Re: [Info-gnuprologjava] gnuprologjava patch: fixes for is_list

[Daniel Thomas]

Hmm,

ISO/IEC DTR 13211–3:2006 defines is_list in its reference implementation
of DCGs as:
is_list([]) :-
!.
is_list([_| Tail]) :-
is_list(Tail).

but SWI-Prolog use:
is_list(X) :- 
var(X), !,
        fail.
is_list([]).
is_list([_|T]) :-
        is_list(T).
and http://www.swi-prolog.org/pldoc/man?predicate=is_list%2F1 claims
this is the de-facto standard.

It seems probable that we should switch to the SWI-Prolog behaviour as
your patch proposes, the subtle failure is particularly nasty. However I
would like to double check that with Michiel Hendriks (hopefully CCed)
who wrote Predicate_is_list.

Daniel

On Mon, 2012-05-07 at 20:30 +0100, Paul Singleton wrote: 
> On 07/05/2012 17:21, Daniel Thomas wrote:
> > Again please can I have a test case to show what this fixes and prevent
> > the same problem happening again. test/inriasuite/extra/list/is_list
> > contains the current tests.
> 
> These inria tests are broken (is_list/1 should never instantiate further):
> 
> [is_list(X),[[X <-- []]]].
> [is_list([a,b|X]),[[X <-- []]]].
> [is_list([1,b|X]),[[X <-- []]]].
> [is_list([3,2|X]),[[X <-- []]]].
> [is_list([3.2,0.1|X]),[[X <-- []]]].
> [is_list([k,82.3|X]),[[X <-- []]]].
> [is_list([a,b,d,y,e,foo(a),2,4,6,dt,9.2|X]),[[X <-- []]]].
> 
> all should yield failure, i.e.
> 
> [is_list(X),failure].
> [is_list([a,b|X]),failure].
> [is_list([1,b|X]),failure].
> [is_list([3,2|X]),failure].
> [is_list([3.2,0.1|X]),failure].
> [is_list([k,82.3|X]),failure].
> [is_list([a,b,d,y,e,foo(a),2,4,6,dt,9.2|X]),failure].
> 
> Fix them and they act as test cases to show up the first bug below.
> 
> NB inria maybe thinks is_list/1 should succeed for var-terminated or 
> proper lists, as it also has is_proper_list/1, but ISO is_list/1 
> demands a proper list, and the tests above are broken anyway.
> 
> The second bug is more subtle:
> 
> [(Q=foo,is_list([1|Q])),failure]
> 
> but unpatched is_list destructively rebinds Q to [] and succeeds!
> 
> > On Fri, 2012-03-23 at 12:08 +0000, Paul Singleton wrote:
> >> fix bugs in vm.buildins.list.Predicate_is_list (unifies var with empty
> >> list, fails to dereference tail of list) and remove redundant arity
> >> check after checking ./2 tag
> >>
> >> ----
> >>
> >> diff --git a/src/gnu/prolog/vm/buildins/list/Predicate_is_list.java
> >> b/src/gnu/prolog/vm/buildins/list/Predicate_is_list.java
> >> index 5ea3ea1..0ab4dbf 100644
> >> --- a/src/gnu/prolog/vm/buildins/list/Predicate_is_list.java
> >> +++ b/src/gnu/prolog/vm/buildins/list/Predicate_is_list.java
> >> @@ -19,6 +19,7 @@
> >>     */
> >>    package gnu.prolog.vm.buildins.list;
> >>
> >> +import gnu.prolog.term.AtomTerm;
> >>    import gnu.prolog.term.CompoundTerm;
> >>    import gnu.prolog.term.Term;
> >>    import gnu.prolog.vm.ExecuteOnlyCode;
> >> @@ -41,7 +42,7 @@
> >>                    Term lst = args[0];
> >>                    while (lst != null)
> >>                    {
> >> -                  if (interpreter.unify(TermConstants.emptyListAtom, lst) 
> >> ==
> >> RC.SUCCESS_LAST)
> >> +                  if (lst instanceof AtomTerm&&  ((AtomTerm) lst) ==
> >> TermConstants.emptyListAtom)
> >>                            {
> >>                                    return RC.SUCCESS_LAST;
> >>                            }
> >> @@ -50,11 +51,7 @@
> >>                                    return RC.FAIL;
> >>                            }
> >>                            CompoundTerm ct = (CompoundTerm) lst;
> >> -                  if (ct.args.length != 2)
> >> -                  {
> >> -                          return RC.FAIL;
> >> -                  }
> >> -                  lst = ct.args[1];
> >> +                  lst = ct.args[1].dereference();
> >>                    }
> >>                    return RC.FAIL;
> >>            }
> >
> 
>

signature.asc
Description: This is a digitally signed message part