Package org.projog.core.term

Class EmptyList

  • All Implemented Interfaces:
    Term
    public final class EmptyList
extends java.lang.Object
implements Term
    Represents a data structure with no Terms.
    See Also:
    List, ListFactory

    • Field Detail

      • EMPTY_LIST

        public static final EmptyList EMPTY_LIST
        Singleton instance

        For performance reasons, use t.getType()==TermType.EMPTY_LIST to check if a term is an empty list, rather than using t.getTerm()==EmptyList.EMPTY_LIST. TODO confirm this is what the code is doing

    • Constructor Detail

      • EmptyList

        private EmptyList()
        Private constructor to force use of EMPTY_LIST

    • Method Detail

      • backtrack

        public void backtrack()
        Description copied from interface: Term
        Reverts this term back to its original state prior to any unifications.

        Makes all Variables that this term consists of uninstantiated.

        Specified by:
        backtrack in interface Term
        See Also:
        Term.unify(Term)

      • copy

        public Term copy​(java.util.Map<Variable,​Variable> sharedVariables)
        Description copied from interface: Term
        Returns a copy of this term.

        The returned copy will share any immutable terms contained in this term. The returned copy will contain new instances for any Variables contained in this term. The sharedVariables parameter keeps track of which Variables have already been copied.

        Specified by:
        copy in interface Term
        Parameters:
        sharedVariables - keeps track of which Variables have already been copied (key = original version, value = version used in copy)
        Returns:
        a copy of this term

      • getTerm

        public EmptyList getTerm()
        Description copied from interface: Term
        Returns the current instantiated state of this term.

        Returns a representation of this term with all instantiated Variables replaced with the terms they are instantiated with.

        Specified by:
        getTerm in interface Term
        Returns:
        a representation of this term with all instantiated Variables replaced with the terms they are instantiated with.

      • isImmutable

        public boolean isImmutable()
        Description copied from interface: Term
        Returns true is this term is immutable.

        A term is considered immutable if its value will never change as a result of executing its Term.unify(Term) or Term.backtrack() methods. A term will not be considered immutable if it is a Variable or any of its arguments are not immutable.

        Specified by:
        isImmutable in interface Term
        Returns:
        true is this term is immutable

      • getArgs

        public Term[] getArgs()
        Description copied from interface: Term
        Returns an array of this terms's arguments.

        Note: for performance reasons the array returned is the same array used internally be the term instance so be careful not to alter the array returned as changes will be reflected in the original term.

        Specified by:
        getArgs in interface Term
        Returns:
        array of this terms's arguments
        See Also:
        Term.getArgument(int)

      • getNumberOfArguments

        public int getNumberOfArguments()
        Description copied from interface: Term
        Returns the number of arguments in this term.
        Specified by:
        getNumberOfArguments in interface Term
        Returns:
        number of arguments in this term

      • getArgument

        public Term getArgument​(int index)
        Description copied from interface: Term
        Returns the term at the specified position in this term's arguments.
        Specified by:
        getArgument in interface Term
        Parameters:
        index - index of the argument to return
        Returns:
        the term at the specified position in this term's arguments
        Throws:
        java.lang.ArrayIndexOutOfBoundsException - as this implementation of Term has no arguments

      • unify

        public boolean unify​(Term t)
        Description copied from interface: Term
        Attempts to unify this term to the specified term.

        The rules for deciding if two terms are unifiable are as follows:

        • An uninstantiated Variable will unify with any term. As a result the Variable will become instantiated to the other term. The instantiaton will be undone when Term.backtrack() is next called on the Variable
        • Non-variable terms will unify with other terms that are of the same TermType and have the same value. The exact meaning of "having the same value" will vary between term types but will include that the two terms being unified have the same number of arguments and that all of their corresponding arguments unify.
        Note: can leave things in "half-state" on failure as neither List or Predicate backtrack earlier args.
        Specified by:
        unify in interface Term
        Parameters:
        t - the term to unify this term against
        Returns:
        true if the attempt to unify this term to the given term was successful
        See Also:
        Term.backtrack()

      • toString

        public java.lang.String toString()
        Overrides:
        toString in class java.lang.Object
        Returns:
        []