From 88cb36e725e49d3724863dd84d65a2cd3e887228 Mon Sep 17 00:00:00 2001
From: Cristiano Calcagno <cristianoc@users.noreply.github.com>
Date: Wed, 8 Jan 2025 09:21:06 +0100
Subject: [PATCH 1/3] Update CHANGELOG.md

---
 CHANGELOG.md | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index afe24e625b..e0d4e2a751 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -23,6 +23,11 @@
 - AST cleanup: Prepare for ast async cleanup: Refactor code for "@res.async" payload handling and clean up handling of type and term parameters, so that now each `=>` in a function definition corresponds to a function. https://github.com/rescript-lang/rescript/pull/7223
 - AST: always put type parameters first in function definitions. https://github.com/rescript-lang/rescript/pull/7233
 
+#### :house: Internal
+
+- AST cleanup: Prepare for ast async cleanup: Refactor code for "@res.async" payload handling and clean up handling of type and term parameters, so that now each `=>` in a function definition corresponds to a function. https://github.com/rescript-lang/rescript/pull/7223
+
+
 # 12.0.0-alpha.7
 
 #### :bug: Bug fix

From 362426d95833100fd45262751585811c27e0f97b Mon Sep 17 00:00:00 2001
From: Cristiano Calcagno <cristianoc@users.noreply.github.com>
Date: Wed, 8 Jan 2025 16:14:57 +0100
Subject: [PATCH 2/3] Extend untyped and types ast with async attribute.

---
 CHANGELOG.md                                  |  1 +
 compiler/frontend/ast_compatible.ml           | 11 ++++-
 compiler/frontend/ast_compatible.mli          |  1 +
 compiler/frontend/ast_uncurry_gen.ml          | 10 ++---
 compiler/frontend/bs_ast_mapper.ml            |  5 ++-
 compiler/frontend/bs_builtin_ppx.ml           |  3 +-
 compiler/ml/ast_async.ml                      | 17 +------
 compiler/ml/ast_helper.ml                     |  4 +-
 compiler/ml/ast_helper.mli                    |  1 +
 compiler/ml/ast_mapper.ml                     |  5 ++-
 compiler/ml/ast_mapper_from0.ml               |  3 +-
 compiler/ml/ast_mapper_to0.ml                 |  8 +++-
 compiler/ml/parsetree.ml                      |  1 +
 compiler/ml/pprintast.ml                      |  7 +--
 compiler/ml/printast.ml                       |  3 +-
 compiler/ml/printtyped.ml                     |  4 +-
 compiler/ml/tast_mapper.ml                    |  5 ++-
 compiler/ml/translcore.ml                     | 13 +++---
 compiler/ml/typecore.ml                       | 24 +++++++---
 compiler/ml/typedtree.ml                      |  1 +
 compiler/ml/typedtree.mli                     |  1 +
 compiler/syntax/src/res_parsetree_viewer.ml   |  8 ++--
 .../expected/UncurriedByDefault.res.txt       |  8 ++--
 .../expressions/expected/async.res.txt        | 45 +++++++++----------
 tests/tools_tests/ppx/TestPpx.res             |  7 +++
 .../src/expected/TestPpx.res.jsout            | 12 +++++
 26 files changed, 124 insertions(+), 84 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index e0d4e2a751..a3936f7618 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -22,6 +22,7 @@
 
 - AST cleanup: Prepare for ast async cleanup: Refactor code for "@res.async" payload handling and clean up handling of type and term parameters, so that now each `=>` in a function definition corresponds to a function. https://github.com/rescript-lang/rescript/pull/7223
 - AST: always put type parameters first in function definitions. https://github.com/rescript-lang/rescript/pull/7233
+- AST cleanup: Remove `@res.async` attribute from the internal representation, and add a flag to untyped and typed ASTs instead. https://github.com/rescript-lang/rescript/pull/7234
 
 #### :house: Internal
 
diff --git a/compiler/frontend/ast_compatible.ml b/compiler/frontend/ast_compatible.ml
index 965c25151b..7dcd98418f 100644
--- a/compiler/frontend/ast_compatible.ml
+++ b/compiler/frontend/ast_compatible.ml
@@ -64,13 +64,20 @@ let app3 ?(loc = default_loc) ?(attrs = []) fn arg1 arg2 arg3 : expression =
       Pexp_apply (fn, [(Nolabel, arg1); (Nolabel, arg2); (Nolabel, arg3)]);
   }
 
-let fun_ ?(loc = default_loc) ?(attrs = []) ~arity pat exp =
+let fun_ ?(loc = default_loc) ?(attrs = []) ?(async = false) ~arity pat exp =
   {
     pexp_loc = loc;
     pexp_attributes = attrs;
     pexp_desc =
       Pexp_fun
-        {arg_label = Nolabel; default = None; lhs = pat; rhs = exp; arity};
+        {
+          arg_label = Nolabel;
+          default = None;
+          lhs = pat;
+          rhs = exp;
+          arity;
+          async;
+        };
   }
 
