[antlr-interest] Re: Local lookahead depth

[leung13512c]

My view about the local lookahead depth is that it is safer to
use than fixed/syntax lookahead which given the alternative priority
over other alternatives while warning would be issued if the 
local lookahead depth is not sufficient.  Anything that
can be done by changing the local lookahead depth can be done
by adding simple fixed lookahead like %LOOKAHEAD(2) to some of
the alternatives.

Chris.
--- In antlr-interest at yahoogroups.com, Oliver Zeigermann <oliver at z...>
wrote:
> Hi Loring!
> 
> lgcraymer wrote:
> > ANTLR is actually quite a bit smarter about lookahead--k is
maximum lookahead, and ANTLR generates minimum lookahead.  If you 
> > set k=20 but don't need more than k=2, then 2 is the maximum
lookahead in the generated code.  I think that Sriram put local 
> > lookahead into JavaCC as a workaround--it does add a step to
grammar debugging that is not necessary for ANTLR.
> 
> Hmmm. Is this true? Definitly not when using greedy blocks... Have a 
> look at this simple grammar
> 
> top : TOKEN0 (sub | top )* TOKEN3 ;
> 
> sub : TOKEN1
>          ( options {greedy=false;} : . )*
>          TOKEN2
>      ;
> 
> k=1 is needed and works ok. Try using k=10 and: Ooops!
> 
> > Also, as to actions in lookahead code:  this is something that Ter
supported in PCCTS under the name "guarded predicates" or some 
> > such.  I don't know that it saw much use, and I suspect that usage
indicates a too early incorporation of semantic information into the 
> > translator--tree transformation helps avoid that.
> 
> 1.) You might really increase the set of parseable languages using this 
> technique
> 2.) Sometimes using tree transformation is too expensive
> 
> > LLK is interesting, but it is a synthesis of implementation
approaches from a number of sources (ANTLR, JavaCC, flex) and not an 
> > extension to the state of the art.  I think that's why Leung
describes it as an ANTLR 2.7.2 derivative despite what seems to be the 
> > complete absence of any ANTLR 2.7.2 source code in the
distribution.  It is also a work in progress.
> 
> Who knows ;) In any case it brings a bit life into discussion :)
> 
> Oliver


 

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