result / optional return type modifiers for functions (one for all returns)

corpus/test.langexp

@@ -219,8 +219,12 @@ parse_number : Unit! = {
   bring ();
 }
 
-: example of or_in and or_out usage for template operators
+: example of or_in and or_out usage for template operators (tuple input and tuple output)
-( & ) |-> 'a |-> 'b <-| 'x <-| 'y = (|-> 'a := <-| 'x) && (|-> 'b := <-| 'y);
+( & ) |-> 'a |-> 'b <-| 'x <-| 'y = ('a := 'x) && ('b := 'y);
+
+: function, that return result ('!' not used on calls)
+: useful when tuples returned
+result_func! 'a 'b -> 'c -> 'd = ?? 'a == 0 => error "some error" !!=> 'c := 'a, 'd := 'b, ()
 
 --------------------------------------------------------------------------------
 
@@ -1194,15 +1198,46 @@ parse_number : Unit! = {
     (argument_name_identifier)
     (operator_expression
       (match
-        (reference_expression
+        (name_expression
           (argument_name_identifier))
         (case
-          (reference_expression
+          (name_expression
             (argument_name_identifier))))
       (operator)
       (match
-        (reference_expression
+        (name_expression
           (argument_name_identifier))
         (case
-          (reference_expression
+          (name_expression
-            (argument_name_identifier)))))))
+            (argument_name_identifier))))))
+  (empty_lines)
+  (ERROR
+    (definition_info
+      (info)
+      (info))
+    (simple_name_identifier)
+    (argument_name_identifier)
+    (argument_name_identifier)
+    (argument_name_identifier)
+    (argument_name_identifier)
+    (comma_expression
+      (condition
+        (operator_expression
+          (name_expression
+            (argument_name_identifier))
+          (operator)
+          (number_literal))
+        (name_expression
+          (simple_name_identifier)
+          (string_literal))
+        (name_expression
+          (argument_name_identifier)))
+      (ERROR
+        (argument_name_identifier))
+      (match
+        (name_expression
+          (argument_name_identifier))
+        (case
+          (name_expression
+            (argument_name_identifier)))))
+    (unit_literal)))

grammar.js

@@ -56,6 +56,7 @@ module.exports = grammar({
       repeat(seq($.constraint, ';')),
       optional($._var_let),
       choice(field('name', $.simple_name_identifier), seq('(', field('name', $.operator), ')')),
+      optional($._optional_result),
       repeat(seq(
         optional($.annotation_identifier),
         optional($._reference),