Fix arrow_function_expression

- Arrow functions cannot contain open tuples, so they should only allow
  closed assignments (like `let_statement`).
- Rename `function_expression` to `arrow_function_expression`.

Author: savq

grammar.js

@@ -223,7 +223,7 @@ module.exports = grammar({
       $._operation,
       $.compound_assignment_expression,
       $.macrocall_expression,
-      $.function_expression,
+      $.arrow_function_expression,
       $.juxtaposition_expression,
       $.ternary_expression,
       $.operator,
@@ -864,7 +864,7 @@ module.exports = grammar({
       $._primary_expression,
     )),
 
-    function_expression: $ => prec.right(PREC.afunc, seq(
+    arrow_function_expression: $ => prec.right(PREC.afunc, seq(
       choice(
         $.identifier,
         $.argument_list,
@@ -873,7 +873,7 @@ module.exports = grammar({
       '->',
       choice(
         $._expression,
-        $.assignment,
+        alias($._closed_assignment, $.assignment),
       ),
     )),
 

queries/highlights.scm

@@ -234,17 +234,17 @@
   ] @keyword.type)
 
 ; Operators & Punctuation
-[
-  "->"
-  (operator)
-] @operator
+(operator) @operator
 
 (adjoint_expression
   "'" @operator)
 
 (range_expression
   ":" @operator)
 
+(arrow_function_expression
+  "->" @operator)
+
 [
   "."
   "..."

test/corpus/expressions.txt

@@ -515,14 +515,15 @@ x -> x^2
 (x,y,z)-> 2*x + y - z
 ()->3
 () -> (sleep(0.1); i += 1; l)
+a -> a = 2, 3
 
 ---
 
 (source_file
-  (function_expression
+  (arrow_function_expression
     (identifier)
     (binary_expression (identifier) (operator) (integer_literal)))
-  (function_expression
+  (arrow_function_expression
     (argument_list (identifier) (identifier) (identifier))
     (binary_expression
       (binary_expression
@@ -531,13 +532,18 @@ x -> x^2
         (identifier))
       (operator)
       (identifier)))
-  (function_expression
+  (arrow_function_expression
     (argument_list)
     (integer_literal))
-  (function_expression
+  (arrow_function_expression
     (argument_list)
     (parenthesized_expression
       (call_expression (identifier) (argument_list (float_literal)))
       (compound_assignment_expression (identifier) (operator) (integer_literal))
-      (identifier))))
+      (identifier)))
+  (open_tuple
+    (arrow_function_expression
+      (identifier)
+      (assignment (identifier) (operator) (integer_literal)))
+    (integer_literal)))