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From: Mild Shock <janburse@fastmail.fm>
Newsgroups: comp.lang.prolog
Subject: My stupid Dogelog Player falls back to _<number> (Was: variable
 ordering guarantees in term_singletons/2)
Date: Sun, 25 May 2025 10:18:43 +0200
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SWI-Prolog has quite an elaborate '_' determiner:

/* SWI-Prolog */
?- length(L,2), (R = L; length(R,2)).
L = R, R = [_, _] ;
L = [_, _],
R = [_, _].

?- length(L,2), (R = f(L); length(R,2)).
L = [_A, _B],
R = f([_A, _B]) ;
L = [_, _],
R = [_, _].

Especially the L = R detection, seems challenging.
Thats something more complex than my listing/[0,1]
and term_singletons/2 usage there.

I have no pure Prolog solutions yet. My stupid
Dogelog Player falls back to _:

/* Dogelog Player */
?- length(L,2), (R = L; length(R,2)).
L = [_309342, _309344], R = [_309342, _309344];
L = [_309342, _309344], R = [_309495, _309497].

?- length(L,2), (R = f(L); length(R,2)).
L = [_310399, _310401], R = f([_310399, _310401]);
L = [_310399, _310401], R = [_310574, _310576].

Mild Shock schrieb:
> Hi,
> 
> People from Vienna were always a little strange.
> Why not adopt term_singletons/2 its already around
> for a while. You can do quite some magic with it.
> 
> Example: Determining singletons during listing,
> from Dogelog Player library(tester/tools):
> 
> % sys_listing_write(+Term, +Stream)
> sys_listing_write(C, T) :-
>     term_variables(C, V),
>     term_singletons(C, A),
>     sys_listing_names(V, A, 0, N),
>     write_term(T, C, [quoted(true), variable_names(N), format(true)]),
>     sys_answer_period(T).
> 
> If term_singletons/2 has the same variable ordering
> guarantees, i.e. left to right, as in term_variables/2
> you can use an algorithm without expensive lookup,
> 
> sys_listing_names([], _, _, []).
> sys_listing_names([X|L], [Y|R], K, ['_'=X|S]) :- X==Y, !,
>     sys_listing_names(L, R, K, S).
> sys_listing_names([X|L], A, K, [N=X|R]) :-
>     sys_listing_name(K, N),
>     J is K+1,
>     sys_listing_names(L, A, J, R).
> 
> Just run along the two lists , if something is both in
> the term_variables/2 and term_singletons/2 list, generate
> a '_' name, otherwise generate a synthetic name.
> 
> Bye
> 
> Mild Shock schrieb:
>> Hi,
>>
>> The development of Trealla Prolog and Scryer Prolog
>> looks like a random search in a mental ocean.
>>
>> It is like Prolog Development à la Sigmund Freud,
>> you have only to dig deep enough, and a solution
>>
>> will pop up. Otherwise blame your mother or other
>> relatives that raised you for supression.
>>
>> LoL
>>
>> Bye
>>
>> Examples: Still clueless how to detect singletons?
>> https://github.com/trealla-prolog/trealla/issues/743