sjm.engine
Class Variable

java.lang.Object
  |
  +--sjm.engine.Variable

Direct Known Subclasses:

Anonymous

public class Variable

extends java.lang.Object

implements ArithmeticTerm, ComparisonTerm

A variable is a named term that can unify with other terms.

A variable has a name, such as "X" or "Person", and an instantiation. When a variable unifies with a term, it "instantiates" to it, taking the term as its value. The instantiation of a variable may be another variable, or a structure.

The scope of a variable is the rule in which it is contained. For example, consider the member program:

     member(X, [X | Rest]);
     member(X, [Y | Rest]) :- member(X, Rest);
 

In this program, the variable "X" in the first rule is the same variable both times it appears in the rule. However, this variable is completely independent of the variable named "X" in the second rule. Variables with the same name in a rule are the same variable, but variables with the same name in different rules are different variables. This is another way of saying that a variable's scope is the rule in which it appears.

To be more specific, the scope of a variable is the dynamic rule in which the variable appears. Since rules may execute recursively, dynamic rules each need an independent copy of a defining rule's variables. In the member program, for example, the second rule may prove itself by reinvoking itself, with a (slightly) different set of variable instantiations.

Consider the query member(c, [a, b, c]). This query will unify with the second rule, and try to prove the second rule's tail, which will be member(c, [b, c]). This structure will try to prove itself, and it too will unify with the second rule. At this point, the proof of member(c, [a, b, c]) will be waiting upon the proof of member(c, [b, c]). That is, the two dynamic copies of the rule, will be in different states, because of their variables. For example, the instantiation of Rest in the first execution of the rule will be [b, c], and the value of Rest in the second rule will [c].

Variables have a name and an instantiation which is unique within a scope; each dynamic version of a rule has a unique Scope.

Field Summary

protected Term	instantiation
java.lang.String	name

Constructor Summary

Variable(java.lang.String name)
Create a variable with the given name.

Method Summary

Term	copyForProof(AxiomSource ignored, Scope scope) Create a copy that uses the provided scope.
java.lang.String	definitionString() Returns string representation of this variable, showing both its name and its value.
boolean	equals(java.lang.Object o) Returns true if the supplied object is an equivalent variable.
java.lang.Object	eval() Returns the value of this variable.
boolean	isList() Returns true if this variable is uninstantiated, or if it contains a list.
java.lang.String	listTailString() Returns a string representation of this variable as the tail of a list.
java.lang.String	toString() Returns a string representation of this variable.
void	unbind() Marks this variable as no longer having an instantiated value.
Unification	unify(Structure s) Instantiates this variable with the supplied structure, or forwards the request to its instantiation if it already has one.
Unification	unify(Term t) Unifies this variable with the supplied term.
Unification	unify(Variable v) Instantiates this variable with the supplied variable, or forwards the request to its instantiation if it already has one.
Unification	variables() Returns a unification containing just this variable.

Methods inherited from class java.lang.Object

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

Field Detail

name

public final java.lang.String name

instantiation

protected Term instantiation

Constructor Detail

Variable

public Variable(java.lang.String name)

Create a variable with the given name.

Method Detail

copyForProof

public Term copyForProof(AxiomSource ignored,
                         Scope scope)

Create a copy that uses the provided scope.

Parameters:

AxiomSource - ignored

Scope - the scope to use for variables in the copy

Returns:

a copy that uses the provided scope

definitionString

public java.lang.String definitionString()

Returns string representation of this variable, showing both its name and its value.

Returns:

a string representation of this variable, showing both its name and its value.

equals

public boolean equals(java.lang.Object o)

Returns true if the supplied object is an equivalent variable.

Overrides:

equals in class java.lang.Object

Parameters:

object - the object to compare

Returns:

true, if the supplied object has the same name, and it the two variables' instantiations are equal

eval

public java.lang.Object eval()

Returns the value of this variable.

Returns:

the value of this variable

Throws:

EvaluationException - if this variable's value is undefined

isList

public boolean isList()

Returns true if this variable is uninstantiated, or if it contains a list.

Tags copied from interface: Term

Returns:

true, if this term is a list

listTailString

public java.lang.String listTailString()

Returns a string representation of this variable as the tail of a list.

Returns:

a string representation of this variable as the tail of a list

toString

public java.lang.String toString()

Returns a string representation of this variable.

Overrides:

toString in class java.lang.Object

Returns:

a string representation of this variable

unbind

public void unbind()

Marks this variable as no longer having an instantiated value.

unify

public Unification unify(Structure s)

Instantiates this variable with the supplied structure, or forwards the request to its instantiation if it already has one.

Parameters:

Structure - a structure to unify with

Returns:

a unification. If this variable is already instantiated, the unification is the result of unifying with the input structure. Otherwise, the result is a new unification containing just this variable, instantiated to the input structure.

unify

public Unification unify(Term t)

Unifies this variable with the supplied term.

This method dispatches the unify request to either a structure or a variable. The receiver will get a signature match from this object as a Variable, not just a Term.

Parameters:

Term - a term to unify with

Returns:

the sum of the variables that bind to values to make the unification work; Returns null if the unification fails.

unify

public Unification unify(Variable v)

Instantiates this variable with the supplied variable, or forwards the request to its instantiation if it already has one.

Parameters:

Variable - a variable to unify with

Returns:

the sum of the variables that bind to values to make the unification work; Returns null if the unification fails.

variables

public Unification variables()

Returns a unification containing just this variable.

Returns:

a unification containing just this variable