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:
  • Field Details

  • Constructor Details

    • Closure

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

    • 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
    • saveContext

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