Question

i am searching for a possibility to transform this code

zero(0).
positive(X) :- \+ zero(X).

so that calls like

?- positive(X).

will generate values for X.

Actually only calls with specific values for X are tested correctly but no values can be generated.

Thanks.

Answer 1

You could write virtually straightforward

positive(1).
positive(X) :- positive(Y), X is Y + 1.

and this indeed will work

?- positive(X).
X = 1 ;
X = 2 ;
X = 3 ;
X = 4 ;
etc

but have in mind, that being not tail-recursive this algorithm is exponentially complex.
I mean:

 ?- time((positive(X),X>4000)).
% 8,010,001 inferences, 3.70 CPU in 3.73 seconds (99% CPU, 2163038 Lips)
X = 4001.

Note the time for generation of 4000 positives.

But if we right it like:

positive1(X) :- from(1,X).

from(From,From).
from(From,X):-From1 is From + 1, from(From1,X).

All works fast now:

 ?- time((positive1(X),X>4000)).
% 8,002 inferences, 0.00 CPU in 0.00 seconds (?% CPU, Infinite Lips)
X = 4001.