Re: freeze/2 and nb_setarg/3 interaction

I'm not convinced SWI is wrong here.  I suspect in your 'broken' case that the second writeln is actually writing [H|T].  

It also seems odd that given X is already [H|T] you would attempt setarg(1,X,H), which is a no-op.  If instead you initialized X=[_|_] the code would make a little more sense.  In fact, it works.

In other words, freeze isn't the problem.
 
Alan Baljeu


________________________________
 From: Michael Hendricks <[email protected]>
To: Prolog mailing list <[email protected]>; Jan Wielemaker <[email protected]> 
Sent: Thursday, April 10, 2014 10:54:17 AM
Subject: Re: [SWIPL] freeze/2 and nb_setarg/3 interaction
 

I spent the morning looking through pl-prims.c and pl-attvar.c to debug
this.  My untrained eye didn't see any obvious problems.   Jan, what's the
best way for me to debug something like this?

Thanks.


-- 
Michael



On Wed, Apr 9, 2014 at 8:58 AM, Michael Hendricks <[email protected]> wrote:

> freeze/2 and nb_setarg/3 seem not to work together.  Consider this example:
>
>
> correct :-
>     X=[_|_],
>     ( nb_setarg(1,X, a),
>       nb_setarg(2,X, [b,c]),
>       writeln(X),
>       fail
>     ; writeln(X)
>     ).
>
> broken :-
>     X=[H|T],
>     freeze(H,nb_setarg(1,X,H)),
>     freeze(T,nb_setarg(2,X,T)),
>     ( H = a,
>       T = [b,c],
>       writeln(X),
>       fail
>     ; writeln(X)
>     ).
>
>
> ?- correct.
> [a,b,c]
> [a,b,c]
> true.
>
> ?- broken.
> [a,b,c]
> [_G1009|_G1010]
> true.
>
> Is that expected behavior?
>
> --
> Michael
>
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