net.sf.saxon.value

Class Closure

Implemented Interfaces:
Serializable, SequenceIterable, ValueRepresentation
Known Direct 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.

Field Summary

protected int
depth
protected Expression
expression
protected SequenceIterator
inputIterator
protected XPathContextMajor
savedXPathContext

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.
protected void
saveContext(Expression expression, XPathContext context)

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

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

Field Details

depth

protected int depth

expression

protected Expression expression

inputIterator

protected SequenceIterator inputIterator

savedXPathContext

protected XPathContextMajor savedXPathContext

Constructor Details

Closure

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

Method Details

getCardinality

public int getCardinality()
Get the cardinality
Overrides:
getCardinality in interface Value

getItemType

public ItemType getItemType(TypeHierarchy th)
Get the static item type
Overrides:
getItemType in interface Value
Parameters:
th - the type hierarchy cache

iterate

public SequenceIterator iterate()
            throws XPathException
Evaluate the expression in a given context to return an iterator over a sequence
Overrides:
iterate in interface Value

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

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

process

public void process(XPathContext context)
            throws XPathException
Process the instruction, without returning any tail calls
Overrides:
process in interface Value
Parameters:
context - The dynamic context, giving access to the current node, the current variables, etc.

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 interface Value

saveContext

protected void saveContext(Expression expression,
                           XPathContext context)
            throws XPathException