Package net.sf.saxon.value

Class Closure

java.lang.Object
net.sf.saxon.value.Closure
All Implemented Interfaces:
ContextOriginator, Sequence
Direct Known Subclasses:
MemoClosure, SingletonClosure
public abstract class Closure extends Object implements Sequence, 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 Details

  • Constructor Details

    • Closure

      public Closure()

  • Method Details

    • saveContext

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

    • setLearningEvaluator

      public void setLearningEvaluator(LearningEvaluator learningEvaluator, int serialNumber)

    • head

      public Item head() throws XPathException
      Get the first item in the sequence.
      Specified by:
      head in interface Sequence
      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.

    • getSavedXPathContext

      public XPathContextMajor getSavedXPathContext()

    • setInputEvaluator

      public void setInputEvaluator(PullEvaluator inputEvaluator)

    • setSavedXPathContext

      public void setSavedXPathContext(XPathContextMajor savedXPathContext)

    • iterate

      public abstract SequenceIterator iterate()
      Evaluate the expression in a given context to return an iterator over a sequence
      Specified by:
      iterate in interface Sequence
      Returns:
      an iterator (specifically, a Saxon SequenceIterator, which is not a Iterator) over all the items

    • 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 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
      Returns:
      An equivalent sequence that can be repeatedly evaluated
      Throws:
      XPathException - if evaluation fails

    • getExpression

      public Expression getExpression()