Question

list_sum([], 0).
list_sum([Head | Tail], TotalSum) :-
    list_sum(Tail, Sum1),
    Total = Head + Sum1.

This code returns true. If I replace Total = Head + Sum1 with Total is Head + Sum1, then it will return the value. But what I should replace it with to get the result like this:

?- list_sum([1,2,0,3], Sum).
Sum = 1+2+0+3 ; % not to return value 6!!!

Answer 1

Note that in the second clause of your procedure TotalSum is never instantiated. You should have received a warning by the interpreter when consulting your code.

Here’s my suggestion:

list_sum([Item], Item).
list_sum([Item1,Item2 | Tail], Total) :-
    list_sum([Item1+Item2|Tail], Total).

The first clause deals with the base case, when there is only one element left in the list, that is your result.

The second clause deals with the recursion step. It grabs the first two items of the list and performs a recursive call replacing those two items with a new term Item1+Item2.