Re: writting anonymous variables in a file

On 02/03/2014 04:38 PM, db wrote:
>
> Hi Jan,
>
> Le 03/02/2014 16:00, Jan Wielemaker a �crit :
>> On 02/03/2014 03:43 PM, db wrote:
>>> Hello,
>>>
>>> I want to write atoms with anonymous variables, like p(_, _), in a
>>> file, but in the file, each anonymous variable (_) have been
>>> replaced by a named anonymous variable, for example _906541.
>>
>> SWI-Prolog's variables are written _GXXXX or _LXXXX.  Is this SWI-Prolog?
>
> I gave a bad example. The true message is :
>
> "Singleton-marked variable appears more than once: _G506341"
>
>
>> This suggests you try to write a single non-ground term using
>> multiple calls to write, for example as below.
>>
>> write('hello('), write(X), write(', '), write(Y), write(').\n').
>
> That is what I have done, because I needed to format
> my list.
> More precisely, I wrote the term
> p(_X, _Y)
> and further (with numerous writings between them)
> the term
> p(_Z, _T).
>
> But _X and _Y have been replaced in my destination file
> by the same variable _G506341.

Yes, that can happed. Similarly, writing p(_X, _Y) and later p(_X, _Z)
might cause the _X not to be the same. It just can't work for non-ground
terms. Using numbervars or some other way to name the variables is the
only thing that can work. The _GXXXX names are simple not stable and,
AFAIK, that is the case in all Prolog system I know about that have a
garbage collector.

Note that numbervars binds your variables, so you typically do

	\+ \+ ( numbervars(Term, 0, _, [singletons(true)]),
		<do the actual writing>
	      ).

	Cheers --- Jan
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