"FailedParse: [...] Expecting end of text" when trying to parse parenthesized expressions in grako

Question

In search_query.ebnf, I have the following grammar definition for grako 3.14.0:

@@grammar :: SearchQuery

start = search_query $;

search_query = parenthesized_query | combined_query | search_term;
parenthesized_query = '(' search_query ')';
combined_query = search_query binary_operator search_query;
binary_operator = '&' | '|';
search_term = /\w+/;

I generate the parser with

grako search_query.ebnf --outfile search_query_parser.py

The result works as I expected for these inputs:

import search_query_parser

parser = search_query_parser.SearchQueryParser()
parser.parse('a')  # -> 'a'
parser.parse('(a)')  # -> ['(', 'a', ')']
parser.parse('a & b')  # -> ['a', '&', 'b']
parser.parse('a | b')  # -> ['a', '|', 'b']
parser.parse('(a|b)&c')  # -> ['(', ['a', '|', 'b'], ')', '&', 'c']

but if I have a parenthesized expression at the right hand side of an operator, the parser gives me an error message:

parser.parse('c&(a|b)')

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/das-g/.virtualenvs/tmp-d0fd5a9428f7612a/search_query_parser.py", line 82, in parse
    return super(SearchQueryParser, self).parse(text, *args, **kwargs)
  File "/home/das-g/.virtualenvs/tmp-d0fd5a9428f7612a/lib/python3.5/site-packages/grako/contexts.py", line 227, in parse
    result = rule()
  File "/home/das-g/.virtualenvs/tmp-d0fd5a9428f7612a/lib/python3.5/site-packages/grako/contexts.py", line 86, in wrapper
    return self._call(rule, name, params, kwparams)
  File "/home/das-g/.virtualenvs/tmp-d0fd5a9428f7612a/lib/python3.5/site-packages/grako/contexts.py", line 475, in _call
    node, newpos, newstate = self._invoke_rule(rule, name, params, kwparams)
  File "/home/das-g/.virtualenvs/tmp-d0fd5a9428f7612a/lib/python3.5/site-packages/grako/contexts.py", line 511, in _invoke_rule
    rule(self)
  File "/home/das-g/.virtualenvs/tmp-d0fd5a9428f7612a/search_query_parser.py", line 87, in _start_
    self._check_eof()
  File "/home/das-g/.virtualenvs/tmp-d0fd5a9428f7612a/lib/python3.5/site-packages/grako/contexts.py", line 650, in _check_eof
    self._error('Expecting end of text.')
  File "/home/das-g/.virtualenvs/tmp-d0fd5a9428f7612a/lib/python3.5/site-packages/grako/contexts.py", line 450, in _error
    item
grako.exceptions.FailedParse: (1:2) Expecting end of text. :
c&(a|b)
 ^
start

Am I doing something wrong?

score 1 · Accepted Answer · answered Sep 12 '16 at 15:44

1

Am I doing something wrong?

I don't think so.

This looks like a known bug in grako concerning "left recursion".

The workaround mentioned in the bug seems to work for your case, too:

@@grammar :: SearchQuery

start = search_query $;

search_query = parenthesized_query | combined_query | search_term;
parenthesized_query = '(' search_query | search_term ')';  ## Workaround
combined_query = search_query binary_operator search_query;
binary_operator = '&' | '|';
search_term = /\w+/;

i.e. mention search_term explicitly inside the parentheses, even though the search_query rule should be able to produce it, too.

answered Sep 12 '16 at 15:44

das-g

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It is a known bug in Grako's handling of (indirect) left recursion. – Apalala Sep 13 '16 at 00:42
@Apalala Isn't that what I wrote? (Or did I get the phrasing wrong? I'm a bit unfamiliar with parsing / grammar jargon in English.) – das-g Sep 13 '16 at 14:46
I was just re-asserting what you wrote. I'm the author of Grako. – Apalala Sep 14 '16 at 02:38

"FailedParse: [...] Expecting end of text" when trying to parse parenthesized expressions in grako

1 Answers1