[antlr-interest] Parameters in fragment lexer rules

[Jean-Christophe Bach]

Hi,

> You need to make the distinction in the lexer via island grammars. Your case
> looks like it will be easy enough but you might need to formulate your lexer
> rules to avoid ambiguous cases. Look at the example island grammar in the
> downloadable example set.
> For your lexer you will need something like this:
> 
> ARROWLBRACE
>    : '->'
>       (
>              (WS* '{')=> WS* '{' SPECIFICPARSER { $type = SPECIFICBLOCK; }
>           |
>       )
>    ;
> 
> COLON
>   : ':'
>        (
>              (WS* '{')=> WS* '{' DIFFERENTPARSER { $type = DIFFERENTBLOCK; }
>           |
>       )
>   ;
> 
> LBRACE 
>   : '{' // Either the parser that calls this lexer knows what to do with
> Java, or you call a java parser here
>   ;

Thank you for your answer. I have already used this example as you
advised, and it is OK.

Now, how would you handle the last case which contains an ambiguity ? : there
are two different situations when a simple '{' is detected. Sometimes I have to
call another parser, sometimes I have to use a simple Java block code.
When I encounter a such situation, no specific character is detected just before
the left brace (contrary to the colon and arrow cases). Passing a parameter to
LBRACE (an int for instance) would be great, but it does not work very well when
used in a parser rule :

anAmbiguousParserRule :
  ... <no specific character> LBRACE[3]... -> ...
  | ... <no specific character> LBRACE[4] ... -> ...
  ;

I obtain this error : "token reference LBRACE may not have parameters"

Regards,

JC


> > -----Original Message-----
> > From: antlr-interest-bounces at antlr.org [mailto:antlr-interest-
> > bounces at antlr.org] On Behalf Of Jean-Christophe Bach
> > Sent: Tuesday, August 03, 2010 5:50 AM
> > To: antlr-interest at antlr.org
> > Subject: [antlr-interest] Parameters in fragment lexer rules
> > 
> > Hi list,
> > 
> > I am rewriting our old parser and I use antlr3 for that.
> > Since I have few problem to handle the '{' and to call the appropriate
> parser
> > depending on the context, I am wondering if fragment lexer rule + a
> > parameter could help me.
> > There are many situations, but I write here 3 cases :
> > ... '->' '{' ... : I need to call a specific parser (#1) ... ':'  '{' ...
> : I need to call
> > ainother specific parser (#2)
> > ...      '{' ... : I need to do a simple Java treatment
> > 
> > I read few articles and the antlr book, and I saw that it was possible to
> do give
> > parameters to a fragment lexer rule. I am wondering if something like that
> is
> > OK :
> > 
> > ARROWLBRACE : '->' LBRACE[2] ;
> > ...
> > <other rules with LBRACE[n]>
> > ...
> > fragment
> > LBRACE[int lbtype] : '{'
> >   {
> >   switch(lbtype) {
> >   case 1:
> >     <Java code1>
> >   case 2:
> >     <Java code2>
> >   case 3:
> >     <Java code3>
> >     ...
> >   }
> >   }
> >   ;
> > 
> > But am I also allowed to write a parser rule containing a LBRACE[n] or is
> it
> > totally illegal ?
> > e.g. :
> > 
> > myRule :
> >  ... LBRACE[1] ... -> ...
> >  |... ARROWLBRACE ... -> ...
> >  ;
> > 
> > When attempting to do that, I have errors :
> > "token reference LBRACE may not have parameters"
> > 
> > Is there any good way to solve this type of problem ?
> > 
> > Thanks in advance,
> > 
> > JC
> > 
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