Dear Stackoverflow Community,
I just wanted to test out the constrained logic programming library (clpfd) for Prolog. So I am including the library by calling
:- use_module(library(clpfd)).
Then I want to do sth like the following.
[X,Y] :: [1..2], X #\= Y, X+Y #\= 3.
But I always get the answer that
ERROR: Syntax error: Operator expected
ERROR: [X,Y]
ERROR: ** here **
ERROR: :: [1..2], X #\= Y, X+Y #\= 3 .
The same happens when executing the following example
? member(X,[42,1,17]), [X,Y] :: [0..20].
ERROR: Syntax error: Operator expected
ERROR: member(X,[42,1,17]), [X,Y]
ERROR: ** here **
ERROR: :: [0..20] .
Seems like Prolog does not recognise the :: operator properly. Any help is appreciated
As far as I know, there is no (::)/2 predicate in the clpfd library. You are probably looking for the ins/2 predicate. For example:
?- [X,Y] ins 1..2, X #\= Y, X+Y #\= 3, label([X,Y]).
false.
?- [X,Y] ins 1..3, X #\= Y, X+Y #\= 3, label([X,Y]).
X = 1,
Y = 3 ;
X = 2,
Y = 3 ;
X = 3,
Y = 1 .
So if X and Y are in 1..2, then there is no solution, since your first constraint says X should be different from Y, and the second constraint says that X + Y should be different from 3.
In case we add 3 to the result, then there are solutions.
We can here use ins/2 to filter as well:
?- member(X,[42,1,17]), [X,Y] ins 0..20.
X = 1,
Y in 0..20 ;
X = 17,
Y in 0..20.