net.sf.saxon.value

Class Closure

java.lang.Object

net.sf.saxon.value.Closure

All Implemented Interfaces:

ContextOriginator, Sequence<Item<?>>

Direct Known Subclasses:

MemoClosure, SingletonClosure

public class Closure
extends java.lang.Object
implements Sequence<Item<?>>, ContextOriginator

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

Fields

Modifier and Type	Field and Description
protected int	depth
protected Expression	expression
protected SequenceIterator<?>	inputIterator
protected XPathContextMajor	savedXPathContext

Constructor Summary

Constructors

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

Method Summary

Modifier and Type	Method and Description
Expression	getExpression()
XPathContextMajor	getSavedXPathContext()
Item<?>	head() Get the first item in the sequence.
SequenceIterator<Item<?>>	iterate() Evaluate the expression in a given context to return an iterator over a sequence
static Sequence<?>	make(Expression expression, XPathContext context, int ref) Construct a Closure by supplying the expression and the set of context variables.
Sequence<Item<?>>	makeRepeatable() Ensure that the sequence is in a form where it can be evaluated more than once.
GroundedValue<?>	reduce() Reduce a value to its simplest form.
void	saveContext(Expression expression, XPathContext context)
void	setExpression(Expression expression)
void	setSavedXPathContext(XPathContextMajor savedXPathContext)

Methods inherited from class java.lang.Object

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

Methods inherited from interface net.sf.saxon.om.Sequence

asIterable, materialize

Field Detail

expression

protected Expression expression

savedXPathContext

protected XPathContextMajor savedXPathContext

depth

protected int depth

inputIterator

protected SequenceIterator<?> inputIterator

Constructor Detail

Closure

public Closure()

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

Method Detail

make

public static Sequence<?> 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 - if a dynamic error occurs

saveContext

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

Throws:

XPathException

head

public Item<?> head()
             throws XPathException

Get the first item in the sequence.

Specified by:

head in interface Sequence<Item<?>>

Returns:

the first item in the sequence if there is one, or null if the sequence is empty

Throws:

XPathException - in the situation where the sequence is evaluated lazily, and evaluation of the first item causes a dynamic error.

getExpression

public Expression getExpression()

getSavedXPathContext

public XPathContextMajor getSavedXPathContext()

setExpression

public void setExpression(Expression expression)

setSavedXPathContext

public void setSavedXPathContext(XPathContextMajor savedXPathContext)

iterate

public SequenceIterator<Item<?>> iterate()
                                  throws XPathException

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

Specified by:

iterate in interface Sequence<Item<?>>

Returns:

an iterator (specifically, a Saxon SequenceIterator, which is not a Iterator) over all the items

Throws:

XPathException - in the situation where the sequence is evaluated lazily, and constructing an iterator over the items causes a dynamic error.

reduce

public GroundedValue<?> 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

Returns:

the simplified value

Throws:

XPathException - if an error occurs doing the lazy evaluation

makeRepeatable

public Sequence<Item<?>> makeRepeatable()
                                 throws XPathException

Description copied from interface: Sequence

Ensure that the sequence is in a form where it can be evaluated more than once. Some sequences (for example LazySequence and Closure can only be evaluated once, and this operation causes these to be grounded. However, making it repeatable is not the same as making it grounded; it does not flush out all errors. Indeed, lazy evaluation relies on this property, because an expression that has been lifted out of a loop must not be evaluated unless the loop is executed at least once, to prevent spurious errors.

Specified by:

makeRepeatable in interface Sequence<Item<?>>

Returns:

An equivalent sequence that can be repeatedly evaluated

Throws:

XPathException - if evaluation fails