How to find the positive numbers in a list in Prolog?

Question

I want to write a code in prolog that gets a list and find its positive numbers and adds them into a new list as below :

?- findPositives([-1,2,3,-5,-7,9],Result)
Result : [2,3,9]

How can I write this code?

Answer 1

Using tfilter/3:

positive_truth(N, true) :-
   N >= 0.
positive_truth(N, false) :-
   N < 0.

?- tfilter(positive_truth, [-1,2,3,-5,-7,9],Result).
   Result = [2,3,9].

Alternatively, using library(clpfd):

pos_truth(Expr, Truth) :-
   Expr #>= 0 #<==> Bool,
   bool01_truth(Bool, Truth).

bool01_truth(0,false).
bool01_truth(1,true).

?- tfilter(pos_truth, [-1,2,3,-5,-7,9],Result).
   Result = [2,3,9].
?- tfilter(pos_truth, [X,Y],Result).
   Result = [],     X in inf.. -1, Y in inf.. -1
;  Result = [Y],    X in inf.. -1, Y in 0..sup
;  Result = [X],    X in 0..sup,   Y in inf.. -1
;  Result = [X, Y], X in 0..sup,   Y in 0..sup.