[antlr-interest] Semantic predicates in v4 lexer rules

[A Z]

Thanks for the response. It's also giving me errors for all syntactic
predicates so I assume it's for the same reason.


On Tue, Jan 24, 2012 at 1:13 AM, Sam Harwell <sam at tunnelvisionlabs.com>wrote:

> Even after this is resolved, semantic predicates are almost surely not the
> correct way to handle this. Instead, you should use an action like the
> following at the end of the rule to reset the keyword to an identifier if
> keywords are disabled. Semantic predicates in a v4 lexer have a major
> performance cost.
>
> K_IF : 'if' {if (!keywords) $type=ID;};
>
> --
> Sam Harwell
> Owner, Lead Developer
> http://tunnelvisionlabs.com
>
>
> -----Original Message-----
> From: Terence Parr [mailto:parrt at cs.usfca.edu]
> Sent: Monday, January 23, 2012 12:40 PM
> To: A Z
> Cc: antlr-interest at antlr.org
> Subject: Re: [antlr-interest] Semantic predicates in v4 lexer rules
>
> ooops. I removed actions from anywhere except the right-hand side but
> forgot
> to leave in predicates.
>
>  adding to the list.
> Ter
> On Jan 23, 2012, at 10:31 AM, A Z wrote:
>
> > Can we still use sementic predicates in lexer rules using v4? The
> > following code works in 3.4 but 4.0ea complains with
> >
> > error(15): TestLexer.g:484:29: '{keywords == true}?' came as a
> > complete surprise to me while looking for lexer rule element
> >
> >
> > K_IF                       : {keywords == true}? => 'if';
> > K_INT                      : {keywords == true}? => 'int';
> >
> >
> > The language I'm using allows changing the keyword set via
> > preprocessor directives so all the keywords need to test a variable at
> > runtime. Is there another way to do this in v4?
> >
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