~debug ~offset ~pos_cursor ~kind_file
                  }
               :: pattern_path)
             ?context_path)
-    | Ppat_construct ({txt}, Some p) ->
-      scope_pattern
-        ~pattern_path:
-          (NVariantPayload
-             {item_num = 0; constructor_name = Utils.get_unqualified_name txt}
-          :: pattern_path)
-        ?context_path p
-    | Ppat_variant (_, None) -> ()
-    | Ppat_variant (txt, Some {ppat_desc = Ppat_tuple pl}) ->
-      pl
+    | Ppat_variant (_, []) -> ()
+    | Ppat_variant (txt, patterns) ->
+      patterns
       |> List.iteri (fun index p ->
           scope_pattern p
             ~pattern_path:
               (NPolyvariantPayload {item_num = index; constructor_name = txt}
               :: pattern_path)
             ?context_path)
-    | Ppat_variant (txt, Some p) ->
-      scope_pattern
-        ~pattern_path:
-          (NPolyvariantPayload {item_num = 0; constructor_name = txt}
-          :: pattern_path)
-        ?context_path p
     | Ppat_record (fields, _, rest) -> (
       Ext_list.iter fields (fun {lid = fname; x = p} ->
           match fname with
@@ -1043,7 +1030,7 @@ let completion_with_parser1 ~debug ~offset ~pos_cursor ~kind_file
                  Pstr_eval
                    ( {
                        pexp_loc;
-                       pexp_desc = Pexp_construct ({txt = path; loc}, None);
+                       pexp_desc = Pexp_construct ({txt = path; loc}, []);
                      },
                      _ );
              };
@@ -1283,36 +1270,41 @@ let completion_with_parser1 ~debug ~offset ~pos_cursor ~kind_file
                          then ValueOrField
                          else Value);
                     }))
-        | Pexp_construct (lid, e_opt) -> (
+        | Pexp_construct (lid, args) ->
           let lid_path = flatten_lid_check_dot lid in
           if debug then
             Printf.printf "Pexp_construct %s:%s %s\n"
               (lid_path |> String.concat "\n")
               (Loc.to_string lid.loc)
-              (match e_opt with
-              | None -> "None"
-              | Some e -> Loc.to_string e.pexp_loc);
+              (match args with
+              | [] -> "None"
+              | args ->
+                args
+                |> List.map (fun (e : Parsetree.expression) ->
+                    Loc.to_string e.pexp_loc)
+                |> String.concat ", ");
           if
-            e_opt = None && (not lid.loc.loc_ghost)
+            args = [] && (not lid.loc.loc_ghost)
             && lid.loc |> Loc.has_pos ~pos:pos_before_cursor
           then
             set_result
               (Cpath
                  (CPId
                     {loc = lid.loc; path = lid_path; completion_context = Value}))
           else
-            match e_opt with
-            | Some e when loc_has_cursor e.pexp_loc -> (
-              match
-                Completion_expressions.complete_constructor_payload
-                  ~pos_before_cursor ~first_char_before_cursor_no_white lid e
-              with
-              | Some result ->
-                (* Check if anything else more important completes before setting this completion. *)
-                Ast_iterator.default_iterator.expr iterator e;
-                set_result result
-              | None -> ())
-            | _ -> ())
+            args
+            |> List.iteri (fun item_num (e : Parsetree.expression) ->
+                if loc_has_cursor e.pexp_loc then
+                  match
+                    Completion_expressions.complete_constructor_payload
+                      ~pos_before_cursor ~first_char_before_cursor_no_white
+                      ~item_num lid e
+                  with
+                  | Some result ->
+                    (* Check if anything else more important completes before setting this completion. *)
+                    Ast_iterator.default_iterator.expr iterator e;
+                    set_result result
+                  | None -> ())
         | Pexp_field (e, field_name) -> (
           if debug then
             Printf.printf "Pexp_field %s %s:%s\n" (Loc.to_string e.pexp_loc)
@@ -1864,10 +1856,7 @@ let completion_with_parser1 ~debug ~offset ~pos_cursor ~kind_file
   in
 
   if kind_file = Files.Res then (
-    let parser =
-      Res_driver.parsing_engine.parse_implementation_from_source
-        ~for_printer:false
-    in
+    let parser = Res_driver.parsing_engine.parse_implementation_from_source in
     let {Res_driver.parsetree = str} = parser ~source:text in
     iterator.structure iterator str |> ignore;
     if blank_after_cursor = Some ' ' || blank_after_cursor = Some '\n' then (
@@ -1878,9 +1867,7 @@ let completion_with_parser1 ~debug ~offset ~pos_cursor ~kind_file
     if !found = false then if debug then Printf.printf "XXX Not found!\n";
     !result)
   else if kind_file = Resi then (
-    let parser =
-      Res_driver.parsing_engine.parse_interface_from_source ~for_printer:false
-    in
+    let parser = Res_driver.parsing_engine.parse_interface_from_source in
     let {Res_driver.parsetree = signature} = parser ~source:text in
     iterator.signature iterator signature |> ignore;
     if blank_after_cursor = Some ' ' || blank_after_cursor = Some '\n' then (