Learning prolog...

gistfile1.prolog

%%%  Why does this return X = [something,something,something,something|_G610].
%%%  when I call it with 
%%%  ?- add_elems(5,X).
%%%
%%%  What is the uninstantiated variable at the end and how do I get rid of it?


add_elems(0,_).
add_elems(N,Out) :-
  NewN is N-1,
	Add = something,
	add_elems(NewN,R),
	Out = [Add|R],!.

That "_G610" at the end is some random garbage, because return "_" when add_elems receives 0.
You should instead return an empty list, because when you add to the head with [Add | R], R is supposed to be a list. You want it to do something like this: ...[something | [something | [] ]]...

Here is the correct version:

add_elems(0,[]).
add_elems(N,Out) :-
    NewN is N-1,
    Add = something,
    add_elems(NewN,R),
    Out = [Add|R].

?- add_elems(5,X).
X = [something, something, something, something, something]