 let const_exp_string ?(loc = default_loc) ?(attrs = []) ?delimiter (s : string)
diff --git a/compiler/frontend/ast_compatible.mli b/compiler/frontend/ast_compatible.mli
index 185d14c93a..63201f9ef8 100644
--- a/compiler/frontend/ast_compatible.mli
+++ b/compiler/frontend/ast_compatible.mli
@@ -74,6 +74,7 @@ val apply_labels :
 val fun_ :
   ?loc:Location.t ->
   ?attrs:attrs ->
+  ?async:bool ->
   arity:int option ->
   pattern ->
   expression ->
diff --git a/compiler/frontend/ast_uncurry_gen.ml b/compiler/frontend/ast_uncurry_gen.ml
index eae466b275..c055f530a7 100644
--- a/compiler/frontend/ast_uncurry_gen.ml
+++ b/compiler/frontend/ast_uncurry_gen.ml
@@ -36,16 +36,16 @@ let to_method_callback loc (self : Bs_ast_mapper.mapper) label
     match Ast_attributes.process_attributes_rev body.pexp_attributes with
     | Nothing, attrs -> (
       match body.pexp_desc with
-      | Pexp_fun {arg_label; lhs = arg; rhs = body} ->
+      | Pexp_fun {arg_label; lhs = arg; rhs = body; async} ->
         Bs_syntaxerr.optional_err loc arg_label;
-        aux ((arg_label, self.pat self arg, attrs) :: acc) body
+        aux ((arg_label, self.pat self arg, attrs, async) :: acc) body
       | _ -> (self.expr self body, acc))
     | _, _ -> (self.expr self body, acc)
   in
-  let result, rev_extra_args = aux [(label, self_pat, [])] body in
+  let result, rev_extra_args = aux [(label, self_pat, [], false)] body in
   let body =
-    Ext_list.fold_left rev_extra_args result (fun e (label, p, attrs) ->
-        Ast_helper.Exp.fun_ ~loc ~attrs ~arity:None label None p e)
+    Ext_list.fold_left rev_extra_args result (fun e (label, p, attrs, async) ->
+        Ast_helper.Exp.fun_ ~loc ~attrs ~arity:None ~async label None p e)
   in
   let arity = List.length rev_extra_args in
   let arity_s = string_of_int arity in
diff --git a/compiler/frontend/bs_ast_mapper.ml b/compiler/frontend/bs_ast_mapper.ml
index 2c0cf55145..99742c849c 100644
--- a/compiler/frontend/bs_ast_mapper.ml
+++ b/compiler/frontend/bs_ast_mapper.ml
@@ -315,8 +315,9 @@ module E = struct
            sub vbs)
         (sub.expr sub e)
     (* #end *)
-    | Pexp_fun {arg_label = lab; default = def; lhs = p; rhs = e; arity} ->
-      fun_ ~loc ~attrs ~arity lab
+    | Pexp_fun {arg_label = lab; default = def; lhs = p; rhs = e; arity; async}
+      ->
+      fun_ ~loc ~attrs ~arity ~async lab
         (map_opt (sub.expr sub) def)
         (sub.pat sub p) (sub.expr sub e)
     | Pexp_apply (e, l) ->
diff --git a/compiler/frontend/bs_builtin_ppx.ml b/compiler/frontend/bs_builtin_ppx.ml
index 34d7835cec..054a9976a3 100644
--- a/compiler/frontend/bs_builtin_ppx.ml
+++ b/compiler/frontend/bs_builtin_ppx.ml
@@ -113,8 +113,7 @@ let expr_mapper ~async_context ~in_function_def (self : mapper)
   | Pexp_newtype (s, body) ->
     let res = self.expr self body in
     {e with pexp_desc = Pexp_newtype (s, res)}
-  | Pexp_fun {arg_label = label; lhs = pat; rhs = body} -> (
-    let async = Ast_async.has_async_payload e.pexp_attributes in
+  | Pexp_fun {arg_label = label; lhs = pat; rhs = body; async} -> (
     match Ast_attributes.process_attributes_rev e.pexp_attributes with
     | Nothing, _ ->
       (* Handle @async x => y => ... is in async context *)
diff --git a/compiler/ml/ast_async.ml b/compiler/ml/ast_async.ml
index f102660066..bd0b8f48ef 100644
--- a/compiler/ml/ast_async.ml
+++ b/compiler/ml/ast_async.ml
@@ -1,29 +1,16 @@
-let has_async_payload attrs =
-  Ext_list.exists attrs (fun ({Location.txt}, _) -> txt = "res.async")
-
 let rec dig_async_payload_from_function (expr : Parsetree.expression) =
   match expr.pexp_desc with
-  | Pexp_fun _ -> has_async_payload expr.pexp_attributes
+  | Pexp_fun {async} -> async
   | Pexp_newtype (_, body) -> dig_async_payload_from_function body
   | _ -> false
 
