I have following clauses:
num_parent(adam, X) :- !, X = 0.
num_parent(eve, X) :- !, X = 0.
num_parent(X, 2).
When I typed the query:
num_parent(eve,X).
It only returns:
X = 0.
which is what I want.
But when I typed this query:
num_parent(X,0).
it only returns:
X = adam.
So how can I modify the clauses to make it return:
X = adam;
X = eve.
Thanks
First, try to formulate what you want in plain English. You probably want to say:
Everyone has two parents except Adam and Eve who have none.
What about Lilith? Never mind, let's stick to your reading.
num_parent(Person, 2) :-
dif(Person, adam),
dif(Person, eve).
num_parent(adam, 0).
num_parent(eve, 0).
As you can see, it is a bit cumbersome to define this: You have to mention each exceptional person twice. Easy to make an error.
With if_/3 available in library(reif)
for
SICStus and
SWI
you can write more succinctly:
num_parent(Person, Num) :-
if_( ( Person = adam ; Person = eve ), Num = 0, Num = 2 ).
And now some uses:
?- num_parent(eve, Num).
Num = 0.
?- num_parent(X, 0).
X = adam
; X = eve
; false.
?- num_parent(X, 2).
dif(X, eve), dif(X, adam).
?- num_parent(lilith, 2).
true.