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From: Mild Shock <janburse@fastmail.fm>
Newsgroups: comp.lang.prolog
Subject: Cannot consult/1 after listing/1 (Was: Scryer Prolog has dethroned
 SWI-Prolog)
Date: Wed, 28 May 2025 12:09:28 +0200
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In-Reply-To: <1011fck$a9rv$1@solani.org>

I retract my claim that Scryer Prolog has dethroned
anything. Its still in a quite bad state.

It also shows that generating annonymous variables,
has also some use cases, where its not only an arbitrary
feature but rather a necessity.

Take this real world example, from a file case.pl:

:- dynamic(sys_get_code_list/6).
sys_get_code_list(-1, _, _, _, [], _) :- !.
sys_get_code_list(H, H, _, _, [H], _) :- !.
sys_get_code_list(H, _, 1, _, [H], _) :- !.
sys_get_code_list(H, D, 0, S, [H|L], F) :- !,
    sys_get_code(S, J, F),
    sys_get_code_list(J, D, 0, S, L, F).
sys_get_code_list(H, D, M, S, [H|L], F) :-
    sys_get_code(S, J, F),
    N is M-1,
    sys_get_code_list(J, D, N, S, L, F).

If I use Scyer Prolog listing/1 I get,
I added the dynamic manually:

:- dynamic(sys_get_code_list/6).
sys_get_code_list(-1,A,B,C,[],D) :-
    !.
sys_get_code_list(A,A,B,C,[A],D) :-
    !.
sys_get_code_list(A,B,1,C,[A],D) :-
    !.
sys_get_code_list(A,B,0,C,[A|D],E) :-
    !,
    sys_get_code(C,F,E),
    sys_get_code_list(F,B,0,C,D,E).
sys_get_code_list(A,B,C,D,[A|E],F) :-
    sys_get_code(D,G,F),
    H is C-1,
    sys_get_code_list(G,B,H,D,E,F).

If I put the generated code into a file case2.pl,
I suddently get a lot of warnings:

$ target/release/scryer-prolog
?- [case2].
% Warning: singleton variables A, B, C, D at line 1 of case2.pl
% Warning: singleton variables B, C, D at line 3 of case2.pl
% Warning: singleton variables B, C, D at line 5 of case2.pl
    true.


Mild Shock schrieb:
> Hi,
> 
> Interestingly Scryer Prolog does many things right.
> I find the following test case:
> 
> /* Scryer Prolog 0.9.4-403 */
> ?- length(L,2), f(L) = R, S = [_A].
> L = [_B, _C],
> R = f([_B, _C]),
> S = [_A].
> 
> While SWI-Prolog still makes the following error. The
> newly generated _A and _B are not checked whether they
> already appear in the answer elsewhere, causing a name clash:
> 
> /* SWI-Prolog 9.3.22 */
> ?- length(L,2), f(L) = R, S = [_A].
>   = [_A, _B],
> R = f([_A, _B]),
> S = [_A].
> 
> Other Prolog systems that avoid the clash as well are
> Trealla Prolog, and since today Dogelog Player.
> 
> Bye
> 
> Mild Shock schrieb:
>> Hi,
>>
>> It is time for the 2025 Prolog Awards.
>> Are you excited yet?
>>
>> https://9gag.com/gag/aPAXyEP
>>
>> Here is the list:
>>
>> 1. Place: Scryer Prolog, it uses rustyline,
>> very convenient:
>>
>> app<tab>     gives append
>> a<tab><tab>  cycles through letter a predicates
>> length(foo)  shows the matching braket in blue
>> Etc..
>>
>> -1. Place: SWI-Prolog, has no line edit at all
>>
>> Bye
>