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delayed_goals_number(?Var, -Number)

Succeeds if Number is the number of goals delayed by the variable Var.
Var
Any term.
Number
Integer or a variable.

Description

Unifies Number with the number of goals delayed by the variable Var. If Var is instantiated, Number is unified with 1000000. If Var is not instantiated and there are no goals delayed by it, Number is unified with 0. This predicate does not construct the list of delayed goals and hence it is faster than delayed_goals/2. Its purpose is to give each variable a weight corresponding to the number of constraints imposed on the variable to be able to select the most constrained one. It is assumed that one million constraints cannot be placed on any variable.

Modes and Determinism

Examples

Success:
    % Make an intelligent permutation choosing
    % the most constrained variable.
    perm([], []).
    perm([Var|List], Values) :-
        delayed_goals_number(Var, C),
        maxval(List, Var, C, Chosen, RestVar),
        delete(Chosen, Values, RestVal),
        perm(RestVar, RestVal).

    maxval(L, Chosen, 1000000, Chosen, L) :- !.
    maxval([], Chosen, _, Chosen, []).
    maxval([X|L], SoFar, MaxVal, Chosen, [V|Rest]) :-
        delayed_goals_number(X, C),
        (C =< MaxVal -> V = X, Next = SoFar, Max = MaxVal ;
            V = SoFar, Next = X, Max = C),
        maxval(L, Next, Max, Chosen, Rest).
    % the values are generated in the listed order
    [eclipse]: perm([A, B, C], [1,2,3]).

    A = 1
    B = 2
    C = 3     More? (;)
    yes.

    % B is more constrained than the others, and so
    % its value will be generated first.
    [eclipse]: B < 3, perm([A, B, C], [1,2,3]).

    A = 2
    B = 1
    C = 3     More? (;)

Fail:
    X > 0, delayed_goals_number(X, 0).



See Also

delayed_goals / 1, delayed_goals / 2, subcall / 2