 let add_async_attribute ~async (body : Parsetree.expression) =
-  let add (exp : Parsetree.expression) =
-    if has_async_payload exp.pexp_attributes then exp
-    else
-      {
-        exp with
-        pexp_attributes =
-          ({txt = "res.async"; loc = Location.none}, PStr [])
-          :: exp.pexp_attributes;
-      }
-  in
   if async then
     let rec add_to_fun (exp : Parsetree.expression) =
       match exp.pexp_desc with
       | Pexp_newtype (txt, e) ->
         {exp with pexp_desc = Pexp_newtype (txt, add_to_fun e)}
-      | Pexp_fun _ -> add exp
+      | Pexp_fun f -> {exp with pexp_desc = Pexp_fun {f with async}}
       | _ -> exp
     in
     add_to_fun body
diff --git a/compiler/ml/ast_helper.ml b/compiler/ml/ast_helper.ml
index b167349f9a..e9ddd8630b 100644
--- a/compiler/ml/ast_helper.ml
+++ b/compiler/ml/ast_helper.ml
@@ -151,9 +151,9 @@ module Exp = struct
   let ident ?loc ?attrs a = mk ?loc ?attrs (Pexp_ident a)
   let constant ?loc ?attrs a = mk ?loc ?attrs (Pexp_constant a)
   let let_ ?loc ?attrs a b c = mk ?loc ?attrs (Pexp_let (a, b, c))
-  let fun_ ?loc ?attrs ~arity a b c d =
+  let fun_ ?loc ?attrs ?(async = false) ~arity a b c d =
     mk ?loc ?attrs
-      (Pexp_fun {arg_label = a; default = b; lhs = c; rhs = d; arity})
+      (Pexp_fun {arg_label = a; default = b; lhs = c; rhs = d; arity; async})
   let apply ?loc ?attrs a b = mk ?loc ?attrs (Pexp_apply (a, b))
   let match_ ?loc ?attrs a b = mk ?loc ?attrs (Pexp_match (a, b))
   let try_ ?loc ?attrs a b = mk ?loc ?attrs (Pexp_try (a, b))
diff --git a/compiler/ml/ast_helper.mli b/compiler/ml/ast_helper.mli
index a8969d2d08..99b5018583 100644
--- a/compiler/ml/ast_helper.mli
+++ b/compiler/ml/ast_helper.mli
@@ -138,6 +138,7 @@ module Exp : sig
   val fun_ :
     ?loc:loc ->
     ?attrs:attrs ->
+    ?async:bool ->
     arity:int option ->
     arg_label ->
     expression option ->
diff --git a/compiler/ml/ast_mapper.ml b/compiler/ml/ast_mapper.ml
index d2ab209b64..eb08e516f0 100644
--- a/compiler/ml/ast_mapper.ml
+++ b/compiler/ml/ast_mapper.ml
@@ -278,8 +278,9 @@ module E = struct
     | Pexp_constant x -> constant ~loc ~attrs x
     | Pexp_let (r, vbs, e) ->
       let_ ~loc ~attrs r (List.map (sub.value_binding sub) vbs) (sub.expr sub e)
-    | Pexp_fun {arg_label = lab; default = def; lhs = p; rhs = e; arity} ->
-      fun_ ~loc ~attrs ~arity lab
+    | Pexp_fun {arg_label = lab; default = def; lhs = p; rhs = e; arity; async}
+      ->
+      fun_ ~loc ~attrs ~arity ~async lab
         (map_opt (sub.expr sub) def)
         (sub.pat sub p) (sub.expr sub e)
     | Pexp_apply (e, l) ->
diff --git a/compiler/ml/ast_mapper_from0.ml b/compiler/ml/ast_mapper_from0.ml
index da1b3ace49..345354d616 100644
--- a/compiler/ml/ast_mapper_from0.ml
+++ b/compiler/ml/ast_mapper_from0.ml
@@ -304,7 +304,8 @@ module E = struct
     | Pexp_let (r, vbs, e) ->
       let_ ~loc ~attrs r (List.map (sub.value_binding sub) vbs) (sub.expr sub e)
     | Pexp_fun (lab, def, p, e) ->
-      fun_ ~loc ~attrs ~arity:None lab
+      let async = Ext_list.exists attrs (fun ({txt}, _) -> txt = "res.async") in
+      fun_ ~loc ~attrs ~async ~arity:None lab
         (map_opt (sub.expr sub) def)
         (sub.pat sub p) (sub.expr sub e)
     | Pexp_function _ -> assert false
diff --git a/compiler/ml/ast_mapper_to0.ml b/compiler/ml/ast_mapper_to0.ml
index 0064aa6afa..c23c292b9b 100644
--- a/compiler/ml/ast_mapper_to0.ml
+++ b/compiler/ml/ast_mapper_to0.ml
@@ -295,7 +295,13 @@ module E = struct
     | Pexp_constant x -> constant ~loc ~attrs (map_constant x)
     | Pexp_let (r, vbs, e) ->
       let_ ~loc ~attrs r (List.map (sub.value_binding sub) vbs) (sub.expr sub e)
-    | Pexp_fun {arg_label = lab; default = def; lhs = p; rhs = e; arity} -> (
+    | Pexp_fun {arg_label = lab; default = def; lhs = p; rhs = e; arity; async}
+      -> (
+      let attrs =
+        if async then
+          ({txt = "res.async"; loc = Location.none}, Pt.PStr []) :: attrs
+        else attrs
+      in
       let e =
         fun_ ~loc ~attrs lab
           (map_opt (sub.expr sub) def)
diff --git a/compiler/ml/parsetree.ml b/compiler/ml/parsetree.ml
index d4a02a6d56..d4a75b32e3 100644
--- a/compiler/ml/parsetree.ml
+++ b/compiler/ml/parsetree.ml
@@ -230,6 +230,7 @@ and expression_desc =
       lhs: pattern;
       rhs: expression;
       arity: arity;
+      async: bool;
     }
     (* fun P -> E1                          (Simple, None)
        fun ~l:P -> E1                       (Labelled l, None)
diff --git a/compiler/ml/pprintast.ml b/compiler/ml/pprintast.ml
index 838a29f24e..9826b3bc87 100644
--- a/compiler/ml/pprintast.ml
+++ b/compiler/ml/pprintast.ml
@@ -605,14 +605,15 @@ and expression ctxt f x =
     | (Pexp_let _ | Pexp_letmodule _ | Pexp_open _ | Pexp_letexception _)
       when ctxt.semi ->
       paren true (expression reset_ctxt) f x
-    | Pexp_fun {arg_label = l; default = e0; lhs = p; rhs = e; arity} ->
+    | Pexp_fun {arg_label = l; default = e0; lhs = p; rhs = e; arity; async} ->
       let arity_str =
         match arity with
         | None -> ""
         | Some arity -> "[arity:" ^ string_of_int arity ^ "]"
       in
-      pp f "@[<2>fun@;%s%a->@;%a@]" arity_str (label_exp ctxt) (l, e0, p)
-        (expression ctxt) e
+      let async_str = if async then "async " else "" in
+      pp f "@[<2>%sfun@;%s%a->@;%a@]" async_str arity_str (label_exp ctxt)
+        (l, e0, p) (expression ctxt) e
     | Pexp_match (e, l) ->
       pp f "@[<hv0>@[<hv0>@[<2>match %a@]@ with@]%a@]" (expression reset_ctxt) e
         (case_list ctxt) l
diff --git a/compiler/ml/printast.ml b/compiler/ml/printast.ml
index d71f798265..b521072bb8 100644
--- a/compiler/ml/printast.ml
+++ b/compiler/ml/printast.ml
@@ -238,8 +238,9 @@ and expression i ppf x =
     line i ppf "Pexp_let %a\n" fmt_rec_flag rf;
     list i value_binding ppf l;
     expression i ppf e
-  | Pexp_fun {arg_label = l; default = eo; lhs = p; rhs = e; arity} ->
+  | Pexp_fun {arg_label = l; default = eo; lhs = p; rhs = e; arity; async} ->
     line i ppf "Pexp_fun\n";
+    let () = if async then line i ppf "async\n" in
     let () =
       match arity with
       | None -> ()
diff --git a/compiler/ml/printtyped.ml b/compiler/ml/printtyped.ml
index af090ac79c..686caa4083 100644
--- a/compiler/ml/printtyped.ml
+++ b/compiler/ml/printtyped.ml
@@ -285,8 +285,10 @@ and expression i ppf x =
     line i ppf "Texp_let %a\n" fmt_rec_flag rf;
     list i value_binding ppf l;
     expression i ppf e
-  | Texp_function {arg_label = p; arity; param; case = case_; partial = _} ->
+  | Texp_function
+      {arg_label = p; arity; async; param; case = case_; partial = _} ->
     line i ppf "Texp_function\n";
+    if async then line i ppf "async\n";
     (match arity with
     | Some arity -> line i ppf "arity: %d\n" arity
     | None -> ());
diff --git a/compiler/ml/tast_mapper.ml b/compiler/ml/tast_mapper.ml
index bce2002a35..24541f60e8 100644
--- a/compiler/ml/tast_mapper.ml
+++ b/compiler/ml/tast_mapper.ml
@@ -199,8 +199,9 @@ let expr sub x =
     | Texp_let (rec_flag, list, exp) ->
       let rec_flag, list = sub.value_bindings sub (rec_flag, list) in
       Texp_let (rec_flag, list, sub.expr sub exp)
-    | Texp_function {arg_label; arity; param; case; partial} ->
-      Texp_function {arg_label; arity; param; case = sub.case sub case; partial}
+    | Texp_function {arg_label; arity; param; case; partial; async} ->
+      Texp_function
+        {arg_label; arity; param; case = sub.case sub case; partial; async}
     | Texp_apply (exp, list) ->
       Texp_apply
         (sub.expr sub exp, List.map (tuple2 id (opt (sub.expr sub))) list)
diff --git a/compiler/ml/translcore.ml b/compiler/ml/translcore.ml
index 0ff4b76a28..73986b5d57 100644
--- a/compiler/ml/translcore.ml
+++ b/compiler/ml/translcore.ml
@@ -555,7 +555,8 @@ let rec push_defaults loc bindings case partial =
    c_lhs = pat;
    c_guard = None;
    c_rhs =
-     {exp_desc = Texp_function {arg_label; arity; param; case; partial}} as exp;
+     {exp_desc = Texp_function {arg_label; arity; param; case; partial; async}}
+     as exp;
   } ->
     let case = push_defaults exp.exp_loc bindings case partial in
 
