CODEWITHSUNDEEP
prolog
Xinglin Qiang
Xinglin Qiang

Reputation: 23

family relation in prolog :descendantTree

I want recursively store a family relation in a list structure. I have the following code:

descendantTree(X,[X]) :-
    \+ parent(X,_).

descendantTree(X,L) :-
    forall(parent(X,Y), descendantTree(Y,YL)),
    append([X],[YL],L).

parent(john, paul).
parent(john, mary).
parent(paul, henry).
parent(paul, june).
parent(mary, adam).
parent(henry, helen).

What I expected is like:

L =[john,[paul,[henry,[helen]],[june]],[mary,[adam]]]

But actually it just return:

L = [john, _9250].

Could you tell me why?

Upvotes: 2

Views: 102

Answers (1)

Paulo Moura
Paulo Moura

Reputation: 18683

The forall/2 predicate implements a generate and test loop using negation. It can be defined as:

% for all solutions of Generate, Test is true
forall(Generate, Test) :-
    \+ (Generate, \+ Test)).

The use of negation means that no bindings will be returned when a call succeeds. In your case, that means that you're calling:

append([X],[_],L)

hence the result you get:

L = [john, _9250].

Look instead into the all-solutions family of predicates: bagof/3, setof/3, and findall/3 and then consider editing your question with your findings.

Upvotes: 1

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