org.projog.core.term

Class EmptyList

java.lang.Object

org.projog.core.term.EmptyList

All Implemented Interfaces:

Term

public final class EmptyList
extends Object
implements Term

Represents a data structure with no Terms.

See Also:

List, ListFactory

Field Summary

Fields

Modifier and Type	Field and Description
static EmptyList	EMPTY_LIST Singleton instance

Method Summary

Modifier and Type	Method and Description
void	backtrack() Reverts this term back to its original state prior to any unifications.
Term	copy(Map<Variable,Variable> sharedVariables) Returns a copy of this term.
Term[]	getArgs() Returns an array of this terms's arguments.
Term	getArgument(int index) Returns the term at the specified position in this term's arguments.
String	getName() Returns ListFactory.LIST_PREDICATE_NAME.
int	getNumberOfArguments() Returns the number of arguments in this term.
EmptyList	getTerm() Returns the current instantiated state of this term.
TermType	getType() Returns TermType.EMPTY_LIST.
boolean	isImmutable() Returns true is this term is immutable.
String	toString()
boolean	unify(Term t) Attempts to unify this term to the specified term.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Methods inherited from interface org.projog.core.term.Term

getBound

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

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(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:

ArrayIndexOutOfBoundsException - as this implementation of Term has no arguments

getName

public String getName()

Returns ListFactory.LIST_PREDICATE_NAME.

Specified by:

getName in interface Term

Returns:

ListFactory.LIST_PREDICATE_NAME

getType

public TermType getType()

Returns TermType.EMPTY_LIST.

Specified by:

getType in interface Term

Returns:

TermType.EMPTY_LIST

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 String toString()

Overrides:

toString in class Object

Returns:

[]