声明一个变量

Question

I didn't touch Prolog since high-school, and even though I've tried to find the info, it didn't help. Below is the example that has to illustrate my problem:

%% everybody():- [dana, cody, bess, abby].
%% Everybody = [dana, cody, bess, abby].

likes(dana, cody).
hates(bess, dana).
hates(cody, abby).

hates(X, Y):- \+ likes(X, Y).

likes_somebody(_, []):- fail.
likes_somebody(X, [girl | others]):-
    likes(X, girl) ; likes_somebody(X, others).

likes_everybody(_, []):- true.
likes_everybody(X, [girl | others]):-
    likes(X, girl) , likes_everybody(X, others).

maplist(likes_somebody, [dana, cody, bess, abby], [dana, cody, bess, abby]).

How do I declare everybody to just be the list of girls? The commented lines are those which I've tried, but I've got bizarre error messages back.

This is the tutorial I followed more or less so far. I'm using GProlog, if it makes any difference. Sorry for such a basic question. GProlog's manual doesn't deal with language syntax, but I've certainly looked there. As an aside, I would be grateful for information on where to look for language documentation (as opposed to implementation documentation).

Answer 1

Every variable in Prolog must begin with an uppercase letter. So for starters, you want Everybody , not everybody .

Second problem, variables in Prolog are not assignables . So probably what you want to do is make a fact and use that instead:

everybody([dana, cody, bess, abby]).

Your bottom line of code is actually a fact definition and will attempt to overwrite maplist/3 . What you probably want to do is put everything above that line into a file (say, called likes.pl ) and then consult it ( [likes]. ). Then you can run a query like this:

?- everybody(Everybody), maplist(likes_somebody, Everybody, Everybody).

This won't work, because likes_somebody/2 processes a list in the second argument. The predicate you have for likes_somebody/2 could be written:

likes_somebody(_, []).

but still won't mean much. It simply unifies anything with the empty list:

?- likes_somebody(chicken_tacos, []).
true.

You really need a predicate to tell you if someone is a girl, like this:

girl(dana).
girl(cody).
girl(bess).
girl(abby).

Then you could do what I think you're trying to do, which is something closer to this:

likes_somebody(X) :- girl(X).

Then the maplist construction would work like this:

everybody(Everybody), maplist(likes_somebody, Everybody).

Which would simply return true. You could simplify and eliminate everybody/1 by instead using findall(Girl, girl(X), Everybody) but it's getting weird.

You're trying to do list processing with likes_everybody/2 , but it's broken because girl is literally girl , not a variable, and others is literally others , not a list of some kind that could be the tail of another list.

I think you still have some old ideas you need to cleanse. Read some more, write some more, and your code will start to make a lot more sense.