Re: Disjunction in phrase/[2,3]

Hi Jan,

Thanks for the quick reply! Your solution indeed works for DCGs with no
extra arguments (only the difference list parameters), as was applicable to
the example I gave. But could one also achieve this result for DCGs with
extra arguments?

I'll give an example:
~~~{.pl}
a(C) -->
  x((b ; c), C).
b(98) --> [98].
c(99) --> [99].

:- meta_predicate(x(//,?,?,?)).
:- meta_predicate(x(3,?,?,?)).
x(M:DCG1, C, A, B):-
  atom(DCG1), !,
  DCG2 =.. [DCG1,C],
  phrase(M:DCG2, A, B).
x(M:DCG1, C, A, B):-
  DCG1 =.. [;|Ps],
  member(P, Ps),
  DCG2 =.. [P,C],
  phrase(M:DCG2, A, B).
~~~

I have tried to comment out either of the meta_predicate/1 directives in
the above example.
- Without the //-variant swipl warns for the use of  disjunction (as
explained in your email, this behavior is expected).
- Without the 3-variant swipl warns for the missing predicates b/2 and c/2
(which is also understandable, since the // does not indicate the number of
arguments the DCG rule has).
Is there a way of combining both directives?

---
Cheers!,
Wouter.

E-mail: [email protected]
WWW: www.wouterbeek.com
Tel.: 0647674624


On Sun, Jul 28, 2013 at 11:03 AM, Jan Wielemaker <[email protected]> wrote:

> On 07/28/2013 10:29 AM, Wouter Beek wrote:
> > Hi all,
> >
> > I get the following warning for the use of disjunction in meta-DCG rules:
> > ~~~
> > Warning: The predicates below are not defined. If these are defined
> > Warning: at runtime using assert/1, use :- dynamic Name/Arity.
> > Warning:
> > Warning: test:;/4, which is referenced by
> > Warning:        c:/prolog/webqr/test.pl:6:5: 1-st clause of test:a/2
> > ~~~
> >
> > Am I doing something wrong or is the missing predicates check too
> > restrictive?
> >
> > Sample code:
> > ~~~{.pl}
> > a -->  x((b ; c)).
> > b --> [98].
> > c --> [99].
> > :- meta_predicate(x(2,?,?)).
>
> Works with this declaration.  DCG bodies are are bit more complicated
> than just '2', so you must tell the system.
>
> :- meta_predicate(x(//,?,?)).
>
>         Cheers --- Jan
>
> > x(DCG, A, B):- phrase(DCG, A, B).
> > ~~~
> >
> > ---
> > Cheers,
> > Wouter Beek.
> >
> > E-mail: [email protected]
> > WWW: www.wouterbeek.com
> > Tel.: 0647674624
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>
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