10.10.9.2 Reflection Predicates

The constraint solving method needs access to information about the current domains of variables. This is provided by the following predicates, which are all constant time operations.

fd_var(?X)

Checks that X is currently an unbound variable that is known to the CLPFD solver.

fd_min(?X, ?Min)

where X is a domain variable. Min is unified with the smallest value in the current domain of X, i.e. an integer or the atom inf denoting minus infinity.

fd_max(?X, ?Max)

where X is a domain variable. Max is unified with the upper bound of the current domain of X, i.e. an integer or the atom sup denoting plus infinity.

fd_size(?X, ?Size)

where X is a domain variable. Size is unified with the size of the current domain of X, if the domain is bounded, or the atom sup otherwise.

fd_set(?X, ?Set)

where X is a domain variable. Set is unified with an FD set denoting the internal representation of the current domain of X; see below.

fd_dom(?X, ?Range)

where X is a domain variable. Range is unified with a ConstantRange (see Syntax of Indexicals) denoting the current domain of X.

fd_degree(?X, ?Degree)

where X is a domain variable. Degree is unified with the number of constraints that are attached to X.

Please note: this number may include some constraints that have been detected as entailed. Also, Degree is not the number of neighbors of X in the constraint network.

The following predicates can be used for computing the set of variables that are (transitively) connected via constraints to some given variable(s).

fd_neighbors(+Var, -Neighbors)

Given a domain variable Var, Neighbors is the set of other variables, domain or otherwise, that occur with Var in some constraint.

fd_closure(+Vars, -Closure)

Given a list Vars of domain variables, Closure is the set of variables (including Vars) that can be transitively reached via constraints. Thus, fd_closure/2 is the transitive closure of fd_neighbors/2.