improver the error message ocaml#4414

Author: bobzhang

typing/typecore.ml

@@ -5066,6 +5066,10 @@ let report_error env ppf = function
  fprintf ppf "it should have type@ %a"
  type_expr ty
  end else begin
+ match ty with 
+ | {desc = Tconstr (Pdot (Pdot(Pident {name = "Js"},"Fn",_),_,_),_,_)} -> 
+ fprintf ppf "This expression is excpeted to have an uncurried function"
+ | _ -> 
  fprintf ppf "This expression should not be a function,@ ";
  fprintf ppf "the expected type is@ %a"
  type_expr ty