grammar.js

const PREC = {
  COMMENT: 1, // Prefer comments over regexes
  STRING: 2,  // In a string, prefer string characters over comments

  COMMA: -1,
  OBJECT: -1,
  DECLARATION: 1,
  ASSIGN: 0,
  TERNARY: 1,
  OR: 2,
  AND: 3,
  REL: 4,
  PLUS: 5,
  TIMES: 6,
  EXP: 7,
  TYPEOF: 8,
  DELETE: 8,
  VOID: 8,
  NOT: 9,
  NEG: 10,
  INC: 11,
  CALL: 12,
  NEW: 13,
  MEMBER: 14
};

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

  externals: $ => [
    $._automatic_semicolon,
    $._template_chars
  ],

  extras: $ => [
    $.comment,
    /[\s\uFEFF\u2060\u200B\u00A0]/
  ],

  supertypes: $ => [
    $._statement,
    $._declaration,
    $._expression,
    $._destructuring_pattern,
  ],

  inline: $ => [
    $._call_signature,
    $._primary_expression,
    $._statement,
    $._expressions,
    $._semicolon,
    $._formal_parameter,
    $._destructuring_pattern,
    $._reserved_identifier,
    $._jsx_attribute,
    $._jsx_element_name,
    $._jsx_child,
    $._jsx_element,
    $._jsx_attribute_name,
    $._jsx_attribute_value,
    $._jsx_identifier,
    $._lhs_expression,
  ],

  conflicts: $ => [
    [$._expression, $._property_name],
    [$._expression, $._property_name, $.arrow_function],
    [$._expression, $.arrow_function],
    [$._expression, $.method_definition],
    [$._expression, $.formal_parameters],
    [$._expression, $.rest_parameter],
    [$.labeled_statement, $._property_name],
    [$.assignment_pattern, $.assignment_expression],
    [$.computed_property_name, $.array],
    [$._for_header, $._expression],
  ],

  word: $ => $.identifier,

  rules: {
    program: $ => seq(
      optional($.hash_bang_line),
      repeat($._statement)
    ),

    hash_bang_line: $ => /#!.*/,

    //
    // Export declarations
    //

    export_statement: $ => choice(
      seq(
        'export',
        choice(
          seq('*', $._from_clause, $._semicolon),
          seq($.export_clause, $._from_clause, $._semicolon),
          seq($.export_clause, $._semicolon)
        )
      ),
      seq(
        repeat(field('decorator', $.decorator)),
        'export',
        choice(
          field('declaration', $._declaration),
          seq(
            'default',
            field('value', $._expression),
            $._semicolon
          )
        )
      ),
    ),

    export_clause: $ => seq(
      '{',
      commaSep(alias($._import_export_specifier, $.export_specifier)),
      optional(','),
      '}'
    ),

    _import_export_specifier: $ => seq(
      field('name', $.identifier),
      optional(seq(
        'as',
        field('alias', $.identifier)
      ))
    ),

    _declaration: $ => choice(
      $.function_declaration,
      $.generator_function_declaration,
      $.class_declaration,
      $.lexical_declaration,
      $.variable_declaration
    ),

    //
    // Import declarations
    //

    import: $ => token('import'),

    import_statement: $ => prec(1, seq(
      'import',
      choice(
        seq($.import_clause, $._from_clause),
        field('source', $.string)
      ),
      $._semicolon
    )),

    import_clause: $ => choice(
      $.namespace_import,
      $.named_imports,
      seq(
        $.identifier,
        optional(seq(
          ',',
          choice(
            $.namespace_import,
            $.named_imports
          )
        ))
      )
    ),

    _from_clause: $ => seq(
      "from", field('source', $.string)
    ),

    namespace_import: $ => seq(
      "*", "as", $.identifier
    ),

    named_imports: $ => seq(
      '{',
      commaSep(alias($._import_export_specifier, $.import_specifier)),
      optional(','),
      '}'
    ),

    //
    // Statements
    //

    _statement: $ => choice(
      $.export_statement,
      $.import_statement,
      $.debugger_statement,
      $.expression_statement,
      $._declaration,
      $.statement_block,

      $.if_statement,
      $.switch_statement,
      $.for_statement,
      $.for_in_statement,
      $.while_statement,
      $.do_statement,
      $.try_statement,
      $.with_statement,

