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Re: Strange variables...
From: |
Salvador Pinto Abreu |
Subject: |
Re: Strange variables... |
Date: |
Thu, 8 Jan 2015 23:51:22 +0000 |
while not really a solution to your problem, a workaround is to use the
compiler (ie. gplc) instead of consulting.
you don't get debugging but performance is way better.
21:13:32$ gplc -o xx xx.pl
21:13:38$ ./xx
GNU Prolog 1.4.4 (64 bits)
Compiled Aug 27 2014, 09:59:03 with gcc
By Daniel Diaz
Copyright (C) 1999-2013 Daniel Diaz
| ?- p5(X).
X = [1,3] ? a
X = [1,3]
X = [2,3]
X = [2,3]
yes
| ?-
> On 08 Jan 2015, at 19h57, Fred Bapst <address@hidden> wrote:
>
> I'm observing a rather strange behaviour (GnuProlog 1.4.4 on Windows and
> Linux,
> idem for older versions) and I'm not able to completely isolate it; it seems
> to
> be related to singleton variables and backtracking. Here is a stupid code
> demonstrating the issue:
>
> %--------------
> z(A,B, A,B).
> z(A,B, B,A).
>
> p5(Ls):-
> _ = [B, C],
> % write(B), % write(C),
> Ls = [A, E],
> z(A, _, 1, 2),
> E=3,
> z(A, _, B, C).
>
> %--------------
>
> Asking for every solution leads to an infinite loop...
>
> ?- p5(X).
> X = [1,3] ? a
> ...... [infinine loop].....
>
> Writing B and C removes the problem; and writing only one of them leads to
> another strange behavior.
>
> Running the call with the tracer also changes the behavior, and reveals
> strange d(...) sub-terms:
>
> {trace}
> | ?- p5(X).
> 1 1 Call: p5(_23) ?
> 2 2 Call: z(_53,_57,1,2) ?
> 2 2 Exit: z(1,2,1,2) ?
> 3 2 Call: z(1,_85,_126,_127) ?
> 3 2 Exit: z(1,_85,1,_85) ?
> 1 1 Exit: p5([1,3]) ?
> X = [1,3] ? a
> 1 1 Redo: p5([1,3]) ?
> 3 2 Redo: z(1,_85,1,_85) ?
> 3 2 Exit: z(1,_85,_85,1) ?
> 1 1 Exit: p5([1,3]) ?
> X = [1,3]
> 1 1 Redo: p5([1,3]) ?
> 2 2 Redo: z(1,2,1,2) ?
> 2 2 Exit: z(2,1,1,2) ?
> 3 2 Call:
> z(2,_85,d(3,[a(z(2,_85,d(3,[a(z(2,_85,...),3,2,...),a(p5(...),1,...)]),2),3,2,144),a(p5([2,3]),1,1,87)]),2)
> ?
> 3 2 Exit:
> z(2,d(3,[a(z(2,d(3,[a(z(2,d(...,...),...),3,2,...),a(p5(...),1,...)]),d(3,[a(z(2,d(...,...),...),3,2,...),a(p5(...),1,...)]),2),3,2,144),a(p5([2,3]),1,1,87)]),d(3,[a(z(2,d(3,[a(z(2,d(...,...),...),3,2,...),a(p5(...),1,...)]),d(3,[a(z(2,d(...,...),...),3,2,...),a(p5(...),1,...)]),2),3,2,144),a(p5([2,3]),1,1,87)]),2)
> ?
> 1 1 Exit: p5([2,3]) ?
> X = [2,3]
> (16 ms) yes
> {trace}
>
>
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