Package net.sf.saxon.value

Class Closure

  • All Implemented Interfaces:
    ContextOriginator, Sequence
    Direct Known Subclasses:
    MemoClosure, SingletonClosure
    public class Closure
extends java.lang.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.

    • 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

      • 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.

      • getExpression

        public Expression getExpression()

      • setExpression

        public void setExpression​(Expression expression)

      • setSavedXPathContext

        public void setSavedXPathContext​(XPathContextMajor savedXPathContext)

      • iterate

        public 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