[antlr-interest] syntax predicate strange behavior

[David Holroyd]

On Tue, May 01, 2007 at 02:03:37AM +0400, Ilia Kantor wrote:
> command_user_body:	
> 	 (LCURL) => LCURL command_arguments? RCURL -> ^(USER_COMMAND command_arguments?) |		 			
> 	 -> USER_COMMAND
> ;
> 
> On input LCURL WS WORD WS MINUS GT...
> this gives me exception from DFA.noViableAlt at first WORD token.

Which decision is it trying to predict when it fails?  That rule has a
'|' decision and a '?' decision.  The command_arguments rule might be
significant to understanding the problem.

Also, why the predicate on LCURL?  Probably makes sense in the wider
grammar, but we can't see that ;)


> By the way, this works fine:
> 
> command_user_body
> options {backtrack=true;}:	
> 	LCURL command_arguments? RCURL -> ^(USER_COMMAND command_arguments?) |		 			
> 	 -> USER_COMMAND
> ;

With backtracking (which I admit I don't wholly understand yet) I guess
if it fails on the first alt, it will back-off to the second, empty
alternative?



-- 
http://david.holroyd.me.uk/