Prolog nested list sum

[BlackIris]

Hi everybody, I'm having a little problem with finding sum of nested list in Prolog. So far what I have is

totalSum([],0).
totalSum([H|T],S):-
      totalSum(T,Sum),
      S is Sum + H.
totalSum([H|T],S):-
     totalSum(H,Sum1),
     totalSum(T,Sum2),
     S is Sum1+Sum2.

With first two predicates, I have no problems getting sum of not-nested list. Anybody with a tip as to how I can modify the third predicate so that it can handle nested list? I've tried passing off [H] instead of just H and many other but it hasn't worked so far. Any comment would be deeply appreciated.

[Rashakil Fol]

Your second predicate is going to match and override whatever you do in your third predicate. You need to handle nested lists before you handle non-nested lists.

[BlackIris]

Thanks for the quick reply. While I agree with you but I have tried with third predicate before second and have encountered the same problem. I think the major problem lies with my logic in third predicate.

[Rashakil Fol]

Well you have more problems than just that.

[BlackIris]

So you mean I have logic problems and more...? Could you point me to a general direction, like where my logic is wrong (I know my logic is flawed but I'm not sure exactly what).

[Rashakil Fol]

All you need to do to fix your problem is look at your code and imagine what a prolog interpreter would do when interpreting it.

[fgmart]

Rashakil was correct that the rule matching a single number needs to come last.

Here is a complete answer. Your first line also had to be changed:

totalSum([],N) :- N is 0.
totalSum([H|T],S):-
    totalSum(H,Sum1),
    totalSum(T,Sum2),
    S is Sum1+Sum2.
totalSum(N,S):-
    S is N.

and here are some examples of using it:

?- totalSum([1, 2, 3], N).
N = 6 .

?- totalSum([1, [2], 3], N).
N = 6 .

?- totalSum([1, [2, 3]], N).
N = 6 .

?- totalSum([1, [2, [3]]], N).
N = 6 .