I’m writing a parser for VBScript/VBA and I’m running into a problem where the parser seems to select the wrong rule and I can’t figure out why. My grammar is way bigger than the one posted here, but I have simplified it to a minimal reproducer.

The grammar is as follows:

module.exports = grammar({
  name: 'vbscript',

  extras: $ => [
    $.horizontal_whitespace
  ],

  rules: {
    source_file: $ => repeat(
      seq($._statement, $._whitespace),
    ),

    _statement: $ => choice(
      prec(2,$.variable_assignment),
      prec(1,$.invocation_statement),
    ),

    variable_assignment: $ => seq(
      $.identifier,
      '=',
      $._expression
    ),

    invocation_statement: $ => seq(
      $.identifier,
      optional(seq($.horizontal_whitespace, $.argument_list))
    ),

    _expression: $ => choice(
      $.identifier,
      $.literal
    ),

    argument_list: $ => seq(
      $.argument,
      repeat(seq(
        ',',
        $.argument
      ))
    ),

    argument: $ => choice(
        $._expression,
    ),

    literal: $ => choice(
      $.string,
    ),

    string: $ => seq('"', /[^"]*/, '"'),

    identifier: $ => /[a-zA-Z_]w*/,

    _whitespace: $ => repeat1(/[nr]/),

    horizontal_whitespace: $=> /[ t]+/,
  }
});

And the test code is as follows:

test = ""
SomeFunction arg1, arg2

The first line should be parsed as variable_assignment and the second line as invocation_statement. However, when I run the parser, I get the following output:

$ tree-sitter parse test.vba
(source_file [0, 0] - [2, 0]
  (invocation_statement [0, 0] - [0, 9]
    (identifier [0, 0] - [0, 4])
    (horizontal_whitespace [0, 4] - [0, 5])
    (ERROR [0, 5] - [0, 6])
    (horizontal_whitespace [0, 6] - [0, 7])
    (argument_list [0, 7] - [0, 9]
      (argument [0, 7] - [0, 9]
        (literal [0, 7] - [0, 9]
          (string [0, 7] - [0, 9])))))
  (invocation_statement [1, 0] - [1, 23]
    (identifier [1, 0] - [1, 12])
    (horizontal_whitespace [1, 12] - [1, 13])
    (argument_list [1, 13] - [1, 23]
      (argument [1, 13] - [1, 17]
        (identifier [1, 13] - [1, 17]))
      (horizontal_whitespace [1, 18] - [1, 19])
      (argument [1, 19] - [1, 23]
        (identifier [1, 19] - [1, 23])))))
test.vba           0.10 ms         330 bytes/ms (ERROR [0, 5] - [0, 6])

Apparently, the first line is parsed as invocation_statement, which doesn’t work, because the = sign is not included in that rule. I cannot figure out why it doesn’t try parsing it as a variable_assignment, which has higher precedence. If I comment out line 15 (prec(1,$.invocation_statement)) or line 26 (optional(seq($.horizontal_whitespace, $.argument_list))), I get the following output:

$ tree-sitter parse test.vba
(source_file [0, 0] - [2, 0]
  (variable_assignment [0, 0] - [0, 9]
    (identifier [0, 0] - [0, 4])
    (horizontal_whitespace [0, 4] - [0, 5])
    (horizontal_whitespace [0, 6] - [0, 7])
    (literal [0, 7] - [0, 9]
      (string [0, 7] - [0, 9])))
  (ERROR [1, 0] - [1, 18]
    (identifier [1, 0] - [1, 12])
    (horizontal_whitespace [1, 12] - [1, 13])
    (identifier [1, 13] - [1, 17]))
  (horizontal_whitespace [1, 18] - [1, 19])
  (invocation_statement [1, 19] - [1, 23]
    (identifier [1, 19] - [1, 23])))
test.vba           0.07 ms         460 bytes/ms (ERROR [1, 0] - [1, 18])

This shows that the variable_assignment rule correctly matches the first line. Obviously, now the second line no longer parses. I hope someone can see what is going on here.