uk.ac.sheffield.vtts.model

Class Expression

java.lang.Object

uk.ac.sheffield.vtts.model.Element

uk.ac.sheffield.vtts.model.Named

uk.ac.sheffield.vtts.model.Expression

Direct Known Subclasses:

Function, Parameter

public abstract class Expression
extends Named

Expression represents a computable expression, with a name and a type. Expression is the abstract root of the Expression library. All subclasses are different kinds of terms that can be evaluated. Expressions break down into Parameters, which store their value, and Functions, which compute their value. The name of an Expression is either the Parameter name, or the name of the root Function dominating the expression. The type of the Expression is usually also declared, but may be inferred for some classes of Expression.

Version:

Broker@Cloud 1.0

Author:

Anthony J H Simons

Field Summary

Fields

Modifier and Type	Field and Description
protected static ModelFactory	factory The singleton ModelFactory for creating objects of different model types.
protected java.lang.String	type The type of this Expression.

Fields inherited from class uk.ac.sheffield.vtts.model.Named

name

Constructor Summary

Constructors

Modifier	Constructor and Description
protected	Expression() Creates a default Expression.
protected	Expression(java.lang.String name) Creates an Expression with a name.
protected	Expression(java.lang.String name, java.lang.String type) Creates an Expression with a name and a type.

Method Summary

Methods

Modifier and Type	Method and Description
abstract void	assign(java.lang.Object value) Assigns a value to this Expression.
boolean	equals(java.lang.Object other) Reports whether this Expression is equal to another object.
abstract java.lang.Object	evaluate() Evaluates this Expression.
java.lang.String	getName() Returns the name of an Expression.
java.lang.String	getType() Returns the type of this Expression.
int	hashCode() Returns a quasi-unique hash code for this expression.
abstract boolean	isAssignable() Reports whether this Expression is re-assignable.
boolean	isBottom() Reports whether this Expression is the bottom element of its type.
abstract boolean	isBound() Reports whether this Expression is bound.
boolean	isOrdered() Reports whether this Expression has a strictly ordered type.
abstract void	rebind() Rebinds this Expression.
abstract void	rebind(java.lang.Object result) Rebinds this unbound Expression to the given result.
abstract Expression	resolve(Scope scope) Causes this Expression to resolve its embedded global and local Parameter references.
Expression	setType(java.lang.String type) Sets the type of this expression.
abstract void	unbind() Unbinds this Expression.

Methods inherited from class uk.ac.sheffield.vtts.model.Named

setName, setName, toString

Methods inherited from class uk.ac.sheffield.vtts.model.Element

safeEquals, safeHashCode, semanticError

Methods inherited from class java.lang.Object

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

Field Detail

factory

protected static ModelFactory factory

The singleton ModelFactory for creating objects of different model types. This is chiefly used by Parameter and Constant, to synthesise models of the different atomic expressions. It is also used by the Predicate subclasses Comparison, Membership and Proposition, to create values satisfying certain constraints.

type

protected java.lang.String type

The type of this Expression.

Constructor Detail

Expression

protected Expression()

Creates a default Expression.

Expression

protected Expression(java.lang.String name)

Creates an Expression with a name. If this Expression is some kind of Parameter, the name is the unique Parameter name. Otherwise, it is the name of some kind of Function at the root of this Expression. Each sub-kind of Function declares a limited set of legal names.

Parameters:

name - the name of this Expression.

Expression

protected Expression(java.lang.String name,
          java.lang.String type)

Creates an Expression with a name and a type. These are either the declared name and type of some kind of Parameter; or the restricted name and result type of some kind of Function.

Parameters:

name - the name of this expression.

type - the type of this expression.

Method Detail

equals

public boolean equals(java.lang.Object other)

Reports whether this Expression is equal to another object. True if the other object is an Expression with the same name and type as this; otherwise false. Note: the type is accessed through a method, to permit type inference if the type field is null.

Overrides:

equals in class Named

Parameters:

other - the other object.

Returns:

true if both Expressions have the same name and type.

hashCode

public int hashCode()

Returns a quasi-unique hash code for this expression. The hash code is based on the meta-type, name and type of the expression. Note: the type is accessed through a method, to permit type inference if the type field is null.