@@ -565,7 +566,8 @@ let rec push_defaults loc bindings case partial =
       c_rhs =
         {
           exp with
-          exp_desc = Texp_function {arg_label; arity; param; case; partial};
+          exp_desc =
+            Texp_function {arg_label; arity; param; case; partial; async};
         };
     }
   | {
@@ -671,8 +673,7 @@ and transl_exp0 (e : Typedtree.expression) : Lambda.lambda =
   | Texp_constant cst -> Lconst (Const_base cst)
   | Texp_let (rec_flag, pat_expr_list, body) ->
     transl_let rec_flag pat_expr_list (transl_exp body)
-  | Texp_function {arg_label = _; arity; param; case; partial} -> (
-    let async = Ast_async.has_async_payload e.exp_attributes in
+  | Texp_function {arg_label = _; arity; param; case; partial; async} -> (
     let directive =
       match extract_directive_for_fn e with
       | None -> None
@@ -1050,11 +1051,11 @@ and transl_function loc partial param case =
              param = param';
              case;
              partial = partial';
+             async;
            };
      } as exp;
   }
-    when Parmatch.inactive ~partial pat
-         && not (Ast_async.has_async_payload exp.exp_attributes) ->
+    when Parmatch.inactive ~partial pat && not async ->
     let params, body, return_unit =
       transl_function exp.exp_loc partial' param' case
     in
diff --git a/compiler/ml/typecore.ml b/compiler/ml/typecore.ml
index 5253cb557e..43d834a8e2 100644
--- a/compiler/ml/typecore.ml
+++ b/compiler/ml/typecore.ml
@@ -2364,7 +2364,14 @@ and type_expect_ ?type_clash_context ?in_function ?(recarg = Rejected) env sexp
         exp_env = env;
       }
   | Pexp_fun
-      {arg_label = l; default = Some default; lhs = spat; rhs = sbody; arity} ->
+      {
+        arg_label = l;
+        default = Some default;
+        lhs = spat;
+        rhs = sbody;
+        arity;
+        async;
+      } ->
     assert (is_optional l);
     (* default allowed only with optional argument *)
     let open Ast_helper in
@@ -2402,10 +2409,13 @@ and type_expect_ ?type_clash_context ?in_function ?(recarg = Rejected) env sexp
         [Vb.mk spat smatch]
         sbody
     in
-    type_function ?in_function ~arity loc sexp.pexp_attributes env ty_expected l
+    type_function ?in_function ~arity ~async loc sexp.pexp_attributes env
+      ty_expected l
       [Exp.case pat body]
-  | Pexp_fun {arg_label = l; default = None; lhs = spat; rhs = sbody; arity} ->
-    type_function ?in_function ~arity loc sexp.pexp_attributes env ty_expected l
+  | Pexp_fun
+      {arg_label = l; default = None; lhs = spat; rhs = sbody; arity; async} ->
+    type_function ?in_function ~arity ~async loc sexp.pexp_attributes env
+      ty_expected l
       [Ast_helper.Exp.case spat sbody]
   | Pexp_apply (sfunct, sargs) ->
     assert (sargs <> []);
@@ -3246,7 +3256,8 @@ and type_expect_ ?type_clash_context ?in_function ?(recarg = Rejected) env sexp
   | Pexp_extension ext ->
     raise (Error_forward (Builtin_attributes.error_of_extension ext))
 
