grako

Basic Grako example gives IndexError

I'd like to get started with Grako (3.6.6) and as a first experience with parsers I wanted to generate an HTML table from a custom syntax. The following basic test

import grako

grammar = """table = { row }+ ;
row = (cell1:cell "|" cell2:cell) "\n";
cell = /[a-z]+/ ;
"""

model = grako.genmodel("model", grammar)

ast = model.parse(
"""a | b
c | d
""", "table")
print(ast)

results in an error

  File "test.py", line 13, in <module>
    """, "table")
  File "grako\grammars.py", line 790, in grako.grammars.Grammar.parse (grako\grammars.c:27773)
  File "grako\grammars.py", line 97, in grako.grammars.GrakoContext.parse (grako\grammars.c:4391)
  File "grako\contexts.py", line 180, in grako.contexts.ParseContext.parse (grako\contexts.c:4313)
  File "grako\grammars.py", line 594, in grako.grammars.Rule.parse (grako\grammars.c:22253)
  File "grako\grammars.py", line 597, in grako.grammars.Rule._parse_rhs (grako\grammars.c:22435)
  File "grako\contexts.py", line 399, in grako.contexts.ParseContext._call (grako\contexts.c:10088)
  File "grako\contexts.py", line 433, in grako.contexts.ParseContext._invoke_rule (grako\contexts.c:11135)
  File "grako\grammars.py", line 435, in grako.grammars.PositiveClosure.parse (grako\grammars.c:17285)
  File "grako\contexts.py", line 695, in grako.contexts.ParseContext._positive_closure (grako\contexts.c:19286)
  File "grako\contexts.py", line 696, in grako.contexts.ParseContext._positive_closure (grako\contexts.c:19240)
  File "grako\grammars.py", line 435, in grako.grammars.PositiveClosure.parse.lambda10 (grako\grammars.c:17195)
  File "grako\grammars.py", line 547, in grako.grammars.RuleRef.parse (grako\grammars.c:20774)
  File "grako\grammars.py", line 594, in grako.grammars.Rule.parse (grako\grammars.c:22253)
  File "grako\grammars.py", line 597, in grako.grammars.Rule._parse_rhs (grako\grammars.c:22435)
  File "grako\contexts.py", line 399, in grako.contexts.ParseContext._call (grako\contexts.c:10088)
  File "grako\contexts.py", line 433, in grako.contexts.ParseContext._invoke_rule (grako\contexts.c:11135)
  File "grako\grammars.py", line 326, in grako.grammars.Sequence.parse (grako\grammars.c:11582)
  File "grako\grammars.py", line 268, in grako.grammars.Token.parse (grako\grammars.c:9463)
  File "grako\contexts.py", line 543, in grako.contexts.ParseContext._token (grako\contexts.c:13772)
  File "grako\buffering.py", line 301, in grako.buffering.Buffer.match (grako\buffering.c:9168)
IndexError: string index out of range

which happens to be partial_match = (token[0].isalpha() and token.isalnum() and self.is_name_char(self.current()) )

Despite me being new to parsers and a little lack of documentation, I'd like to stick to Grako.

Can you help me set up a basic example which outputs the HTML for a table?

Solution

  • Grako is not seeing the "\n" in the grammar correctly because newlines are not allowed in tokens, and the \n is being evaluated in the context of the outer, triple-quote ("""), string. Things work fine if you use /\n/ instead.

    Also note that if \n will be part of the language, then you should probably write a @@whitespace clause so the parser doesn't skip over the character:

    @@whitespace :: /[\t ]+/

    This is the correct grammar for your language:

    grammar = """
@@whitespace :: /[\t ]+/
table = { row }+ ;
row = (cell1:cell "|" cell2:cell) "\\n";
cell = /[a-z]+/ ;
"""

    I'm currently patching Grako to detect and report errors like the one in your grammar. The changes are already in the Bitbucket repository. I'll make a release after I finish testing.