Digit sum in Prolog

Question

How can I define in Prolog the rule to calculate the digit sum of an integer?

Example:

digitsum[Integer, Result] :- ...

so that the query digitsum(123, X). results in X = 6 (=1+2+3). A solution running under SWI-Prolog is appreciated.

Answer 1

This problem naturally leads to an implementation using library(clpfd):

:- use_module(library(clpfd)).

digitsum(Z,S) :-
   Z #= 0,
   S #= 0.
digitsum(Z0, S0) :-
   Z0 #> 0, S0 #> 0,
   S0 #= Z0 mod 10 + S1,
   Z1 #= Z0 // 10,  % should be rather div
   digitsum(Z1, S1).

?- digitsum(D,S).
   D = S, S = 0
;  D = S, S in 1..9
;  D in 10..99, D/10#=_A, D mod 10#=_B,
   _A in 1..9, _B+_A#=S, _B in 0..9, S in 1..18
;  ... .

The first answer is about zero. The second says that a number 1..9 is the same as its sum. Then it talks about the numbers 10..99 etc.