Prolog - How can I save results from recursive calls?

Question

I am still trying to understand the Prolog logic and have stumbled upon a problem. I am trying to save values found within recursive calls, to pass on or gather.

As such:

main([]) :- !.
main([H|Tail]) :- findall(X,something(_,_,X),R),
                  getValueReturn(R,H,Lin, Lout),
                  main(Tail).


% X is the Head from main
getValueReturn([H|Tail],X,Lin, Lout) :- subset(X, H) ->
                                findall(A,something(A,_,H),L1),
                                append(Lin,L1,Lout), 
                                getValueReturn(Tail,X,Lout,L)
                                ;
                                getValueReturn(Tail,X,Lin,Lout).  

I would like to gather the results from findall in getValueReturn, combine them, and send them back to main, which can then use them. How do I create and add to a list within getValueReturn? Similarly, how can I save the list in my main for all recursive calls?

EDIT:

I edited the code above as per a comment reply, however when I run this through trace, the list deletes all elements when the empty list is found.

What am I doing wrong? This is the first time I try to use the concept of building a list through recursion.

Answer 1

You should post complete code that can be run, with example data. I have not tested this.

You need to pass L around on the top-level also. Using the same variable names for different parameters in adjacent procedures does not improve readability.

main([E|Es],L0,L) :-
  findall(X,something(_,_,X),Rs),
  getValueReturn(Rs,E,L0,L1),
  main(Es,L1,L).
main([],L,L).

getValueReturn([R|Rs],E,L0,L) :-
  ( subset(E,R) ->
    findall(A,something(A,_,R),New),
    append(L0,New,L1), 
    getValueReturn(Rs,E,L1,L)
  ; getValueReturn(Rs,E,L0,L) ).
getValueReturn([],_,L,L).

A variable can only have one value in Prolog. In your code, for example, Lout is the output from append/3 , an input to a recursive call of getValueReturn/4 , and then also the output on the top-level. This is probably not going to do what you want.

Answer 2

I have found the best way to do what I was trying to was to use asserta/z when a result was found, and then gather these results later on.

Otherwise the code became overly complicated and did not function as intended.