Question

For some reason, this is not working. I am getting:
ERROR: is/2: Arguments are not sufficiently instantiated

1 add_list([]).
2 add_list([H|T]):-
3                 Sum2 is Sum1 + H,
4                 add_list(T).

I am trying to add the contents of a list (containing only numbers).

Answer 1

I’m not sure what you are trying to do. But if you are trying to calc total sum it will be this way (changed name to list_sum as add_list doesn’t make any sense):

list_sum([], 0).
list_sum([H|T], Sum):-
    list_sum(T, SubSum),
    Sum is SubSum + H.