Introduce automatic curried application.

Using the uncurried-always mode is difficult when calling compiler libraries, as those are uncurried.
This introduces automatic curried application in uncurried-only mode: when a curried function is used in application, the application is treated as curried application.
This means that even in uncurried-always mode, one can use compiler libraries in most cases. Exceptions are functions that take callbacks, such as `Array.map`.

Author: cristianoc

jscomp/bsb/bsb_ninja_rule.ml

@@ -102,6 +102,10 @@ let make_custom_rules ~(gentype_config : Bsb_config_types.gentype_config)
       since the default is already good -- it does not*)
   let buf = Ext_buffer.create 100 in
   let ns_flag = match namespace with None -> "" | Some n -> " -bs-ns " ^ n in
+  let add_uncurried_flag = function
+    | Res_uncurried.Legacy -> ()
+    | Default -> Ext_buffer.add_string buf " -uncurried default"
+    | Always -> Ext_buffer.add_string buf " -uncurried always" in
   let mk_ml_cmj_cmd ~(read_cmi : [ `yes | `is_cmi | `no ]) ~is_dev ~postbuild :
       string =
     Ext_buffer.clear buf;
@@ -121,6 +125,7 @@ let make_custom_rules ~(gentype_config : Bsb_config_types.gentype_config)
     (match gentype_config with
     | false -> ()
     | true -> Ext_buffer.add_string buf " -bs-gentype");
+    add_uncurried_flag uncurried;
     if read_cmi <> `is_cmi then (
       Ext_buffer.add_string buf " -bs-package-name ";
       Ext_buffer.add_string buf (Ext_filename.maybe_quote package_name);
@@ -171,10 +176,7 @@ let make_custom_rules ~(gentype_config : Bsb_config_types.gentype_config)
     | None -> ()
     | Some Classic -> Ext_buffer.add_string buf " -bs-jsx-mode classic"
     | Some Automatic -> Ext_buffer.add_string buf " -bs-jsx-mode automatic");
-    (match uncurried with
-    | Legacy -> ()
-    | Default -> Ext_buffer.add_string buf " -uncurried default"
-    | Always -> Ext_buffer.add_string buf " -uncurried always");
+    add_uncurried_flag uncurried;
 
     Ext_buffer.add_char_string buf ' ' bsc_flags;
     Ext_buffer.add_string buf " -absname -bs-ast -o $out $i";

jscomp/bsc/rescript_compiler_main.ml

@@ -406,7 +406,7 @@ let buckle_script_flags : (string * Bsc_args.spec * string) array =
     "-uncurried", string_call (fun i ->
       match i with
       | "default" -> Res_uncurried.init := Default
-      | "always" -> Res_uncurried.init := Always
+      | "always" -> Res_uncurried.init := Always; Config.use_automatic_curried_application := true
       | "legacy" -> Res_uncurried.init := Legacy
       | _ -> Bsc_args.bad_arg (" Not supported -uncurried option : " ^ i)
   ),

jscomp/build_tests/uncurried/src/UncurriedAlways.res

@@ -8,4 +8,6 @@ let b = bar(3, 4)
 
 let w = 3->foo(4)
 
-let a = 3->foo(. 4)
+let a = 3->foo(. 4)
+
+Js.log(a) // Test automatic uncurried applicatin

jscomp/ext/config.ml

@@ -13,6 +13,8 @@ let bs_only = ref true
 
 let unsafe_empty_array = ref false
 
+let use_automatic_curried_application = ref false
+
 and cmi_magic_number = "Caml1999I022"
 
 and ast_impl_magic_number = "Caml1999M022"

jscomp/ext/config.mli

@@ -46,3 +46,5 @@ val cmt_magic_number : string
 (* Magic number for compiled interface files *)
 
 val print_config : out_channel -> unit
+
+val use_automatic_curried_application : bool ref

jscomp/ml/typecore.ml

@@ -2016,7 +2016,9 @@ and type_expect_ ?in_function ?(recarg=Rejected) env sexp ty_expected =
       end_def ();
       wrap_trace_gadt_instances env (lower_args env []) ty;
       begin_def ();
-      let uncurried = Ext_list.exists sexp.pexp_attributes (fun ({txt },_) -> txt = "res.uapp") in
+      let uncurried =
+        Ext_list.exists sexp.pexp_attributes (fun ({txt },_) -> txt = "res.uapp")
+        && not (is_automatic_curried_application env funct) in
       let (args, ty_res, fully_applied) = type_application uncurried env funct sargs in
       end_def ();
       unify_var env (newvar()) funct.exp_type;
@@ -2975,7 +2977,12 @@ and type_argument ?recarg env sarg ty_expected' ty_expected =
       let texp = type_expect ?recarg env sarg ty_expected' in
       unify_exp env texp ty_expected;
       texp
-
+and is_automatic_curried_application env funct =
+  (* When a curried function is used with uncurried application, treat it as a curried application *)
+  !Config.use_automatic_curried_application &&
+  match (expand_head env funct.exp_type).desc with
+  | Tarrow _ -> true
+  | _ -> false
 and type_application uncurried env funct (sargs : sargs) : targs * Types.type_expr * bool =
   (* funct.exp_type may be generic *)
   let result_type omitted ty_fun =