      $.break_statement,
      $.continue_statement,
      $.return_statement,
      $.throw_statement,
      $.empty_statement,
      $.labeled_statement
    ),

    expression_statement: $ => seq(
      $._expressions,
      $._semicolon
    ),

    variable_declaration: $ => seq(
      'var',
      commaSep1($.variable_declarator),
      $._semicolon
    ),

    lexical_declaration: $ => seq(
      choice('let', 'const'),
      commaSep1($.variable_declarator),
      $._semicolon
    ),

    variable_declarator: $ => seq(
      field('name', choice($.identifier, $._destructuring_pattern)),
      optional($._initializer)
    ),

    statement_block: $ => prec.right(seq(
      '{',
      repeat($._statement),
      '}',
      optional($._automatic_semicolon)
    )),

    else_clause: $ => seq('else', $._statement),

    if_statement: $ => prec.right(seq(
      'if',
      field('condition', $.parenthesized_expression),
      field('consequence', $._statement),
      optional(field('alternative', $.else_clause))
    )),

    switch_statement: $ => seq(
      'switch',
      field('value', $.parenthesized_expression),
      field('body', $.switch_body)
    ),

    for_statement: $ => seq(
      'for',
      '(',
      field('initializer', choice(
        $.lexical_declaration,
        $.variable_declaration,
        $.expression_statement,
        $.empty_statement
      )),
      field('condition', choice(
        $.expression_statement,
        $.empty_statement
      )),
      field('increment', optional($._expressions)),
      ')',
      field('body', $._statement)
    ),

    for_in_statement: $ => seq(
      'for',
      optional('await'),
      $._for_header,
      field('body', $._statement)
    ),

    _for_header: $ => seq(
      '(',
      optional(choice('var', 'let', 'const')),
      field('left', choice($.parenthesized_expression, $._lhs_expression)),
      choice('in', 'of'),
      field('right', $._expressions),
      ')',
    ),

    while_statement: $ => seq(
      'while',
      field('condition', $.parenthesized_expression),
      field('body', $._statement)
    ),

    do_statement: $ => seq(
      'do',
      field('body', $._statement),
      'while',
      field('condition', $.parenthesized_expression),
      $._semicolon
    ),

    try_statement: $ => seq(
      'try',
      field('body', $.statement_block),
      optional(field('handler', $.catch_clause)),
      optional(field('finalizer', $.finally_clause))
    ),

    with_statement: $ => seq(
      'with',
      field('object', $.parenthesized_expression),
      field('body', $._statement)
    ),

    break_statement: $ => seq(
      'break',
      field('label', optional(alias($.identifier, $.statement_identifier))),
      $._semicolon
    ),

    continue_statement: $ => seq(
      'continue',
      field('label', optional(alias($.identifier, $.statement_identifier))),
      $._semicolon
    ),

    debugger_statement: $ => seq(
      'debugger',
      $._semicolon
    ),

    return_statement: $ => seq(
      'return',
      optional($._expressions),
      $._semicolon
    ),

    throw_statement: $ => seq(
      'throw',
      $._expressions,
      $._semicolon
    ),

    empty_statement: $ => ';',

    labeled_statement: $ => prec.dynamic(-1, seq(
      field('label', alias(choice($.identifier, $._reserved_identifier), $.statement_identifier)),
      ':',
      $._statement
    )),

    //
    // Statement components
    //

    switch_body: $ => seq(
      '{',
      repeat(choice($.switch_case, $.switch_default)),
      '}'
    ),

    switch_case: $ => seq(
      'case',
      field('value', $._expressions),
      ':',
      repeat($._statement)
    ),

    switch_default: $ => seq(
      'default',
      ':',
      repeat($._statement)
    ),

    catch_clause: $ => seq(
      'catch',
      optional(seq('(', field('parameter', choice($.identifier, $._destructuring_pattern)), ')')),
      field('body', $.statement_block)
    ),

    finally_clause: $ => seq(
      'finally',
      field('body', $.statement_block)
    ),

    parenthesized_expression: $ => seq(
      '(',
      $._expressions,
      ')'
    ),

    //
    // Expressions
    //
    _expressions: $ => choice(
      $._expression,
      $.sequence_expression
    ),

