[Yap-users] More fun problems!

[Edward S. <esc...@ce...>]

Hi all,

Thanks again for the help so far.  I removed clpfd from our program, and
that particular issue went away.  Unfortunately, we have another one.

Here's a snippet of the problem:

certainInstanceCallsMethod(Constructor, Method) :-
    someStuff(Constructor, Method),
    writeln('ok'), writeln(Constructor), writeln(Method).

certainInstanceCallsMethod(Constructor, Method) :-
    writeln('hey'),
    writeln(Method),
    certainConstructor(Constructor),
    writeln('hey2'),
    writeln(Constructor),
    certainVFTableEntry(VFTable, _, Method),
    writeln('here we go'),
    writeln(Method),
    % And the method is in that virtual function table.
    certainVFTableWrite(_, Constructor, _, VFTable),
    writeln('done!').

certainInstanceCallsMethodSet(Constructor, Set) :-
    certainConstructor(Constructor),
    setof(Method, certainInstanceCallsMethod(Constructor, Method), Set).

Here is the problematic output:
ok
method(101)
method(101)
ok
method(101)
method(102)
ok
method(101)
method(103)
hey
_164154
hey2
method(101)
C = method(101),
S = [method(101),method(102),method(103)] ? ;
here we go
method(104)
done!
false.

So the first rule is used to prove
certainInstanceCallsMethod(method(101), method(101)),
certainInstanceCallsMethod(method(101), method(102)), and
certainInstanceCallsMethod(method(101), method(103)).

The second rule begins evaluating, but stops when it reaches
certainVFTableEntry.  AFTER I hit semicolon some type of backtracking
occurs and the second rule finishes, proving
certainInstanceCallsMethod(method(101), method(104)).  If I execute the
query again, all four methods are included in the resulting set.

It's possible and even likely that certainVFTableEntry is recursing and
calling certainInstanceCallsMethod again; this is why we are using
tabling.  Assuming I have rid my program of all coroutining, is this a
bug, or is it some type of expected behavior?  In particular, why does
the backtracking happen outside of the setof()?

Thanks again,

Ed