[antlr-interest] How to force error recovery?

[Jim Idle]

If you are using too many syntactic predicates, then you can end up with a
case that you cannot recover from without actually trying to parse
(backtrack mode). However, this usually means that your grammar needs more
left factoring than it has at the moment.

However, that said, the followset should only include tokens that can start
one of the alts in your statement rule. Hence, having found such a token,
your statement rule should handle it and if not, it should throw the
exception and allow you to try again. You may just need to apply the
technique within subrules. There are also cases where the input is so out of
whack that there is nothing that can be recovered.

So, without seeing your grammar, I can’t really guide you to be honest, but
now you know how to do this in general, you should find that you can work
through the specific cases and either re-jig the grammar, or write very
specific recovery routines for very specific situations. First rule of thumb
is that if you have predicates with more than one or two tokens, then your
grammar is very likely in need of some work.

Jim

> -----Original Message-----
> From: ed.tirelli at gmail.com [mailto:ed.tirelli at gmail.com] On Behalf Of
Edson
> Tirelli
> Sent: Tuesday, October 05, 2010 9:07 AM
> To: Jim Idle
> Cc: antlr-interest at antlr.org
> Subject: Re: [antlr-interest] How to force error recovery?
> 
>    Hi Jim,
> 
>    Yes, I found the wiki after sending the e-mail yesterday. Thanks for
taking
> the time on writing that as it was really helpful.
> 
>    Now, continuing on the subject, I need to go a step further for my use
case.
> Just so you understand, in my case, due to syntactic predicates, even if
the
> next token is on the follow set, the "statement" rule can still fail. So,
the
> question is: how to stay in the loop, skipping/deleting tokens, until it
either
> succeeds in parsing the rest of the input or EOF is found?
> 
> compilationUnit
>    : resync (statement resync)* EOF
>    ;
> 
>    Thanks,
>      Edson
> 
> 
> 
> 2010/10/5 Jim Idle <jimi at temporal-wave.com>:
> > Please read the article in the wiki on error recovery methods. You can
> > see there how to keep a parse loop going instead of it breaking out.
> > You can also see a real world example if you download the source code
> > for the JavaFX compiler, as I wrote the error recovery article after
writing
> that parser.
> >
> >
> http://www.antlr.org/wiki/display/ANTLR3/Custom+Syntax+Error+Recovery
> >
> > Jim
> >
> >> -----Original Message-----
> >> From: antlr-interest-bounces at antlr.org [mailto:antlr-interest-
> >> bounces at antlr.org] On Behalf Of Edson Tirelli
> >> Sent: Monday, October 04, 2010 3:27 PM
> >> To: antlr-interest at antlr.org
> >> Subject: [antlr-interest] How to force error recovery?
> >>
> >>    Hi all,
> >>
> >>    Look at this simple grammar:
> >>
> >> grammar testGrammar;
> >> options {
> >>   output=AST;
> >> }
> >>
> >> compilationUnit
> >>   : statement* EOF
> >>   ;
> >>
> >> statement
> >>   :   A^
> >>   |   B^ C
> >>   ;
> >>
> >> A   :   'a';
> >>
> >> B   : 'b';
> >>
> >> C   :   'c';
> >>
> >> WS  :   ( ' '
> >>         | '\t'
> >>         | '\r'
> >>         | '\n'
> >>         ) {$channel=HIDDEN;}
> >>     ;
> >>
> >>
> >>     Using the above grammar, it will successfully parse an input like:
> >>
> >> a b c a
> >>
> >>     Now, if the input is:
> >>
> >> a c a
> >>
> >>     The generated parser will parse "a", and will fail at "c", as it
> >> is
> > not a valid
> >> statement. Reading the error recovery chapter on the ANTLR book, I
> >> would imagine ANTLR would delete/skip the "c" token and try to
> >> recover, successfully parsing the second "a", as that is a valid
statement
> again.
> > But it is
> >> not working like this. It is aborting the parsing with an error at "c".
> >>
> >>     Question: how do I force it to recover from the error and
> >> continue
> > parsing?
> >>
> >>     The actual scenario is that the parser I am working on is used by
> >> an
> > IDE
> >> environment (eclipse), so we need it to continue parsing and
> >> presenting
> > the
> >> users with all the errors found in the file, not just the first one.
> >> The
> > error
> >> recovery seems to work on some rules, but not on the top rule
> >> (compilationUnit).
> >>
> >>     Thanks,
> >>        Edson
> >>
> >> --
> >>   Edson Tirelli
> >>   JBoss Drools Core Development
> >>   JBoss by Red Hat @ www.jboss.com
> >>
> >> List: http://www.antlr.org/mailman/listinfo/antlr-interest
> >> Unsubscribe:
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> >
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> 
> 
> 
> --
>   Edson Tirelli
>   JBoss Drools Core Development
>   JBoss by Red Hat @ www.jboss.com