[antlr-interest] Context-sensitive lexer
John B. Brodie
jbb at acm.org
Fri Jun 17 14:28:00 PDT 2011
On Fri, 2011-06-17 at 15:29 +0200, Jonas wrote:
> Hi John!
>
> I believed that using the semantic predicate would hinder ANTLR from
> trying to match TITLE_TEXT in other situations than when lexerState
> indicates that we are in the title section. Anyway, changing the TEXT
> fragment to (~('\r' | '\n'))+ does not prevent the infinite loop. Keep
> the good ideas coming!
When I run your example from the command line I get this message printed
to the console continuously...
line 4:0 rule TITLE_TEXT failed predicate: {lexerState==STATE_IN_TITLE}?
perhaps predicates in the Lexer do not actually perform as you are
expecting? (look at the generated lexer code....)
>
> Best Regards,
> Jonas
>
> On Fri, Jun 17, 2011 at 3:06 PM, John B. Brodie <jbb at acm.org> wrote:
> > Greetings!
> >
> > Your TEXT fragment (and therefore your TITLE_TEXT token) can be empty!
> >
> > Thus, I think your lexer is trying to recognize infinitely many
> > TITLE_TEXT tokens.
> >
> > Hope this helps...
> > -jbb
> >
> > On Fri, 2011-06-17 at 14:15 +0200, Jonas wrote:
> >> Hi,
> >>
> >> I'm developing a parser for a file format where context is very
> >> important. I'm looking to
> >> 1) understand why my ANTLR parser gets into infinite loops
> >> 2) find out if there is any better way to implement context
> >> sensitivity than what I am doing with semantic predicates.
> >>
> >> A typical beginning of a file looks like this:
> >> TITLE
> >> some title text
> >>
> >> SECTION1
> >> a=b*c
> >> END
> >>
> >> SECTION2
> >> ...
> >>
> >> SECTION3
> >> ...
> >>
> >> The syntax differs from section to section; the 'TITLE' section is
> >> terminated by the newline after the title text line, while other
> >> sections can e.g. use single quote string literals and be terminated
> >> by a keyword like 'END'. Here is a sample grammar, that gets into an
> >> infinite loop:
> >>
> >> grammar test;
> >>
> >> options {
> >> output=AST;
> >> }
> >>
> >> @lexer::members {
> >> static final int STATE_AT_BEGINNING = 0;
> >> static final int STATE_IN_TITLE = 1;
> >> static final int STATE_AFTER_TITLE = 2;
> >> int lexerState = STATE_AT_BEGINNING;
> >> }
> >>
> >> file : title;
> >>
> >> title : BEGIN_TITLE TITLE_TEXT END_TITLE;
> >>
> >> BEGIN_TITLE
> >> : {(lexerState == STATE_AT_BEGINNING)}? 'TITLE' WS_NL
> >> {lexerState=STATE_IN_TITLE;}
> >> ;
> >>
> >> TITLE_TEXT
> >> : {lexerState == STATE_IN_TITLE}? TEXT
> >> ;
> >>
> >> END_TITLE
> >> : {lexerState == STATE_IN_TITLE}? NL {lexerState=STATE_AFTER_TITLE;}
> >> ;
> >>
> >> BLANK_ROW
> >> : {!(lexerState == STATE_IN_TITLE)}? WS_NL
> >> ;
> >>
> >> REMARK : {!(lexerState == STATE_IN_TITLE)}? 'REMA' .* NL
> >> ;
> >>
> >> fragment
> >> WS_NL : (' ' | '\t')* NL;
> >>
> >> fragment
> >> NL : '\r'? '\n';
> >>
> >> fragment
> >> TEXT : (~('\r' | '\n'))*;
> >>
> >
> >
> >
>
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