net.sf.saxon.value
Class Closure

java.lang.Object
  net.sf.saxon.value.Value
      net.sf.saxon.value.Closure

All Implemented Interfaces:

Serializable, SequenceIterable, ValueRepresentation

Direct Known Subclasses:

MemoClosure, SingletonClosure

public class Closure
extends Value

A Closure represents a value that has not yet been evaluated: the value is represented by an expression, together with saved values of all the context variables that the expression depends on.

This Closure is designed for use when the value is only read once. If the value is read more than once, a new iterator over the underlying expression is obtained each time: this may (for example in the case of a filter expression) involve significant re-calculation.

The expression may depend on local variables and on the context item; these values are held in the saved XPathContext object that is kept as part of the Closure, and they will always be read from that object. The expression may also depend on global variables; these are unchanging, so they can be read from the Bindery in the normal way. Expressions that depend on other contextual information, for example the values of position(), last(), current(), current-group(), should not be evaluated using this mechanism: they should always be evaluated eagerly. This means that the Closure does not need to keep a copy of these context variables.

See Also:

Serialized Form

Field Summary

Fields inherited from class net.sf.saxon.value.Value

EMPTY_CLASS_ARRAY, INDETERMINATE_ORDERING

Fields inherited from interface net.sf.saxon.om.ValueRepresentation

EMPTY_VALUE_ARRAY

Constructor Summary

Closure()
Constructor should not be called directly, instances should be made using the make() method.

Method Summary

int	getCardinality() Get the cardinality
ItemType	getItemType(TypeHierarchy th) Get the static item type
SequenceIterator	iterate() Evaluate the expression in a given context to return an iterator over a sequence
static Value	make(Expression expression, XPathContext context, int ref) Construct a Closure by supplying the expression and the set of context variables.
static Closure	makeIteratorClosure(SequenceIterator iterator) Construct a Closure over an existing SequenceIterator.
void	process(XPathContext context) Process the instruction, without returning any tail calls
Value	reduce() Reduce a value to its simplest form.

Methods inherited from class net.sf.saxon.value.Value

asItem, asItem, asIterator, asValue, checkPermittedContents, convertJavaObjectToXPath, convertToJava, convertToJava, effectiveBooleanValue, equals, fromItem, getCanonicalLexicalRepresentation, getIterator, getLength, getSchemaComparable, getStringValue, getStringValueCS, itemAt, iterate, makeQNameValue, stringToNumber, toString

Methods inherited from class java.lang.Object

getClass, hashCode, notify, notifyAll, wait, wait, wait

Constructor Detail

Closure

public Closure()

Constructor should not be called directly, instances should be made using the make() method.

Method Detail

makeIteratorClosure

public static Closure makeIteratorClosure(SequenceIterator iterator)

Construct a Closure over an existing SequenceIterator. This is used when an extension function returns a SequenceIterator as its result (it replaces the old SequenceIntent class for this purpose). There is no known expression in this case. Note that the caller must ensure this is a "clean" iterator: it must be positioned at the start, and must not be shared by anyone else.

Parameters:

iterator - the supplied iterator

Returns:

the Closure over this iterator

make

public static Value make(Expression expression,
                         XPathContext context,
                         int ref)
                  throws XPathException

Construct a Closure by supplying the expression and the set of context variables.

Parameters:

expression - the expression to be lazily evaluated

context - the dynamic context of the expression including for example the variables on which it depends

ref - the number of references to the value being lazily evaluated; this affects the kind of Closure that is created

Returns:

the Closure, a virtual value that can later be materialized when its content is required

Throws:

XPathException

getItemType

public ItemType getItemType(TypeHierarchy th)

Get the static item type

Overrides:

getItemType in class Value

Parameters:

th - the type hierarchy cache

Returns:

for the default implementation: AnyItemType (not known)

getCardinality

public int getCardinality()

Get the cardinality

Overrides:

getCardinality in class Value

Returns:

the cardinality

iterate

public SequenceIterator iterate()
                         throws XPathException

Evaluate the expression in a given context to return an iterator over a sequence

Specified by:

iterate in class Value

Returns:

an iterator over the sequence of items

Throws:

XPathException - if a dynamic error occurs. This is possible only in the case of values that are materialized lazily, that is, where the iterate() method leads to computation of an expression that delivers the values.

process

public void process(XPathContext context)
             throws XPathException

Process the instruction, without returning any tail calls

Overrides:

process in class Value

Parameters:

context - The dynamic context, giving access to the current node, the current variables, etc.

Throws:

XPathException

reduce

public Value reduce()
             throws XPathException

Reduce a value to its simplest form. If the value is a closure or some other form of deferred value such as a FunctionCallPackage, then it is reduced to a SequenceExtent. If it is a SequenceExtent containing a single item, then it is reduced to that item. One consequence that is exploited by class FilterExpression is that if the value is a singleton numeric value, then the result will be an instance of NumericValue

Overrides:

reduce in class Value

Returns:

the value in simplified form

Throws:

XPathException