I'm reading "Seven languages in seven weeks" atm, and I'm stumped over some Prolog query that I don't understand the 'no' response to.
The friends.pl file looks like this:
likes(wallace, cheese).
likes(grommit, cheese).
likes(wendolene, sheep).
friend(X, Y) :- \+(X = Y), likes(X, Z), likes(Y, Z).
I can do some trivial queries on it, such as:
| ?- ['friends'].
compiling /home/marc/btlang-code/code/prolog/friends.pl for byte code...
/home/marc/btlang-code/code/prolog/friends.pl compiled, 12 lines read - 994 bytes written, 8 ms
yes
| ?- friend(wallace,grommit).
yes
| ?- friend(wallace,wendolene).
no
This is all as expected. Now, I want to introduce a variable in the query. My intent being that Prolog will give me a list of all of Wallace's friends. I'm expecting X = grommit, but I'm getting no:
| ?- trace.
The debugger will first creep -- showing everything (trace)
yes
{trace}
| ?- friend(wallace,X).
1 1 Call: friend(wallace,_16) ?
2 2 Call: \+wallace=_16 ?
3 3 Call: wallace=_16 ?
3 3 Exit: wallace=wallace ?
2 2 Fail: \+wallace=_16 ?
1 1 Fail: friend(wallace,_16) ?
no
{trace}
It doesn't even try to unify X (_16) with grommit. Why?
It is the definition of friend:
friend(X, Y) :- \+(X = Y), likes(X, Z), likes(Y, Z).
The important thing here is that you start with \+(X = Y) which is normally defined as:
\+ Goal :- Goal,!,fail
Note that this means that if goal succeeds, you are sure to fail. Free variables (ones that havent been assigned) will always unify, and thus be equal, so you will always fail with a free variable. It will thus never assign a value to X or Y if it doesn't already have one.
Instead
friend(X, Y) :- likes(X, Z), likes(Y, Z), \+(X = Y)
will behave more as you expect.
The problem here is that prolog gives you powerful ways to control the flow of programs, but those dont really fit nicely with its more logic oriented design. It should be possible to express "negation as failure" type constraints in a way that does not produce these problems. I'm not a huge prolog fan for this reason.