multithreading - Prolog find all paths Implementation

Question

I've been tasked to implement a version of findall in Prolog without using any Prolog built-ins except for not and cut - so basically in pure Prolog.

I'm trying to search a tree for all direct descendants and return the results in a list

parent(a, b).
parent(b, c).
parent(b, d).
parent(e, d).

What I have so far is:

find(X, L) :- find2(X, [], L).
find2(X, Acc, L) :- parent(Y, X), find2(Y, [Y|Acc], L).
find2(_, Acc, Acc).

What I want to be getting when I enter for example:

find(a,X).

would be:

X = [b, c, d]

(Order not important)

However instead I am getting:

X = [b, c] ;
X = [b, d] ;
X = [b] ;
X = [].

I'm new to Prolog so any help on this would be much appreciated.

Thanks

Answer 1

Besides asserting data as you go, you can also use an extra-logical predicate such as nb_setarg/3. Then once a parent is found, you fail back past nb_setarg and find another parent. All previously found solutions should stay in the term you did nb_setarg on, then after all results are exhausted, the nb_setarg term is the answer. The SWI-Prolog example is good, but its just a counter. Try doing it with a list (or better yet: difference list) that builds as you go.