net.sf.saxon.value

Class Closure

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 intdepth
protected Expressionexpression
protected SequenceIteratorinputIterator
protected XPathContextMajorsavedXPathContext
Constructor Summary
Closure()
Constructor should not be called directly, instances should be made using the make() method.
Method Summary
voiddisplay(int level, NamePool pool, PrintStream out)
intgetCardinality()
Get the cardinality
intgetImplementationMethod()
An implementation of Expression must provide at least one of the methods evaluateItem(), iterate(), or process().
ItemTypegetItemType(TypeHierarchy th)
Get the static item type
intgetSpecialProperties()
Get the static properties of this expression (other than its type).
booleanisIndexable()
Determine whether this Closure is indexable
SequenceIteratoriterate(XPathContext context)
Evaluate the expression in a given context to return an iterator over a sequence
static Valuemake(Expression expression, XPathContext context, int ref)
Construct a Closure by supplying the expression and the set of context variables.
static ClosuremakeIteratorClosure(SequenceIterator iterator)
Construct a Closure over an existing SequenceIterator.
voidprocess(XPathContext context)
Process the instruction, without returning any tail calls
Valuereduce()
Reduce a value to its simplest form.

Field Detail

depth

protected int depth

expression

protected Expression expression

inputIterator

protected SequenceIterator inputIterator

savedXPathContext

protected XPathContextMajor savedXPathContext

Constructor Detail

Closure

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

Method Detail

display

public void display(int level, NamePool pool, PrintStream out)

getCardinality

public int getCardinality()
Get the cardinality

getImplementationMethod

public int getImplementationMethod()
An implementation of Expression must provide at least one of the methods evaluateItem(), iterate(), or process(). This method indicates which of these methods is provided. This implementation provides both iterate() and process() methods natively.

getItemType

public ItemType getItemType(TypeHierarchy th)
Get the static item type

Parameters: th

getSpecialProperties

public int getSpecialProperties()
Get the static properties of this expression (other than its type). The result is bit-signficant. These properties are used for optimizations. In general, if property bit is set, it is true, but if it is unset, the value is unknown.

isIndexable

public boolean isIndexable()
Determine whether this Closure is indexable

iterate

public SequenceIterator iterate(XPathContext context)
Evaluate the expression in a given context to return an iterator over a sequence

Parameters: context the evaluation context. This is ignored; we use the context saved as part of the Closure instead.

make

public static Value make(Expression expression, XPathContext context, int ref)
Construct a Closure by supplying the expression and the set of context variables.

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.

process

public void process(XPathContext context)
Process the instruction, without returning any tail calls

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

reduce

public Value reduce()
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