Re: Feature wishlist for P::RD 2.0 ore perhaps 1.81

Hi Damian,

Damian Conway schrieb:
> 
> Karl Gaissmaier wrote:
> 
> ...some suggestions for additional features of Parse::RecDescent, specifically
> the ability to specify "required alternatives" within a repeated subrule:
> 
>    > statement: A! | B! | C | D
> 
> and "mutually exclusive alternations":
> 
>    > rule     : A ^ B ^ C
> 
> The problem with these proposed features is that they are not *localized*.
> 
> That is, the presence of an A! doesn't affect the rule that the A! is in,
> it affects the rule that calls the rule that the A! is in. And that is difficult
> to implement in a recursive descent parser.
> 

as I feared already in my first mail about adding an additional state

> > But I think that P::RD is by far no way just a CFG ParserGenerator, it's a beast.
> 
> <grin> I should quote you in the documentation. ;-)

you're welcome

> 
> > I stumbled so many times over these two missing features (mandatory and xor)
> > and my "feeling" tells me, there must be a gentler(general) solution
> > than just doing this in action codes.
> 
> Clearly this has been a problem for you. But I would have solved it like so:
> 
>         $grammar = << 'EOGRAMMAR';
>         contact   : statement(s)
>                                                 { $return = { map %$_ @{$item[-1]} } }
>                     <reject: do{!($return->{email} && $return->{name})) >
>         statement : email | name |  phone | fax
>         email     : 'email' '=' value  {{email=>$item[-1]}}
>         name      : 'name' '=' value   {{name=>$item[-1]}}
>         phone     : 'phone' '=' value  {{phone=>$item[-1]}}
>         fax       : 'fax' '=' value    {{fax=>$item[-1]}}
>          value     : /".*?"/
>         EOGRAMMAR
> 
> Since you needed to collect the data anyway, the actions would have had to be
> there no matter what. So the overhead for the requirements testing is just the single
> <reject> directive.

again it's just a question of maintainance. If I introduce a different
subrule in statement: I have to change action code logic. I think
therefore you introduced for example the %item hash, because we are
always not perfect. My stupid thinking:

Everything already done very well by Damian Conway will not hurt
me any more by my imperfection :-)

> 
> Likewise, if (for example) phone and fax were mutually exclusive, you could
> just extend that to:
> 
>         $grammar = << 'EOGRAMMAR';
>         contact   : statement(s)
>                                                 { $return = { map %$_ @{$item[-1]} } }
>                     <reject: do{!($return->{email} && $return->{name})} >
>                     <reject: do{  $return->{phone} && $return->{fax}  } >
>         statement : email | name |  phone | fax
>         email     : 'email' '=' value  {{email=>$item[-1]}}
>         name      : 'name' '=' value   {{name=>$item[-1]}}
>         phone     : 'phone' '=' value  {{phone=>$item[-1]}}
>         fax       : 'fax' '=' value    {{fax=>$item[-1]}}
>          value     : /".*?"/
>         EOGRAMMAR
> 

sure, the same comments as before.

> > Perhaps Damian can bring some light in the darkness.
> 
> Perhaps. But I doubt it. ;-)
> 
> However, since you've taken the trouble to mention (and obviously think about)
> this problem, I will certainly devote some time to it myself and see if I can
> develop a better solution; one that's consistent with a recursive descent implementation.
> 

thanks in advance, even if P::RD stays at it is already!

Best Regards
	Charly
-- 
Karl Gaissmaier          Computing Center,University of Ulm,Germany
Email:[email protected]          Network Administration
Tel.: ++49 731 50-22499