-and type_function ?in_function ~arity loc attrs env ty_expected_ l caselist =
+and type_function ?in_function ~arity ~async loc attrs env ty_expected_ l
+    caselist =
   let state = Warnings.backup () in
   (* Disable Unerasable_optional_argument for uncurried functions *)
   let unerasable_optional_argument =
@@ -3304,7 +3315,8 @@ and type_function ?in_function ~arity loc attrs env ty_expected_ l caselist =
   Warnings.restore state;
   re
     {
-      exp_desc = Texp_function {arg_label = l; arity; param; case; partial};
+      exp_desc =
+        Texp_function {arg_label = l; arity; param; case; partial; async};
       exp_loc = loc;
       exp_extra = [];
       exp_type;
diff --git a/compiler/ml/typedtree.ml b/compiler/ml/typedtree.ml
index 43d13cf450..c3af7028fd 100644
--- a/compiler/ml/typedtree.ml
+++ b/compiler/ml/typedtree.ml
@@ -82,6 +82,7 @@ and expression_desc =
       param: Ident.t;
       case: case;
       partial: partial;
+      async: bool;
     }
   | Texp_apply of expression * (arg_label * expression option) list
   | Texp_match of expression * case list * case list * partial
diff --git a/compiler/ml/typedtree.mli b/compiler/ml/typedtree.mli
index d6c6988078..2aabedb4aa 100644
--- a/compiler/ml/typedtree.mli
+++ b/compiler/ml/typedtree.mli
@@ -136,6 +136,7 @@ and expression_desc =
       param: Ident.t;
       case: case;
       partial: partial;
+      async: bool;
     }
       (** [Pexp_fun] and [Pexp_function] both translate to [Texp_function].
             See {!Parsetree} for more details.
diff --git a/compiler/syntax/src/res_parsetree_viewer.ml b/compiler/syntax/src/res_parsetree_viewer.ml
index 7331d89ef4..a2578e5ade 100644
--- a/compiler/syntax/src/res_parsetree_viewer.ml
+++ b/compiler/syntax/src/res_parsetree_viewer.ml
@@ -214,7 +214,7 @@ let filter_parsing_attrs attrs =
       | ( {
             Location.txt =
               ( "res.braces" | "ns.braces" | "res.iflet" | "res.namedArgLoc"
-              | "res.ternary" | "res.async" | "res.await" | "res.template"
+              | "res.ternary" | "res.await" | "res.template"
               | "res.taggedTemplate" | "res.patVariantSpread"
               | "res.dictPattern" );
           },
@@ -365,7 +365,7 @@ let has_attributes attrs =
       | ( {
             Location.txt =
               ( "res.braces" | "ns.braces" | "res.iflet" | "res.ternary"
-              | "res.async" | "res.await" | "res.template" );
+              | "res.await" | "res.template" );
           },
           _ ) ->
         false
@@ -548,8 +548,8 @@ let is_printable_attribute attr =
   match attr with
   | ( {
         Location.txt =
-          ( "res.iflet" | "res.braces" | "ns.braces" | "JSX" | "res.async"
-          | "res.await" | "res.template" | "res.ternary" );
+          ( "res.iflet" | "res.braces" | "ns.braces" | "JSX" | "res.await"
+          | "res.template" | "res.ternary" );
       },
       _ ) ->
     false
diff --git a/tests/syntax_tests/data/parsing/grammar/expressions/expected/UncurriedByDefault.res.txt b/tests/syntax_tests/data/parsing/grammar/expressions/expected/UncurriedByDefault.res.txt
index d2a3bf9c58..2b8c88feb1 100644
--- a/tests/syntax_tests/data/parsing/grammar/expressions/expected/UncurriedByDefault.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/expressions/expected/UncurriedByDefault.res.txt
@@ -37,9 +37,9 @@ type nonrec unested = (string -> unit (a:1)) -> unit (a:1)
 let (uannpoly : 'a -> string (a:1)) = xx
 let (uannint : int -> string (a:1)) = xx
 let _ = ((fun [arity:1]x -> 34)[@att ])
-let _ = ((fun [arity:1]x -> 34)[@res.async ][@att ])
+let _ = ((async fun [arity:1]x -> 34)[@att ])
 let _ = preserveAttr ((fun [arity:1]x -> 34)[@att ])
-let _ = preserveAttr ((fun [arity:1]x -> 34)[@res.async ][@att ])
+let _ = preserveAttr ((async fun [arity:1]x -> 34)[@att ])
 let t0 (type a) (type b) [arity:2](l : a list) (x : a) = x :: l
 let t1 (type a) (type b) [arity:2](l : a list) (x : a) = x :: l
 let t2 (type a) (type b) [arity:2](l : a list) (x : a) = x :: l
@@ -95,9 +95,9 @@ let pipe1 = 3 |.u f
 let (uannpoly : 'a -> string (a:1)) = xx
 let (uannint : int -> string (a:1)) = xx
 let _ = ((fun [arity:1]x -> 34)[@att ])
-let _ = ((fun [arity:1]x -> 34)[@res.async ][@att ])
+let _ = ((async fun [arity:1]x -> 34)[@att ])