    _expression: $ => choice(
      $._primary_expression,
      $._jsx_element,
      $.jsx_fragment,
      $.assignment_expression,
      $.augmented_assignment_expression,
      $.await_expression,
      $.unary_expression,
      $.binary_expression,
      $.ternary_expression,
      $.update_expression,
      $.new_expression,
      $.yield_expression,
    ),

    _primary_expression: $ => choice(
      $.this,
      $.super,
      $.identifier,
      alias($._reserved_identifier, $.identifier),
      $.number,
      $.string,
      $.template_string,
      $.regex,
      $.true,
      $.false,
      $.null,
      $.undefined,
      $.import,
      $.object,
      $.array,
      $.function,
      $.arrow_function,
      $.generator_function,
      $.class,
      $.parenthesized_expression,
      $.subscript_expression,
      $.member_expression,
      $.meta_property,
      $.call_expression,
    ),

    yield_expression: $ => prec.right(seq(
      'yield',
      choice(
        seq('*', $._expression),
        optional($._expression)
      ))),

    object: $ => prec(PREC.OBJECT, seq(
      '{',
      commaSep(optional(choice(
        $.pair,
        $.spread_element,
        $.method_definition,
        $.assignment_pattern,
        alias(
          choice($.identifier, $._reserved_identifier),
          $.shorthand_property_identifier
        )
      ))),
      '}'
    )),

    assignment_pattern: $ => seq(
      field('left', choice(
        alias(choice($._reserved_identifier, $.identifier), $.shorthand_property_identifier),
        $._destructuring_pattern
      )),
      '=',
      field('right', $._expression)
    ),

    array: $ => seq(
      '[',
      commaSep(optional(choice(
        $._expression,
        $.spread_element
      ))),
      ']'
    ),

    _jsx_element: $ => choice($.jsx_element, $.jsx_self_closing_element),

    jsx_element: $ => seq(
      field('open_tag', $.jsx_opening_element),
      repeat($._jsx_child),
      field('close_tag', $.jsx_closing_element)
    ),

    jsx_fragment: $ => seq('<', '>', repeat($._jsx_child), '<', '/', '>'),

    jsx_text: $ => /[^{}<>]+/,

    jsx_expression: $ => seq(
      '{',
      optional(choice(
        $._expression,
        $.sequence_expression,
        $.spread_element
      )),
      '}'
    ),

    _jsx_child: $ => choice(
      $.jsx_text,
      $._jsx_element,
      $.jsx_fragment,
      $.jsx_expression
    ),

    jsx_opening_element: $ => prec.dynamic(-1, seq(
      '<',
      field('name', $._jsx_element_name),
      repeat(field('attribute', $._jsx_attribute)),
      '>'
    )),

    jsx_identifier: $ => /[a-zA-Z_$][a-zA-Z\d_$]*-[a-zA-Z\d_$\-]*/,

    _jsx_identifier: $ => choice(
      alias($.jsx_identifier, $.identifier),
      $.identifier
    ),

    nested_identifier: $ => prec(PREC.MEMBER, seq(
      choice($.identifier, $.nested_identifier),
      '.',
      $.identifier
    )),

    jsx_namespace_name: $ => seq($._jsx_identifier, ':', $._jsx_identifier),

    _jsx_element_name: $ => choice(
      $._jsx_identifier,
      $.nested_identifier,
      $.jsx_namespace_name,
    ),

    jsx_closing_element: $ => seq(
      '<',
      '/',
      field('name', $._jsx_element_name),
      '>'
    ),

    jsx_self_closing_element: $ => seq(
      '<',
      field('name', $._jsx_element_name),
      repeat(field('attribute', $._jsx_attribute)),
      '/',
      '>'
    ),

    _jsx_attribute: $ => choice($.jsx_attribute, $.jsx_expression),

    _jsx_attribute_name: $ => choice(alias($._jsx_identifier, $.property_identifier), $.jsx_namespace_name),

    jsx_attribute: $ => seq(
      $._jsx_attribute_name,
      optional(seq(
        '=',
        $._jsx_attribute_value
      ))
    ),

