How to calculate a sum of sequence of numbers in Prolog

Question

The task is to calculate a sum of natural numbers from 0 to M. I wrote the following code using SWI-Prolog:

my_sum(From, To, _) :- From > To, !.
my_sum(From, To, S) :-
  From = 0,
  Next is 1,
  S is 1,
  my_sum(Next, To, S).
my_sum(From, To, S) :-
  From > 0,
  Next is From + 1,
  S is S + Next,
  my_sum(Next, To, S).

But when I try to calculate:

my_sum(0,10,S), writeln(S).

I got False instead of correct number. What is going wrong with this example?

Answer 1

this is surely false for Next \= 0: S is S + Next. Another more fundamental problem is that you're doing the computation in 'reverse' order. That is, when From > To and the program stop, you don't 'get back' the result. Then you should add an accumulator (another parameter, to be propagated to all recursive calls) and unify it with the partial sum at that last step...

Anyway, should be simpler:

my_sum(From, To, S) :-
  From < To,
  Next is From + 1,
  my_sum(Next, To, T),
  S is T + From.
my_sum(N, N, N).

| ?- my_sum(2, 4, N).
N = 9