Class Once

java.lang.Object
org.projog.core.predicate.builtin.compound.Once
All Implemented Interfaces:
PredicateFactory
public final class Once extends Object implements PredicateFactory
once(X) - calls the goal represented by a term.

The once(X) goal succeeds if an attempt to satisfy the goal represented by the term X succeeds. No attempt is made to retry the goal during backtracking - it is only evaluated once.

  • Constructor Details

  • Method Details

    • preprocess

      public PredicateFactory preprocess(Term term)
      Specified by:
      preprocess in interface PredicateFactory

    • getPredicate

      public Predicate getPredicate(Term t)
      Description copied from interface: PredicateFactory
      Returns a Predicate to be used in the evaluation of a goal.
      Specified by:
      getPredicate in interface PredicateFactory
      Parameters:
      t - the term containing the arguments to use in the evaluation of the goal
      Returns:
      Predicate to be used in the evaluation of the goal
      See Also:

    • isRetryable

      public boolean isRetryable()
      Description copied from interface: PredicateFactory
      Should instances of this implementation be re-evaluated when backtracking?

      Some goals (e.g. X is 1) are only meant to be evaluated once (the statement is either true or false) while others (e.g. repeat(3)) are meant to be evaluated multiple times. For instances of Predicate that are designed to possibly have Predicate.evaluate() called on them multiple times for the same individual query this method should return true. For instances of Predicate that are designed to only be evaluated once per individual query this method should return false.

      Specified by:
      isRetryable in interface PredicateFactory
      Returns:
      true if an attempt should be made to re-evaluate instances of implementing classes when backtracking, false otherwise