[antlr-interest] Lookahead question

[Terence Parr]

On Dec 8, 2004, at 9:45 AM, jose.sanleandro at ventura24.es wrote:
> The reason is that some of the grammar rules allow arbitrary texts, 
> which
> ocassionally triggered conflicts with literals. I thought I had just 
> the
> lookahead option to defeat such conflicts.
> Finally, I decided to use a lookahead of one character, and explictly 
> solve
> the conflicts. That ended up in a grammar which doesn't seem so :(. 
> Take a
> look at a fragment:
>
> STARTS_WITH_B:
>     'b'
>      (({ if  (   (LA(1) == 'r')
>               && (LA(2) == 'a')
>               && (LA(3) == 'n')
>               && (LA(4) == 'c')
>               && (LA(5) == 'h')
>               && (LA(6) == ':'))
>          {
>            mRESERVED_BRANCH(false);
>            $setType(LITERAL_BRANCH);
>          }
>          else
>          {
>            mSTRING(false);
>            $setType(STRING);
>          }
>        })
>     )
>     ;

Isn't that the same as looking up the literal in the literals table?  
I.e., just have rule:

ID : ('a'..'z')+ ;

and no rules like

BR : "branch" ;

and ANTLR will do the right thing albeit not super fast ;)

> Moreover, which is the main drawback of explicitly resolving the 
> ambiguous
> situations for the lexer using inline LA(x) checks?

Well, only that ANTLR should really be saving you the trouble. :)

Ter
--
CS Professor & Grad Director, University of San Francisco
Creator, ANTLR Parser Generator, http://www.antlr.org
Cofounder, http://www.jguru.com
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