Overrides:

hashCode in class Named

Returns:

the hash code for this expression.

getName

public java.lang.String getName()

Returns the name of an Expression. It is an error if the name has not already been set, when this access-method is invoked.

Overrides:

getName in class Named

Returns:

the name.

setType

public Expression setType(java.lang.String type)

Sets the type of this expression.

Parameters:

type - the type to set.

Returns:

this Parameter.

getType

public java.lang.String getType()

Returns the type of this Expression.

Returns:

the type of this Expression.

isAssignable

public abstract boolean isAssignable()

Reports whether this Expression is re-assignable. If this Expression may be updated by an Assignment expression, returns true; otherwise returns false.

Returns:

true, if this Expression is re-assignable.

isOrdered

public boolean isOrdered()

Reports whether this Expression has a strictly ordered type. All number types are ordered arithmetically; Character and String are lexicographically ordered. List and Set types are only partially ordered by inclusion, so are not considered strictly ordered. This judgement is used when refining the partitions of a comparison, to decide, for example, whether equals(x, y) should only imply the existence of a single complementary partition notEquals(x, y), or the two ordered partitions lessThan(x, y) and moreThan(x, y).

Returns:

true, if this Expression has a strictly ordered type.

isBottom

public boolean isBottom()

Reports whether this Expression is the bottom element of its type. This is false for all Expressions, except for Constants whose value is the empty or bottom value for its declared type. This judgement is used when refining the partitions of a comparison, to decide whether lessThan(x, y) is a meaningful partition.

Returns:

true, if this Expression is the bottom element of its type.

assign

public abstract void assign(java.lang.Object value)

Assigns a value to this Expression. If this Expression is assignable, binds it to the supplied value. All Parameters, apart from Constants, are assignable. Functions and Predicates are not assignable, so the request is ignored in these kinds of Expression.

Parameters:

value - the value to assign.

evaluate

public abstract java.lang.Object evaluate()

Evaluates this Expression. This Expression is either a Parameter or some kind of Function with further Expression operands. Converts a Parameter's stored value into a Java object, according to the declared type. Evaluates a Function on each of its operands, after evaluating these in turn. The result is a Java object of a cognate type that corresponds to the Expression's declared type. Evaluation, unbinding and rebinding are used during model simulation.

Returns:

the Java object denoting the value of this Expression.

isBound

public abstract boolean isBound()

Reports whether this Expression is bound. This Expression is either a Parameter or some kind of Function with further Expression operands. If this is a Parameter, tests whether the bound flag is set. If this is a Function, tests whether every operand is recursively bound.

Returns:

true, if this Expression is bound.

unbind

public abstract void unbind()

Unbinds this Expression. This Expression is either a Parameter or some kind of Function with further Expression operands. Releases a Parameter's stored value, unless it is a Constant. For a Function, propagates the unbinding request to all operands. Evaluation, unbinding and rebinding are used during model simulation.

rebind

public abstract void rebind()

Rebinds this Expression. This Expression is either a Parameter or some kind of Function with further Expression operands. Restores a Parameter's value to its default value, unless it is a Constant. For a Function, propagates the rebinding request to all operands. For a Predicate, attempts to satisfy the Predicate's constraint when rebinding. Evaluation, unbinding and rebinding are used during model simulation.

rebind

public abstract void rebind(java.lang.Object result)

Rebinds this unbound Expression to the given result. This Expression is either a Parameter or some kind of Function with further Expression operands. Sets an unbound Parameter to the result value; or attempts to ensure that the Function's result value is equal to the result.

Parameters:

result - the desired bound value.

resolve

public abstract Expression resolve(Scope scope)

Causes this Expression to resolve its embedded global and local Parameter references. Resolution is used when building a model, to ensure that duplicate copies of named Parameters constructed by the parser are replaced by references to a single copy of each uniquely-named Parameter. Expressions are resolved within a Scope, where this is a table of global and local Parameter declarations that are currently in scope.

Parameters:

scope - a table of global and local Parameters.

Returns:

this Expression.