org.projog.core.term

Class Variable

java.lang.Object

org.projog.core.term.Variable

All Implemented Interfaces:

Term

public final class Variable
extends Object
implements Term

Represents an unspecified Term.

A Variable can be either instantiated (representing another single Term) or uninstantiated (not representing any other Term). Variables are not constants. What Term, if any, a Variable is instantiated with can vary during its life time. A Variable becomes instantiated by calls to unify(Term) and becomes uninstantiated again by calls to backtrack().

Field Summary

Fields

Modifier and Type	Field and Description
static String	ANONYMOUS_VARIABLE_ID

Constructor Summary

Constructors

Constructor and Description
Variable() Creates an anonymous variable.
Variable(String id)

Method Summary

Modifier and Type	Method and Description
void	backtrack() Reverts this variable to an uninstantiated state.
Term	copy(Map<Variable,Variable> sharedVariables) Returns a copy of this term.
Term[]	getArgs() Calls Term.getArgs() on the Term this variable is instantiated with.
Term	getArgument(int index) Calls Term.getArgument(int) on the Term this variable is instantiated with.
Term	getAttributeOrDefault(VariableAttribute attributeKey, Term defaultValue)
Term	getBound() Returns the term this object is bound to.
String	getId()
String	getName() Calls Term.getName() on the Term this variable is instantiated with.
int	getNumberOfArguments() Calls Term.getNumberOfArguments() on the Term this variable is instantiated with.
Term	getTerm() Returns the current instantiated state of this term.
TermType	getType() Returns TermType.VARIABLE if uninstantiated else TermType of instantiated Term.
boolean	isAnonymous()
boolean	isImmutable() Always returns false even if instantiated with an immutable Term.
void	putAttribute(VariableAttribute attributeKey, Term attributeValue)
void	removeAttribute(VariableAttribute attributeKey)
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

Field Detail

ANONYMOUS_VARIABLE_ID

public static final String ANONYMOUS_VARIABLE_ID

See Also:

Constant Field Values

Constructor Detail

Variable

public Variable()

Creates an anonymous variable. The ID of the variable will be an underscore.

Variable

public Variable(String id)

Parameters:

id - value by which this variable can be identified

Method Detail

getAttributeOrDefault

public Term getAttributeOrDefault(VariableAttribute attributeKey,
                                  Term defaultValue)

putAttribute

public void putAttribute(VariableAttribute attributeKey,
                         Term attributeValue)

removeAttribute

public void removeAttribute(VariableAttribute attributeKey)

getName

public String getName()

Calls Term.getName() on the Term this variable is instantiated with.

Specified by:

getName in interface Term

Returns:

a string representation of this term

Throws:

NullPointerException - if the Variable is currently uninstantiated

getId

public String getId()

Returns:

value provided in constructor by which this variable can be identified

isAnonymous

public boolean isAnonymous()

getArgs

public Term[] getArgs()

Calls Term.getArgs() on the Term this variable is instantiated with.

Specified by:

getArgs in interface Term

Returns:

array of this terms's arguments

Throws:

NullPointerException - if the Variable is currently uninstantiated

See Also:

Term.getArgument(int)

getNumberOfArguments

public int getNumberOfArguments()

Calls Term.getNumberOfArguments() on the Term this variable is instantiated with.

Specified by:

getNumberOfArguments in interface Term

Returns:

number of arguments in this term

Throws:

NullPointerException - if the Variable is currently uninstantiated

getArgument

public Term getArgument(int index)

Calls Term.getArgument(int) on the Term this variable is instantiated with.

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:

NullPointerException - if the Variable is currently uninstantiated

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()

getType

public TermType getType()

Returns TermType.VARIABLE if uninstantiated else TermType of instantiated Term.

Specified by:

getType in interface Term

Returns:

TermType.VARIABLE if this variable is uninstantiated else calls Term.getType() on the Term this variable is instantiated with.

isImmutable

public boolean isImmutable()

Always returns false even if instantiated with an immutable Term.

Specified by:

isImmutable in interface Term

Returns:

false

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

getBound

public Term getBound()

Description copied from interface: Term

Returns the term this object is bound to.

For anything other than a Variable this method will return this. TODO think of a better name and explanation.

Specified by:

getBound in interface Term

getTerm

public Term 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:

itself if this variable is uninstantiated else calls Term.getType() on the Term this variable is instantiated with.

backtrack

public void backtrack()

Reverts this variable to an uninstantiated state.

Specified by:

backtrack in interface Term

See Also:

Term.unify(Term)

toString

public String toString()

Overrides:

toString in class Object

Returns:

if this variable is uninstantiated then returns this variable's id else calls toString() on the Term this variable is instantiated with.