 let _ = preserveAttr ((fun [arity:1]x -> 34)[@att ])
-let _ = preserveAttr ((fun [arity:1]x -> 34)[@res.async ][@att ])
+let _ = preserveAttr ((async fun [arity:1]x -> 34)[@att ])
 let t0 (type a) (type b) [arity:2](l : a list) (x : a) = x :: l
 let t1 (type a) (type b) [arity:2](l : a list) (x : a) = x :: l
 let t2 (type a) (type b) [arity:2](l : a list) (x : a) = x :: l
diff --git a/tests/syntax_tests/data/parsing/grammar/expressions/expected/async.res.txt b/tests/syntax_tests/data/parsing/grammar/expressions/expected/async.res.txt
index 532bd33f9e..3410ad4918 100644
--- a/tests/syntax_tests/data/parsing/grammar/expressions/expected/async.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/expressions/expected/async.res.txt
@@ -1,17 +1,12 @@
-let greetUser =
-  ((fun [arity:1]userId ->
-      ((let name = ((getUserName userId)[@res.await ]) in
-        ({js|Hello |js} ^ name) ^ {js|!|js})
-      [@res.braces ]))
-  [@res.async ])
-;;((fun [arity:1]() -> 123)[@res.async ])
-let fetch = ((fun [arity:1]url -> browserFetch url)
-  [@res.braces ][@res.async ])
+let greetUser [arity:1]userId =
+  ((let name = ((getUserName userId)[@res.await ]) in
+    ({js|Hello |js} ^ name) ^ {js|!|js})
+  [@res.braces ])
+;;async fun [arity:1]() -> 123
+let fetch = ((async fun [arity:1]url -> browserFetch url)[@res.braces ])
 let fetch2 =
-  (((((fun [arity:1]url -> browserFetch url))
-    [@res.async ]);
-    (((fun [arity:1]url -> browserFetch2 url))
-    [@res.async ]))
+  (((async fun [arity:1]url -> browserFetch url);
+    (async fun [arity:1]url -> browserFetch2 url))
   [@res.braces ])
 let async =
   ((let f = async () in
@@ -23,27 +18,27 @@ let async =
   [@res.braces ])
 let f =
   ((if isPositive
-    then ((fun [arity:2]a -> fun b -> (a + b : int))[@res.async ])
-    else (((fun [arity:2]c -> fun d -> (c - d : int)))[@res.async ]))
+    then async fun [arity:2]a -> fun b -> (a + b : int)
+    else (async fun [arity:2]c -> fun d -> (c - d : int)))
   [@res.ternary ])
 let foo = async ~a:((34)[@res.namedArgLoc ])
-let bar = ((fun [arity:1]~a:((a)[@res.namedArgLoc ]) -> a + 1)[@res.async ])
+let bar [arity:1]~a:((a)[@res.namedArgLoc ])  = a + 1
 let ex1 = ((3)[@res.await ]) + ((4)[@res.await ])
 let ex2 = ((3)[@res.await ]) ** ((4)[@res.await ])
 let ex3 = ((foo |.u (bar ~arg:((arg)[@res.namedArgLoc ])))[@res.await ])
 let ex4 = (((foo.bar).baz)[@res.await ])
-let attr1 = ((fun [arity:1]x -> x + 1)[@res.async ][@a ])
+let attr1 = ((async fun [arity:1]x -> x + 1)[@a ])
 let attr2 = ((fun (type a) ->
-  ((fun [arity:1]() -> fun (type b) -> fun (type c) -> fun [arity:1]x -> 3)
-  [@res.async ]))[@a ])
+  async fun [arity:1]() -> fun (type b) -> fun (type c) ->
+    fun [arity:1]x -> 3)
+  [@a ])
 let attr3 = ((fun (type a) ->
-  fun [arity:1]() -> fun (type b) -> fun (type c) -> ((fun [arity:1]x -> 3)
-    [@res.async ]))
+  fun [arity:1]() -> fun (type b) -> fun (type c) ->
+    async fun [arity:1]x -> 3)
   [@a ])
 let attr4 = ((fun (type a) ->
-  fun [arity:1]() -> ((fun (type b) -> fun (type c) -> ((fun [arity:1]x -> 3)
-    [@res.async ]))[@b ]))
+  fun [arity:1]() -> ((fun (type b) -> fun (type c) ->
+    async fun [arity:1]x -> 3)[@b ]))
   [@a ])
 let (attr5 : int) = ((fun (type a) -> fun (type b) -> fun (type c) ->
-  ((fun [arity:1]() -> fun [arity:1](x : a) -> x)[@res.async ]))[@a ]
-  [@b ])
\ No newline at end of file
+  async fun [arity:1]() -> fun [arity:1](x : a) -> x)[@a ][@b ])
\ No newline at end of file
diff --git a/tests/tools_tests/ppx/TestPpx.res b/tests/tools_tests/ppx/TestPpx.res
index a3288f33e2..641821fb6d 100644
--- a/tests/tools_tests/ppx/TestPpx.res
+++ b/tests/tools_tests/ppx/TestPpx.res
@@ -35,3 +35,10 @@ module Uncurried = {
   type f1 = int => string
   type f2 = (int, int) => string
 }
+
+let async_succ = async x => x + 1
+let async_foo = async (x, y) => {
+  let a: promise<int> = async_succ(x)
+  let b: promise<int> = async_succ(y)
+  (await a) + (await b)
+}
diff --git a/tests/tools_tests/src/expected/TestPpx.res.jsout b/tests/tools_tests/src/expected/TestPpx.res.jsout
index 8235042c40..bcd68f7e14 100644
--- a/tests/tools_tests/src/expected/TestPpx.res.jsout
+++ b/tests/tools_tests/src/expected/TestPpx.res.jsout
@@ -41,6 +41,16 @@ async function fpromise(promise, _x) {
 
