Question

I want to multiply two lists, where I take the left list and multiply it by each element of
the right list.

For example:

?- multLists([3,4,2], [4,7,8], R).
R = [[12,16,8],[21,28,14],[24,32,16]].

For that I wrote a helper predicate that takes a list and multiplies it
by a single scalar:

multListElem([], _, _).
multListElem([H|T], Result, Elem) :- 
    multListElem(T, W, Elem),
    Z is H*Elem,
    Result = [Z|W].

But now, when I run ?- multListElem([1,2,3], X, 3).
I get:

1 ?- multListElem([1,2,3], X, 3).
X = [3, 6, 9|_G1840].

What is that weird tail _G1840?

Answer 1

The bug is here: multListElem([],_,_). When the first list is empty, the result is empty, so you must write multListElem([],[],_).

When you work with lists, you can use functional idioms like
maplist:

multLists(L1, L2, R) :-
    maplist(mult_one_list(L1), L2, R).

mult_one_list(L1, Elem, R) :-
    maplist(mult_2_numbers(Elem), L1, R).

mult_2_numbers(V1, V2, R) :-
    R is V1 * V2.

maplist applies the first argument to each element of each list (passed to it as an argument).