    _jsx_attribute_value: $ => choice(
      $.string,
      $.jsx_expression,
      $._jsx_element,
      $.jsx_fragment
    ),

    class: $ => seq(
      repeat(field('decorator', $.decorator)),
      'class',
      field('name', optional($.identifier)),
      optional($.class_heritage),
      field('body', $.class_body)
    ),

    class_declaration: $ => prec(PREC.DECLARATION, seq(
      repeat(field('decorator', $.decorator)),
      'class',
      field('name', $.identifier),
      optional($.class_heritage),
      field('body', $.class_body),
      optional($._automatic_semicolon)
    )),

    class_heritage: $ => seq('extends', $._expression),

    function: $ => seq(
      optional('async'),
      'function',
      field('name', optional($.identifier)),
      $._call_signature,
      field('body', $.statement_block)
    ),

    function_declaration: $ => prec.right(PREC.DECLARATION, seq(
      optional('async'),
      'function',
      field('name', $.identifier),
      $._call_signature,
      field('body', $.statement_block),
      optional($._automatic_semicolon)
    )),

    generator_function: $ => seq(
      optional('async'),
      'function',
      '*',
      field('name', optional($.identifier)),
      $._call_signature,
      field('body', $.statement_block)
    ),

    generator_function_declaration: $ => prec.right(PREC.DECLARATION, seq(
      optional('async'),
      'function',
      '*',
      field('name', $.identifier),
      $._call_signature,
      field('body', $.statement_block),
      optional($._automatic_semicolon)
    )),

    arrow_function: $ => seq(
      optional('async'),
      choice(
        field('parameter', choice(
          alias($._reserved_identifier, $.identifier),
          $.identifier,
        )),
        $._call_signature
      ),
      '=>',
      field('body', choice(
        $._expression,
        $.statement_block
      ))
    ),

    // Override
    _call_signature: $ => seq(
      field('parameters', $.formal_parameters)
    ),

    call_expression: $ => choice(
      prec(PREC.CALL, seq(
        field('function', $._expression),
        field('arguments', choice($.arguments, $.template_string))
      )),
      prec(PREC.MEMBER, seq(
        field('function', $._primary_expression),
        '?.',
        field('arguments', $.arguments)
      ))
    ),

    new_expression: $ => prec.right(PREC.NEW, seq(
      'new',
      field('constructor', $._primary_expression),
      field('arguments', optional(prec.dynamic(1, $.arguments)))
    )),

    await_expression: $ => seq(
      'await',
      $._expression
    ),

    member_expression: $ => prec(PREC.MEMBER, seq(
      field('object', choice($._expression, $._primary_expression)),
      choice('.', '?.'),
      field('property', alias($.identifier, $.property_identifier))
    )),

    subscript_expression: $ => prec.right(PREC.MEMBER, seq(
      field('object', choice($._expression, $._primary_expression)),
      optional('?.'),
      '[', field('index', $._expressions), ']'
    )),

    _lhs_expression: $ => choice(
      $.member_expression,
      $.subscript_expression,
      $.identifier,
      alias($._reserved_identifier, $.identifier),
      $._destructuring_pattern
    ),

    assignment_expression: $ => prec.right(PREC.ASSIGN, seq(
      field('left', choice($.parenthesized_expression, $._lhs_expression)),
      '=',
      field('right', $._expression)
    )),

    augmented_assignment_expression: $ => prec.right(PREC.ASSIGN, seq(
      field('left', choice(
        $.member_expression,
        $.subscript_expression,
        alias($._reserved_identifier, $.identifier),
        $.identifier,
        $.parenthesized_expression,
      )),
      choice('+=', '-=', '*=', '/=', '%=', '^=', '&=', '|=', '>>=', '>>>=',
             '<<=', '**=', '&&=', '||=', '??='),
      field('right', $._expression)
    )),

    _initializer: $ => seq(
      '=',
      field('value', $._expression)
    ),

    _destructuring_pattern: $ => choice(
      alias($.object, $.object_pattern),
      alias($.array, $.array_pattern)
    ),

    spread_element: $ => seq('...', $._expression),

    ternary_expression: $ => prec.right(PREC.TERNARY, seq(
      field('condition', $._expression),
      '?',
      field('consequence', $._expression),
      ':',
      field('alternative', $._expression)
    )),

