Heisenbug in if/3

[Brian DeVries]

Howdy all,

I'm trying to use the if/3 predicate, because I'd like to be able to
backtrack over the first predicate in the test, rather than committing
to it as is done with ->/3. Unfortunately, I've hit a Heisenbug in
using it.

My simple sample code is:
bar(X) :- if(foo(3), true, true).
foo(1).
foo(2).
foo(3).

I expect it to bind X = 1 and return that answer, then X = 2 and X = 3
when I hit semicolon. When I run it, however, it gives me this error:

?- bar(X).
     ERROR at  clause 1 of prolog:call/1 !!
     INSTANTIATION ERROR- call/1: expected bound value

If I trace the execution, this happens:

?- trace.
% Trace mode on.
yes
% trace
   ?- bar(X).
          (1)    call:bar(_1055) ?
          (2)    call:if(user:foo(_1055),user:true,user:true) ?
          (3)    call:foo(_1055) ?
?         (3)    exit:foo(1) ?
          (4)    call:foo(1) ? l
X = 1 ? ;
X = 2 ? ;
X = 3 ? ;
no
% trace

Basically, if I trace the execution of bar/1, so long as I wait until
I see line (2) before leaping, I can execute without problem. If I
don't (i.e., I leap at line (1) ), then it gives me the above error. I
can provide additional traces if necessary, but I figure these should
be sufficient.

Additionally, if I turn trace back off, it goes back to the same error:

?- notrace.
% Debug mode off.
yes
   ?- bar(X).
     ERROR at  clause 1 of prolog:call/1 !!
     INSTANTIATION ERROR- call/1: expected bound value

X-(


Additionally, I can encounter a really weird trace with the following
sequence of steps (after exiting and reconsulting my test file):
   ?- trace.
% Trace mode on.
yes
% trace
   ?- bar(X).
          (1)    call:bar(_1055) ? l
     ERROR at  clause 1 of prolog:call/1 !!
     INSTANTIATION ERROR- call/1: expected bound value
          (1)    exception:bar(_1055) ? a
% YAP execution aborted.
   ?- bar(X).
     ERROR at  clause 1 of prolog:call/1 !!
     INSTANTIATION ERROR- call/1: expected bound value
   ?- trace.
% Trace mode on.
yes
% trace
   ?- bar(X).
          (1)    call:bar(_1055) ?
          (2)    call:call(user:_1126) ?
          (2)    exit:call(user:!) ?
          (1)    exit:bar(_1055) ?

true ?
yes
% trace

So now I'm really feeling X-( .

I am running YAP version Yap-5.1.3 .

Is there anything I am doing wrong? I don't see where I'm using if/3
improperly or something, and it works when I trace it (sometimes
anyways), so I'm pretty confused now.

Thanks!
~Brian W. DeVries

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[Paulo Moura]

On 2008/10/18, at 01:17, Brian DeVries wrote:

> I am running YAP version Yap-5.1.3 .

The bug is also present in the latest git version of YAP 5.1.4.

> Is there anything I am doing wrong? I don't see where I'm using if/3
> improperly or something, and it works when I trace it (sometimes
> anyways), so I'm pretty confused now.


The problem doesn't seem to be in the implementation of the if/3  
predicate:

    ?- if(foo(X), true, true).
X = 1 ? ;
X = 2 ? ;
X = 3 ? ;
no

Cheers,

Paulo


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

Office 4.3  Ext. 3257
Phone: [phone] Fax: [phone]
Email: <mailto:pmoura@...>

Home page: <http://www.di.ubi.pt/~pmoura>
Research:  <http://logtalk.org/>
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[Vitor Santos Costa-2]

Hi Brian

That was a bug in the compilation of if/3. I wrote a quick fix and I
pushed it to the git repository. The patch is at:

http://gitorious.org/projects/yap-git/repos/mainline/commits/38c97bd331d47baaa8a19328f336352a54128fc2

Thanks!

Vitor

On Sat, Oct 18, 2008 at 1:17 AM, Brian DeVries
<contingencyplan@...> wrote:

> Howdy all,
>
> I'm trying to use the if/3 predicate, because I'd like to be able to
> backtrack over the first predicate in the test, rather than committing
> to it as is done with ->/3. Unfortunately, I've hit a Heisenbug in
> using it.
>
> My simple sample code is:
> bar(X) :- if(foo(3), true, true).
> foo(1).
> foo(2).
> foo(3).
>
> I expect it to bind X = 1 and return that answer, then X = 2 and X = 3
> when I hit semicolon. When I run it, however, it gives me this error:
>
> ?- bar(X).
>     ERROR at  clause 1 of prolog:call/1 !!
>     INSTANTIATION ERROR- call/1: expected bound value
>
> If I trace the execution, this happens:
>
> ?- trace.
> % Trace mode on.
> yes
> % trace
>   ?- bar(X).
>          (1)    call:bar(_1055) ?
>          (2)    call:if(user:foo(_1055),user:true,user:true) ?
>          (3)    call:foo(_1055) ?
> ?         (3)    exit:foo(1) ?
>          (4)    call:foo(1) ? l
> X = 1 ? ;
> X = 2 ? ;
> X = 3 ? ;
> no
> % trace
>
> Basically, if I trace the execution of bar/1, so long as I wait until
> I see line (2) before leaping, I can execute without problem. If I
> don't (i.e., I leap at line (1) ), then it gives me the above error. I
> can provide additional traces if necessary, but I figure these should
> be sufficient.
>
> Additionally, if I turn trace back off, it goes back to the same error:
>
> ?- notrace.
> % Debug mode off.
> yes
>   ?- bar(X).
>     ERROR at  clause 1 of prolog:call/1 !!
>     INSTANTIATION ERROR- call/1: expected bound value
>
> X-(
>
>
> Additionally, I can encounter a really weird trace with the following
> sequence of steps (after exiting and reconsulting my test file):
>   ?- trace.
> % Trace mode on.
> yes
> % trace
>   ?- bar(X).
>          (1)    call:bar(_1055) ? l
>     ERROR at  clause 1 of prolog:call/1 !!
>     INSTANTIATION ERROR- call/1: expected bound value
>          (1)    exception:bar(_1055) ? a
> % YAP execution aborted.
>   ?- bar(X).
>     ERROR at  clause 1 of prolog:call/1 !!
>     INSTANTIATION ERROR- call/1: expected bound value
>   ?- trace.
> % Trace mode on.
> yes
> % trace
>   ?- bar(X).
>          (1)    call:bar(_1055) ?
>          (2)    call:call(user:_1126) ?
>          (2)    exit:call(user:!) ?
>          (1)    exit:bar(_1055) ?
>
> true ?
> yes
> % trace
>
> So now I'm really feeling X-( .
>
> I am running YAP version Yap-5.1.3 .
>
> Is there anything I am doing wrong? I don't see where I'm using if/3
> improperly or something, and it works when I trace it (sometimes
> anyways), so I'm pretty confused now.
>
> Thanks!
> ~Brian W. DeVries
>
> -------------------------------------------------------------------------
> This SF.Net email is sponsored by the Moblin Your Move Developer's challenge
> Build the coolest Linux based applications with Moblin SDK & win great prizes
> Grand prize is a trip for two to an Open Source event anywhere in the world
> http://moblin-contest.org/redirect.php?banner_id=100&url=/
> _______________________________________________
> Yap-users mailing list
> Yap-users@...
> https://lists.sourceforge.net/lists/listinfo/yap-users
>

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Grand prize is a trip for two to an Open Source event anywhere in the world
http://moblin-contest.org/redirect.php?banner_id=100&url=/
_______________________________________________
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Yap-users@...
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