 let Uncurried = {};
 
+async function async_succ(x) {
+  return x + 1 | 0;
+}
+
+async function async_foo(x, y) {
+  let a = async_succ(x);
+  let b = async_succ(y);
+  return await a + await b | 0;
+}
+
 let a = "A";
 
 let b = "B";
@@ -55,4 +65,6 @@ exports.OptionalFields = OptionalFields;
 exports.Arity = Arity;
 exports.fpromise = fpromise;
 exports.Uncurried = Uncurried;
+exports.async_succ = async_succ;
+exports.async_foo = async_foo;
 /*  Not a pure module */

From 56e5aea37ac4a4417a808fc48de37f276808143b Mon Sep 17 00:00:00 2001
From: Cristiano Calcagno <cristianoc@users.noreply.github.com>
Date: Wed, 8 Jan 2025 17:12:01 +0100
Subject: [PATCH 3/3] Simplify ast async: add async flag directly where it's
 needed.

---
 CHANGELOG.md                                       |  5 -----
 compiler/ml/ast_async.ml                           | 12 ------------
 compiler/ml/ast_uncurried.ml                       |  4 ++--
 compiler/ml/pprintast.ml                           |  8 +++++---
 compiler/syntax/src/jsx_v4.ml                      |  3 +--
 compiler/syntax/src/res_core.ml                    | 14 ++++++++------
 .../grammar/expressions/expected/async.res.txt     |  4 ++--
 7 files changed, 18 insertions(+), 32 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index a3936f7618..8f81f0d1ab 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -24,11 +24,6 @@
 - AST: always put type parameters first in function definitions. https://github.com/rescript-lang/rescript/pull/7233
 - AST cleanup: Remove `@res.async` attribute from the internal representation, and add a flag to untyped and typed ASTs instead. https://github.com/rescript-lang/rescript/pull/7234
 
-#### :house: Internal
-
-- AST cleanup: Prepare for ast async cleanup: Refactor code for "@res.async" payload handling and clean up handling of type and term parameters, so that now each `=>` in a function definition corresponds to a function. https://github.com/rescript-lang/rescript/pull/7223
-
-
 # 12.0.0-alpha.7
 
 #### :bug: Bug fix
diff --git a/compiler/ml/ast_async.ml b/compiler/ml/ast_async.ml
index bd0b8f48ef..d5494ebfba 100644
--- a/compiler/ml/ast_async.ml
+++ b/compiler/ml/ast_async.ml
@@ -4,18 +4,6 @@ let rec dig_async_payload_from_function (expr : Parsetree.expression) =
   | Pexp_newtype (_, body) -> dig_async_payload_from_function body
   | _ -> false
 
-let add_async_attribute ~async (body : Parsetree.expression) =
-  if async then
-    let rec add_to_fun (exp : Parsetree.expression) =
-      match exp.pexp_desc with
-      | Pexp_newtype (txt, e) ->
-        {exp with pexp_desc = Pexp_newtype (txt, add_to_fun e)}
-      | Pexp_fun f -> {exp with pexp_desc = Pexp_fun {f with async}}
-      | _ -> exp
-    in
-    add_to_fun body
-  else body
-
 let add_promise_type ?(loc = Location.none) ~async
     (result : Parsetree.expression) =
   if async then
diff --git a/compiler/ml/ast_uncurried.ml b/compiler/ml/ast_uncurried.ml
index 98b898675b..9834af1087 100644
--- a/compiler/ml/ast_uncurried.ml
+++ b/compiler/ml/ast_uncurried.ml
@@ -6,11 +6,11 @@ let uncurried_type ~arity (t_arg : Parsetree.core_type) =
     {t_arg with ptyp_desc = Ptyp_arrow (l, t1, t2, Some arity)}
   | _ -> assert false
 
