net.sf.saxon.value
Class Closure

java.lang.Object
  extended by net.sf.saxon.value.Value
      extended by 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


Copyright (C) Michael H. Kay. All rights reserved.