Hint about partial application in error where required labels are missing (#7807)

* hint about partial application

* examplify partial application

* changelog

Author: zth

CHANGELOG.md

@@ -33,6 +33,7 @@
 - Make parser less strict around leading attributes. https://github.com/rescript-lang/rescript/pull/7787
 - Dedicated error message for ternary type mismatch. https://github.com/rescript-lang/rescript/pull/7804
 - Dedicated error message for passing a braced ident to something expected to be a record. https://github.com/rescript-lang/rescript/pull/7806
+- Hint about partial application when missing required argument in function call. https://github.com/rescript-lang/rescript/pull/7807
 
 #### :house: Internal
 

compiler/ml/typecore.ml

@@ -4667,12 +4667,18 @@ let report_error env loc ppf error =
 
     if not is_fallback then fprintf ppf "@,";
 
-    if List.length missing_required_args > 0 then
+    if List.length missing_required_args > 0 then (
       fprintf ppf "@,- Missing arguments that must be provided: %s"
         (missing_required_args
         |> List.map (fun v -> "~" ^ v)
         |> String.concat ", ");
 
+      fprintf ppf
+        "@,\
+         - Hint: Did you want to partially apply the function? You can do that \
+         by putting `...` just before the closing parens of the function call. \
+         Example: @{<info>yourFn(~arg1=someVar, ...)@}");
+
     if List.length superfluous_args > 0 then
       fprintf ppf "@,- Called with arguments it does not take: %s"
         (superfluous_args |> String.concat ", ");

tests/build_tests/super_errors/expected/moreArguments1.res.expected

@@ -10,4 +10,5 @@
   The function has type:
   (~a: int, ~b: int) => int
 
-  - Missing arguments that must be provided: ~b
+  - Missing arguments that must be provided: ~b
+  - Hint: Did you want to partially apply the function? You can do that by putting `...` just before the closing parens of the function call. Example: yourFn(~arg1=someVar, ...)

tests/build_tests/super_errors/expected/moreArguments4.res.expected

@@ -10,4 +10,5 @@
   The function has type:
   (int, ~b: int, ~c: 'a, ~d: 'b) => int
 
-  - Missing arguments that must be provided: ~d, ~c, ~b
+  - Missing arguments that must be provided: ~d, ~c, ~b
+  - Hint: Did you want to partially apply the function? You can do that by putting `...` just before the closing parens of the function call. Example: yourFn(~arg1=someVar, ...)