Parse for loop identifier as a variable name

Also remove some duplication in expression and concatenation

corpus/control_flow.txt

@@ -28,7 +28,7 @@ done
 
 (program
   (for_statement
-    (word)
+    (variable_name)
     (word)
     (word)
     (command_substitution (command (command_name (word)) (word) (word)))

grammar.js

@@ -16,6 +16,7 @@ module.exports = grammar({
     $._statement,
     $._terminator,
     $._expression,
+    $._primary_expression,
     $._variable_name,
   ],
 
@@ -27,7 +28,7 @@ module.exports = grammar({
     $.file_descriptor,
     $._empty_value,
     $._concat,
-    $.variable_name,
+    $.variable_name, // Variable name followed by an operator like '=' or '+='
     '\n',
   ],
 
@@ -63,7 +64,7 @@ module.exports = grammar({
 
     for_statement: $ => seq(
       'for',
-      $.word,
+      $._variable_name,
       'in',
       repeat1($._expression),
       $._terminator,
@@ -242,44 +243,29 @@ module.exports = grammar({
     // Expressions
 
     _expression: $ => choice(
+      $.concatenation,
+      $._primary_expression
+    ),
+
+    _primary_expression: $ => choice(
       $.word,
       $.string,
       $.raw_string,
       $.expansion,
       $.simple_expansion,
       $.command_substitution,
-      $.process_substitution,
-      $.concatenation
+      $.process_substitution
     ),
 
     concatenation: $ => prec(-1, seq(
-      choice(
-        $.word,
-        $.string,
-        $.raw_string,
-        $.expansion,
-        $.simple_expansion,
-        $.command_substitution,
-        $.process_substitution
-      ),
-      repeat1(seq(
-        $._concat,
-        choice(
-          $.word,
-          $.string,
-          $.raw_string,
-          $.expansion,
-          $.simple_expansion,
-          $.command_substitution,
-          $.process_substitution
-        )
-      ))
+      $._primary_expression,
+      repeat1(seq($._concat, $._primary_expression))
     )),
 
     string: $ => seq(
       '"',
       repeat(choice(
-        /[^"`$]+/,
+        $._string_content,
         $.expansion,
         $.simple_expansion,
         $.command_substitution
@@ -287,6 +273,8 @@ module.exports = grammar({
       '"'
     ),
 
+    _string_content: $ => /[^"`$]+/,
+
     array: $ => seq(
       '(',
       repeat($._expression),

src/grammar.json

@@ -83,7 +83,7 @@
         },
         {
           "type": "SYMBOL",
-          "name": "word"
+          "name": "_variable_name"
         },
         {
           "type": "STRING",
@@ -319,16 +319,14 @@
       "members": [
         {
           "type": "CHOICE",
+          "members": [
+            {
+              "type": "SEQ",
               "members": [
                 {
                   "type": "STRING",
                   "value": "function"
                 },
-            {
-              "type": "BLANK"
-            }
-          ]
-        },
                 {
                   "type": "SYMBOL",
                   "name": "word"
@@ -353,6 +351,27 @@
                       "type": "BLANK"
                     }
                   ]
+                }
+              ]
+            },
+            {
+              "type": "SEQ",
+              "members": [
+                {
+                  "type": "SYMBOL",
+                  "name": "word"
+                },
+                {
+                  "type": "STRING",
+                  "value": "("
+                },
+                {
+                  "type": "STRING",
+                  "value": ")"
+                }
+              ]
+            }
+          ]
         },
         {
           "type": "SYMBOL",
@@ -859,47 +878,17 @@
     "_expression": {
       "type": "CHOICE",
       "members": [
-        {
-          "type": "SYMBOL",
-          "name": "word"
-        },
-        {
-          "type": "SYMBOL",
-          "name": "string"
-        },
-        {
-          "type": "SYMBOL",
-          "name": "raw_string"
-        },
-        {
-          "type": "SYMBOL",
-          "name": "expansion"
-        },
-        {
-          "type": "SYMBOL",
-          "name": "simple_expansion"
-        },
-        {
-          "type": "SYMBOL",
-          "name": "command_substitution"
-        },
-        {
-          "type": "SYMBOL",
-          "name": "process_substitution"
-        },
         {
           "type": "SYMBOL",
           "name": "concatenation"
+        },
+        {
+          "type": "SYMBOL",
+          "name": "_primary_expression"
         }
       ]
     },
-    "concatenation": {
-      "type": "PREC",
-      "value": -1,
-      "content": {
-        "type": "SEQ",
-        "members": [
-          {
+    "_primary_expression": {
       "type": "CHOICE",
       "members": [
         {
@@ -932,6 +921,16 @@
         }
       ]
     },
+    "concatenation": {
+      "type": "PREC",
+      "value": -1,
+      "content": {
+        "type": "SEQ",
+        "members": [
+          {
+            "type": "SYMBOL",
+            "name": "_primary_expression"
+          },
           {
             "type": "REPEAT1",
             "content": {
@@ -941,38 +940,9 @@
                   "type": "SYMBOL",
                   "name": "_concat"
                 },
-                {
-                  "type": "CHOICE",
-                  "members": [
                 {
                   "type": "SYMBOL",
-                      "name": "word"
-                    },
-                    {
-                      "type": "SYMBOL",
-                      "name": "string"
-                    },
-                    {
-                      "type": "SYMBOL",
-                      "name": "raw_string"
-                    },
-                    {
-                      "type": "SYMBOL",
-                      "name": "expansion"
-                    },
-                    {
-                      "type": "SYMBOL",
-                      "name": "simple_expansion"
-                    },
-                    {
-                      "type": "SYMBOL",
-                      "name": "command_substitution"
-                    },
-                    {
-                      "type": "SYMBOL",
-                      "name": "process_substitution"
-                    }
-                  ]
+                  "name": "_primary_expression"
                 }
               ]
             }
@@ -993,8 +963,8 @@
             "type": "CHOICE",
             "members": [
               {
-                "type": "PATTERN",
-                "value": "[^\"`$]+"
+                "type": "SYMBOL",
+                "name": "_string_content"
               },
               {
                 "type": "SYMBOL",
@@ -1017,6 +987,10 @@
         }
       ]
     },
+    "_string_content": {
+      "type": "PATTERN",
+      "value": "[^\"`$]+"
+    },
     "array": {
       "type": "SEQ",
       "members": [
@@ -1443,6 +1417,7 @@
     "_statement",
     "_terminator",
     "_expression",
+    "_primary_expression",
     "_variable_name"
   ]
 }