-let uncurried_fun ~arity fun_expr =
+let uncurried_fun ?(async = false) ~arity fun_expr =
   let fun_expr =
     match fun_expr.Parsetree.pexp_desc with
     | Pexp_fun f ->
-      {fun_expr with pexp_desc = Pexp_fun {f with arity = Some arity}}
+      {fun_expr with pexp_desc = Pexp_fun {f with arity = Some arity; async}}
     | _ -> assert false
   in
   fun_expr
diff --git a/compiler/ml/pprintast.ml b/compiler/ml/pprintast.ml
index 9826b3bc87..ea826c2689 100644
--- a/compiler/ml/pprintast.ml
+++ b/compiler/ml/pprintast.ml
@@ -993,17 +993,19 @@ and binding ctxt f {pvb_pat = p; pvb_expr = x; _} =
     if x.pexp_attributes <> [] then pp f "=@;%a" (expression ctxt) x
     else
       match x.pexp_desc with
-      | Pexp_fun {arg_label = label; default = eo; lhs = p; rhs = e; arity} ->
+      | Pexp_fun
+          {arg_label = label; default = eo; lhs = p; rhs = e; arity; async} ->
         let arity_str =
           match arity with
           | None -> ""
           | Some arity -> "[arity:" ^ string_of_int arity ^ "]"
         in
+        let async_str = if async then "async " else "" in
         if label = Nolabel then
-          pp f "%s%a@ %a" arity_str (simple_pattern ctxt) p
+          pp f "%s%s%a@ %a" async_str arity_str (simple_pattern ctxt) p
             pp_print_pexp_function e
         else
-          pp f "%s%a@ %a" arity_str (label_exp ctxt) (label, eo, p)
+          pp f "%s%s%a@ %a" async_str arity_str (label_exp ctxt) (label, eo, p)
             pp_print_pexp_function e
       | Pexp_newtype (str, e) ->
         pp f "(type@ %s)@ %a" str.txt pp_print_pexp_function e
diff --git a/compiler/syntax/src/jsx_v4.ml b/compiler/syntax/src/jsx_v4.ml
index c0b31afe31..e4c853e31a 100644
--- a/compiler/syntax/src/jsx_v4.ml
+++ b/compiler/syntax/src/jsx_v4.ml
@@ -1125,7 +1125,7 @@ let map_binding ~config ~empty_loc ~pstr_loc ~file_name ~rec_flag binding =
       | _ -> Pat.record (List.rev patterns_with_label) Open
     in
     let expression =
-      Exp.fun_ ~arity:(Some 1) Nolabel None
+      Exp.fun_ ~arity:(Some 1) ~async:is_async Nolabel None
         (Pat.constraint_ record_pattern
            (Typ.constr ~loc:empty_loc
               {txt = Lident "props"; loc = empty_loc}
@@ -1140,7 +1140,6 @@ let map_binding ~config ~empty_loc ~pstr_loc ~file_name ~rec_flag binding =
                 | _ -> [Typ.any ()]))))
         expression
     in
-    let expression = Ast_async.add_async_attribute ~async:is_async expression in
     let expression =
       (* Add new tupes (type a,b,c) to make's definition *)
       newtypes
diff --git a/compiler/syntax/src/res_core.ml b/compiler/syntax/src/res_core.ml
index 7a1fbeb610..8c3095ebb0 100644
--- a/compiler/syntax/src/res_core.ml
+++ b/compiler/syntax/src/res_core.ml
@@ -1537,7 +1537,7 @@ and parse_ternary_expr left_operand p =
   | _ -> left_operand
 
 and parse_es6_arrow_expression ?(arrow_attrs = []) ?(arrow_start_pos = None)
-    ?context ?term_parameters p =
+    ?context ?term_parameters ~async p =
   let start_pos = p.Parser.start_pos in
   Parser.leave_breadcrumb p Grammar.Es6ArrowExpr;
   (* Parsing function parameters and attributes:
@@ -1609,7 +1609,9 @@ and parse_es6_arrow_expression ?(arrow_attrs = []) ?(arrow_start_pos = None)
       term_parameters body
   in
   let arrow_expr =
-    Ast_uncurried.uncurried_fun ~arity:(List.length term_parameters) arrow_expr
+    Ast_uncurried.uncurried_fun
+      ~arity:(List.length term_parameters)
+      ~async arrow_expr
   in
   let arrow_expr =
     match type_param_opt with
@@ -2159,7 +2161,7 @@ and parse_operand_expr ~context p =
       then
         let arrow_attrs = !attrs in
         let () = attrs := [] in
-        parse_es6_arrow_expression ~arrow_attrs ~context p
+        parse_es6_arrow_expression ~async:false ~arrow_attrs ~context p
       else parse_unary_expr p
   in
   (* let endPos = p.Parser.prevEndPos in *)
@@ -3000,7 +3002,7 @@ and parse_braced_or_record_expr p =
         let loc = mk_loc start_pos ident_end_pos in
         let ident = Location.mkloc (Longident.last path_ident.txt) loc in
         let a =
-          parse_es6_arrow_expression
+          parse_es6_arrow_expression ~async:false
             ~term_parameters:
               [
                 {
@@ -3303,8 +3305,8 @@ and parse_expr_block ?first p =
 and parse_async_arrow_expression ?(arrow_attrs = []) p =
   let start_pos = p.Parser.start_pos in
   Parser.expect (Lident "async") p;
-  Ast_async.add_async_attribute ~async:true
-    (parse_es6_arrow_expression ~arrow_attrs ~arrow_start_pos:(Some start_pos) p)
+  parse_es6_arrow_expression ~async:true ~arrow_attrs
+    ~arrow_start_pos:(Some start_pos) p
 
 and parse_await_expression p =
   let await_loc = mk_loc p.Parser.start_pos p.end_pos in
diff --git a/tests/syntax_tests/data/parsing/grammar/expressions/expected/async.res.txt b/tests/syntax_tests/data/parsing/grammar/expressions/expected/async.res.txt
index 3410ad4918..0601fabd7b 100644
--- a/tests/syntax_tests/data/parsing/grammar/expressions/expected/async.res.txt
+++ b/tests/syntax_tests/data/parsing/grammar/expressions/expected/async.res.txt
@@ -1,4 +1,4 @@
-let greetUser [arity:1]userId =
+let greetUser async [arity:1]userId =
   ((let name = ((getUserName userId)[@res.await ]) in
     ({js|Hello |js} ^ name) ^ {js|!|js})
   [@res.braces ])
@@ -22,7 +22,7 @@ let f =
     else (async fun [arity:2]c -> fun d -> (c - d : int)))
   [@res.ternary ])
 let foo = async ~a:((34)[@res.namedArgLoc ])
-let bar [arity:1]~a:((a)[@res.namedArgLoc ])  = a + 1
+let bar async [arity:1]~a:((a)[@res.namedArgLoc ])  = a + 1
 let ex1 = ((3)[@res.await ]) + ((4)[@res.await ])
 let ex2 = ((3)[@res.await ]) ** ((4)[@res.await ])
 let ex3 = ((foo |.u (bar ~arg:((arg)[@res.namedArgLoc ])))[@res.await ])