Re: Perhaps a FAQ: How to shortcut an alternation if one already commited subrule fails

Hi Sean and other P::RD participants,

Sean O'Rourke schrieb:

> Barring Perl 6, one thing you can do is lift the unique prefixes up
> into the rule you want to fail, e.g.:
> 
> ENTITY : ( '#' <commit> COMMENT
>          | IDENT '=' <commit> OPTION
>          | TYPE NAME '{' <commit> DECLARATION
>          | 'scope' <commit> SCOPE
>          )(s)
> 
> COMMENT : m/.*/
> 
> OPTION  : VALUE
>            { store($item{IDENT}, $item{VALUE}) }
> 
> DECL    : DECL_BODY '}'
>            { store($item{TYPE}, $ITEM{NAME}, $ITEM{DECL_BODY}) }
> 
> SCOPE   : '{' ENTITY(s) '}'
> 
> On the other hand, this _does_ highlight exactly what prefixes you're
> using to distinguish between rules.  And it suggests you might want
> to put SCOPE right after comment, since it is accepted or rejected
> based on a single, simple token.


sure this is one possibility with a lot of drawbacks.

In my example I need the prefixes for processing it in
the subrule actions and the parent rule consumed it
already. OK, I could send it downstream as arguments
but this makes the grammar very uply and hard to maintain.

The biggest drawback is the error message:
With commiting the different productions already
in the ENTITY rule, I get just an error message, stating
that a ENTITY failed, not if it was already a commited
OPTION or DECLARATION etc.

I was thinking about unique prefix lookahead, but this
failed due to the multi token prefixes like TYPE VALUE
in the declaration. OK, again you could collapse this
to one more indirection, but this makes the grammar again
more unreadable and hard to mainatin (the example is just
a shrink down extract in a huge grammar).

Hmm, any solutions with this - now more detailed - problem
in mind?

Best Regards
	Charly

-- 
Karl Gaissmaier       KIZ/Infrastructure, University of Ulm, Germany
Email:[email protected]           Service Group Network
Tel.: ++49 731 50-22499