Prolog Goal Query Returns False

1
votes

I'm a high school student studying Prolog and am getting an error on a goal.

I have the following facts:

character(jim).
character(jenny).
character_type(jim, prince).
charcter_type(jenny, princess).
skill(fly).
skill(invisibility).
has_skill(jim, fly).
has_skill(jenny_invisibility).
pet(jim, horse).
pet(jenny, bird).
animal(horse).
animal(bird).

I would like to get all the pets of characters who are princesses. I am trying:

pet(character_type(_, princess), X).

Without successful results. Any help is appreciated.

2
prolog
Anything starting with lower case letter in Prolog is atomic and not a variable. So pet(character_type(_, princess), X). fails because neither of the two pet facts you have will unify with that term. They won't unify because jim does not match character_type(_, princess) and jenny does not match character_type(_, princess). - lurker

2 Answers

2
votes

In Prolog, arguments to predicates and functions can only be terms. A term is combinations of variables, constants, and functions. Terms cannot contain predicates.The Predicates are names for functions that are true or false.The functions are something that return non-Boolean values.

The argument passed in pet predicate i.e character_type is a predicate, thus cannot be written as pet(character_type(_, princess), X).

Instead writing the query as character_type(X,princess), pet(X,Y). will give you the desired result. X= jenny Y = bird.

1
votes

you can not use prolog predicate like C-Function



    character_type(_, princess)

returns nothing.

I think this is what you intend to do.


    character_type(C, princess),pet(C, X).