SWI-Prolog -- fd

Documentation
- Reference manual
  - The SWI-Prolog library
    - library(clpfd): CLP(FD): Constraint Logic Programming over Finite Domains
      - CLP(FD) predicate index
        
        Reflection predicates
        
        fd_var/1
        
        fd_inf/2
        
        fd_sup/2
        
        fd_size/2
        
        fd_dom/2
- Packages

Availability::- use_module(library(clpfd)).

fd_dom(+Var, -Dom)

Dom is the current domain (see in/2) of Var. This predicate is useful if you want to reason about domains. It is not needed if you only want to display remaining domains; instead, separate your model from the search part and let the toplevel display this information via residual goals.

For example, to implement a custom labeling strategy, you may need to inspect the current domain of a finite domain variable. With the following code, you can convert a finite domain to a list of integers:

dom_integers(D, Is) :- phrase(dom_integers_(D), Is).

dom_integers_(I)      --> { integer(I) }, [I].
dom_integers_(L..U)   --> { numlist(L, U, Is) }, Is.
dom_integers_(D1\/D2) --> dom_integers_(D1), dom_integers_(D2).

Example:

?- X in 1..5, X #\= 4, fd_dom(X, D), dom_integers(D, Is).
D = 1..3\/5,
Is = [1,2,3,5],
X in 1..3\/5.