[SOLVED] gprolog: having trouble getting rule to work

Robhogg · 10-12-2013, 07:22 AM

I don't know whether this is a bug, or something I've misunderstood. I came across prolog in a book I was reading, so decided to give it a try. I installed the default gprolog package in Debain Wheezy (version 1.3.0), and was trying some basic examples. What I've found, though, is that "not equals" works with atoms, but not with variables.

In 'family.pl', I have the following:

Code:

person(becky).
person(frances).
person(john).
person(jules).

father(john, jules).
father(john, becky).
mother(frances, jules).
mother(frances, becky).

parent(X,Y):- father(X,Y).
parent(X,Y):- mother(X,Y).

sibling(X,Y):- \+(X = Y),
        parent(Z,X),
        parent(Z,Y).

Querying sibling gives me:

Code:

| ?- sibling(becky,jules).

true ? ;

true ? ;

no
| ?- sibling(Who,jules).

no
| ?-

However, if I replace \+(X = Y) with (X = Y), it is able to tell me that jules is jules' own sibling:

Code:

| ?- sibling(becky,jules).

no
| ?- sibling(jules,jules).

true ? ;

true ? ;

no
| ?- sibling(Who,jules).

Who = jules ? ;

Who = jules ? ;

no
| ?-

Is it something I'm doing, or should I try a different version of Prolog?

ntubski · 10-12-2013, 10:05 AM

(=)/2 doesn't mean equal, it means unifiable. A variable always unifies with an atom so Who = jules succeeds. I think you want (==)/2 (term identical), or probably (\==)/2 (term not identical).

Robhogg · 10-12-2013, 02:37 PM

Thanks for the response, ntubski. It still seems to be misbehaving when it comes to variables, though:

Code:

| ?- [user].
compiling user for byte code...
sibling(X,Y):- (X \== Y),
  parent(Z,X),
  parent(Z,X).

user compiled, 4 lines read - 489 bytes written, 61657 ms
warning: user:1: redefining procedure sibling/2
         /home/rob/prolog/family.pl:14: previous definition

yes
| ?- sibling(jules,becky).

true ? ;

true ? ;

no
| ?- sibling(jules,jules).

no
| ?- sibling(Who,jules).

Who = jules ? ;

Who = becky ? ;

Who = jules ? ;

Who = becky ? ;

yes
| ?-

Does look like a bug...

Update:
Or not so much, now. Just tried with swi-prolog instead, and am getting the same behaviour. *confused*

ntubski · 10-12-2013, 04:04 PM

Ah, the problem is that you are the checking X (or Who) before it has been unified:

Code:

| ?- ['/home/npostavs/tmp/sib.prolog'].
compiling /home/npostavs/tmp/sib.prolog for byte code...
/home/npostavs/tmp/sib.prolog compiled, 17 lines read - 1533 bytes written, 38 ms

yes
| ?-trace.
The debugger will first creep -- showing everything (trace)

yes
{trace}
| ?-sibling(Who, jules).
      1    1  Call: sibling(_16,jules) ? 
      2    2  Call: _16\==jules ? s
      2    2  Exit: _16\==jules ? s
      3    2  Call: parent(_110,_16) ? s
      3    2  Exit: parent(john,jules) ? l

Who = jules ? 

(4 ms) yes
{trace}

_16 is Who, and _110 is Z. You can see _16 is unified to jules after it was checked. Moving that check to the end seems to work.

Robhogg · 10-12-2013, 04:46 PM

Thanks, ntubski. Yes, that fixed it. But tricky...