[antlr-interest] @init actions executed during lookahead, @after actions not

Ron Hunter-Duvar ron.hunter-duvar at oracle.com
Mon Apr 19 08:21:52 PDT 2010


Hi Thomas,

I suspected that might be the explanation. If it's intended for use in 
declaring local variables, putting it inside an if would break it, at 
least in Java.

I do need a local variable declaration (I record the stack depth at the 
start to ensure the stack pop at the end is to the right level) so I 
can't just move it into an action at the start of each rule. I know I 
could leave the variable declaration in the @init, and do the push in an 
action. But the solution I have now of checking for backtracking within 
the method that does the push works well, and keeps the grammar simpler.

Thanks,
Ron


tcorbat at hsr.ch wrote:
> Hi Ron,
>
> I think this behavior is as intended. The @init block is intended to declare local variables. Therefore, it will always be executed. 
> You could move the action, manipulating your stack, out of the @init block into an action inside the rule. Then, it would not get executed during lookahead.
> I don't know if it is favorable, but if this manipulation of the stack is required in the subrules to correctly decide on alternatives, you could leave the action in @init and put the cleanup action (form @after) to the "finally" block, which will always get executed, regardless of the backtracking state.
>
> I hope this helps.
> Thomas
>
>
> ________________________________________
> Von: antlr-interest-bounces at antlr.org [antlr-interest-bounces at antlr.org] im Auftrag von Ron Hunter-Duvar [ron.hunter-duvar at oracle.com]
> Gesendet: Samstag, 17. April 2010 00:59
> An: antlr-interest at antlr.org
> Betreff: [antlr-interest] @init actions executed during lookahead,      @after actions not
>
> Hi,
>
> I just ran into something a little odd. I'm using @init actions in some
> parser rules to stack some information and @after to pop it again. In
> the generated Java code, the @after action gets wrapped in an "if (
> state.backtracking==0 ) {...}", so that it only gets executed when other
> actions are being executed, not during lookahead. This is what I
> expected. But I noticed that the @init actions are executed
> unconditionally, including during lookahead. I didn't expect this. The
> result was a lot of junk on the stack when it went into a dfa. The fix
> was easy enough, just checking state.backtracking myself. But I was
> wondering if this is an Antlr bug or if it's supposed to work this way.
>
> Ron
>
> --
> Ron Hunter-Duvar | Software Developer V | 403-272-6580
> Oracle Service Engineering
> Gulf Canada Square 401 - 9th Avenue S.W., Calgary, AB, Canada T2P 3C5
>
> All opinions expressed here are mine, and do not necessarily represent
> those of my employer.
>
>
> List: http://www.antlr.org/mailman/listinfo/antlr-interest
> Unsubscribe: http://www.antlr.org/mailman/options/antlr-interest/your-email-address
>
> List: http://www.antlr.org/mailman/listinfo/antlr-interest
> Unsubscribe: http://www.antlr.org/mailman/options/antlr-interest/your-email-address
>
>   

-- 
Ron Hunter-Duvar | Software Developer V | 403-272-6580
Oracle Service Engineering
Gulf Canada Square 401 - 9th Avenue S.W., Calgary, AB, Canada T2P 3C5

All opinions expressed here are mine, and do not necessarily represent
those of my employer.



More information about the antlr-interest mailing list