Re: predicate_property help.

[Paulo Moura]

On 2007/10/02, at 15:20, Amine Marref wrote:

> The above code works with Sicstus for instance. However, GNU Prolog  
> needs A to be a  predicate indicator e.g. length/2 and not a goal.

True.

> so:
>
> | ?- predicate_property(length/2, built_in).
>
> yes
> | ?- predicate_property(length([a,b],2), built_in).
> uncaught exception: error(type_error(predicate_indicator,length 
> ([a,b],2)),predicate_property/2)
>
> My question is then: how do I go from the goal length([a,b],2) to  
> the predicate indicator length/2?
>
> The best I could do was:
>
> | ?- functor(length([a,b],2), Functor, Arity), atom_concat 
> (Functor,/,Temp), number_atom(Arity, Arity1), atom_concat(Temp,  
> Arity1, Predicate_Indicator).
>
> Arity = 2
> Arity1 = '2'
> Functor = length
> Predicate_Indicator = 'length/2'
> Temp = 'length/'
>
> yes
>
> However, 'length/2' does not work as first argument to  
> predicate_property.

Of course not, 'length/2' is an atom. Try:

| ?- functor(length([a,b],2), Functor, Arity), predicate_property 
(Functor/Arity, built_in).

All the best,

Paulo


-----------------------------------------------------------------
Paulo Jorge Lopes de Moura
Dep. of Computer Science, University of Beira Interior
6201-001 Covilhã, Portugal

Office 4.3  Ext. 3257
Phone: +351 275319891 Fax: +351 275319899
Email: <mailto:address@hidden>

Home page: <http://www.di.ubi.pt/~pmoura>
Research: <http://logtalk.org/>
-----------------------------------------------------------------