    binary_expression: $ => choice(
      ...[
        ['&&', PREC.AND],
        ['||', PREC.OR],
        ['>>', PREC.TIMES],
        ['>>>', PREC.TIMES],
        ['<<', PREC.TIMES],
        ['&', PREC.AND],
        ['^', PREC.OR],
        ['|', PREC.OR],
        ['+', PREC.PLUS],
        ['-', PREC.PLUS],
        ['*', PREC.TIMES],
        ['/', PREC.TIMES],
        ['%', PREC.TIMES],
        ['**', PREC.EXP],
        ['<', PREC.REL],
        ['<=', PREC.REL],
        ['==', PREC.REL],
        ['===', PREC.REL],
        ['!=', PREC.REL],
        ['!==', PREC.REL],
        ['>=', PREC.REL],
        ['>', PREC.REL],
        ['??', PREC.TERNARY],
        ['instanceof', PREC.REL],
        ['in', PREC.REL],
      ].map(([operator, precedence]) =>
        prec.left(precedence, seq(
          field('left', $._expression),
          field('operator', operator),
          field('right', $._expression)
        ))
      )
    ),

    unary_expression: $ => choice(...[
      ['!', PREC.NOT],
      ['~', PREC.NOT],
      ['-', PREC.NEG],
      ['+', PREC.NEG],
      ['typeof', PREC.TYPEOF],
      ['void', PREC.VOID],
      ['delete', PREC.DELETE],
    ].map(([operator, precedence]) =>
      prec.left(precedence, seq(
        field('operator', operator),
        field('argument', $._expression)
      ))
    )),

    update_expression: $ => prec.left(PREC.INC, choice(
      seq(
        field('argument', $._expression),
        field('operator', choice('++', '--'))
      ),
      seq(
        field('operator', choice('++', '--')),
        field('argument', $._expression)
      ),
    )),

    sequence_expression: $ => prec(PREC.COMMA, seq(
      field('left', $._expression),
      ',',
      field('right', choice($.sequence_expression, $._expression))
    )),

    //
    // Primitives
    //

    string: $ => choice(
      seq(
        '"',
        repeat(choice(
          token.immediate(prec(PREC.STRING, /[^"\\\n]+|\\\r?\n/)),
          $.escape_sequence
        )),
        '"'
      ),
      seq(
        "'",
        repeat(choice(
          token.immediate(prec(PREC.STRING, /[^'\\\n]+|\\\r?\n/)),
          $.escape_sequence
        )),
        "'"
      )
    ),

    escape_sequence: $ => token.immediate(seq(
      '\\',
      choice(
        /[^xu0-7]/,
        /[0-7]{1,3}/,
        /x[0-9a-fA-F]{2}/,
        /u[0-9a-fA-F]{4}/,
        /u{[0-9a-fA-F]+}/
      )
    )),

    // http://stackoverflow.com/questions/13014947/regex-to-match-a-c-style-multiline-comment/36328890#36328890
    comment: $ => token(prec(PREC.COMMENT, choice(
      seq('//', /.*/),
      seq(
        '/*',
        /[^*]*\*+([^/*][^*]*\*+)*/,
        '/'
      )
    ))),

    template_string: $ => seq(
      '`',
      repeat(choice(
        $._template_chars,
        $.escape_sequence,
        $.template_substitution
      )),
      '`'
    ),

    template_substitution: $ => seq(
      '${',
      $._expressions,
      '}'
    ),

    regex: $ => seq(
      '/',
      field('pattern', $.regex_pattern),
      token.immediate('/'),
      optional(field('flags', $.regex_flags))
    ),

    regex_pattern: $ => token.immediate(
      repeat1(choice(
        seq(
          '[',
          repeat(choice(
            seq('\\', /./), // escaped character
            /[^\]\n\\]/       // any character besides ']' or '\n'
          )),
          ']'
        ),              // square-bracket-delimited character class
        seq('\\', /./), // escaped character
        /[^/\\\[\n]/    // any character besides '[', '\', '/', '\n'
      ))
    ),

    regex_flags: $ => token.immediate(/[a-z]+/),

    number: $ => {
      const hex_literal = seq(
        choice('0x', '0X'),
        /[\da-fA-F](_?[\da-fA-F])*/
      )

      const decimal_digits = /\d(_?\d)*/
      const signed_integer = seq(optional(choice('-', '+')), decimal_digits)
      const exponent_part = seq(choice('e', 'E'), signed_integer)

      const binary_literal = seq(choice('0b', '0B'), /[0-1](_?[0-1])*/)

      const octal_literal = seq(choice('0o', '0O'), /[0-7](_?[0-7])*/)

      const bigint_literal = seq(choice(hex_literal, binary_literal, octal_literal, decimal_digits), 'n')

      const decimal_integer_literal = choice(
        '0',
        seq(optional('0'), /[1-9]/, optional(seq(optional('_'), decimal_digits)))
      )

      const decimal_literal = choice(
        seq(decimal_integer_literal, '.', optional(decimal_digits), optional(exponent_part)),
        seq('.', decimal_digits, optional(exponent_part)),
        seq(decimal_integer_literal, exponent_part),
        seq(decimal_digits),
      )

      return token(choice(
        hex_literal,
        decimal_literal,
        binary_literal,
        octal_literal,
        bigint_literal,
      ))
    },

    identifier: $ => {
      const alpha = /[^\x00-\x1F\s0-9:;`"'@#.,|^&<=>+\-*/\\%?!~()\[\]{}\uFEFF\u2060\u200B\u00A0]|\\u[0-9a-fA-F]{4}|\\u\{[0-9a-fA-F]+\}/
      const alphanumeric = /[^\x00-\x1F\s:;`"'@#.,|^&<=>+\-*/\\%?!~()\[\]{}\uFEFF\u2060\u200B\u00A0]|\\u[0-9a-fA-F]{4}|\\u\{[0-9a-fA-F]+\}/
      return token(seq(alpha, repeat(alphanumeric)))
    },

    meta_property: $ => seq('new', '.', 'target'),

    this: $ => 'this',
    super: $ => 'super',
    true: $ => 'true',
    false: $ => 'false',
    null: $ => 'null',
    undefined: $ => 'undefined',

    //
    // Expression components
    //

    arguments: $ => prec(PREC.CALL, seq(
      '(',
      commaSep(optional(choice($._expression, $.spread_element))),
      ')'
    )),

    decorator: $ => seq(
      '@',
      choice(
        $.identifier,
        alias($.decorator_member_expression, $.member_expression),
        alias($.decorator_call_expression, $.call_expression)
      )
    ),

    decorator_member_expression: $ => prec(PREC.MEMBER, seq(
      field('object', choice(
        $.identifier,
        alias($.decorator_member_expression, $.member_expression)
      )),
      '.',
      field('property', alias($.identifier, $.property_identifier))
    )),

    decorator_call_expression: $ => prec(PREC.CALL, seq(
      field('function', choice(
        $.identifier,
        alias($.decorator_member_expression, $.member_expression)
      )),
      field('arguments', $.arguments)
    )),

    class_body: $ => seq(
      '{',
      repeat(choice(
        seq(field('member', $.method_definition), optional(';')),
        seq(field('member', $.public_field_definition), $._semicolon)
      )),
      '}'
    ),

    public_field_definition: $ => seq(
      optional('static'),
      field('property', $._property_name),
      optional($._initializer)
    ),

    formal_parameters: $ => seq(
      '(',
      optional(seq(
        commaSep1($._formal_parameter),
        optional(',')
      )),
      ')'
    ),

    _formal_parameter: $ => choice(
      $.identifier,
      alias($._reserved_identifier, $.identifier),
      $._destructuring_pattern,
      $.assignment_pattern,
      $.rest_parameter
    ),

    rest_parameter: $ => seq(
      '...',
      choice(
        $.identifier,
        $._destructuring_pattern,
      )
    ),

    method_definition: $ => seq(
      repeat(field('decorator', $.decorator)),
      optional('static'),
      optional('async'),
      optional(choice('get', 'set', '*')),
      field('name', $._property_name),
      field('parameters', $.formal_parameters),
      field('body', $.statement_block)
    ),

    pair: $ => seq(
      field('key', $._property_name),
      ':',
      field('value', $._expression)
    ),

    _property_name: $ => choice(
      alias(choice(
        $.identifier,
        $._reserved_identifier
      ), $.property_identifier),
      $.string,
      $.number,
      $.computed_property_name
    ),

    computed_property_name: $ => seq(
      '[',
      $._expression,
      ']'
    ),

    _reserved_identifier: $ => choice(
      'get',
      'set',
      'async',
      'static',
    ),

    _semicolon: $ => choice($._automatic_semicolon, ';')
  }
});

function commaSep1(rule) {
  return seq(rule, repeat(seq(',', rule)));
}

function commaSep(rule) {
  return optional(commaSep1(rule));
}