Class SingletonClosure

  • All Implemented Interfaces:
    ContextOriginator, Sequence

    public class SingletonClosure
    extends Closure
    implements Sequence
    A SingletonClosure 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. The value of a SingletonClosure is always either a single item or an empty sequence.

    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

      • SingletonClosure

        public SingletonClosure​(Expression exp,
                                XPathContext context)
                         throws XPathException
        Constructor should not be called directly, instances should be made using the make() method.
        exp - the expression to be lazily evaluated
        context - the context in which the expression should be evaluated
        XPathException - if an error occurs saving the dynamic context
    • Method Detail

      • iterate

        public UnfailingIterator iterate()
                                  throws XPathException
        Evaluate the expression in a given context to return an iterator over a sequence
        Specified by:
        iterate in interface Sequence
        iterate in class Closure
        an iterator (specifically, a Saxon SequenceIterator, which is not a Iterator) over all the items
        XPathException - in the situation where the sequence is evaluated lazily, and constructing an iterator over the items causes a dynamic error.
      • itemAt

        public Item itemAt​(int n)
                    throws XPathException
        Get the n'th item in the sequence (starting from 0). This is defined for all SequenceValues, but its real benefits come for a SequenceValue stored extensionally
      • materialize

        public ZeroOrOne materialize()
                              throws XPathException
        Return a value containing all the items in the sequence returned by this SequenceIterator
        Specified by:
        materialize in interface Sequence
        the corresponding value
        XPathException - if evaluating the contents of the sequence fails with a dynamic error.
      • makeRepeatable

        public SingletonClosure makeRepeatable()
        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
        makeRepeatable in class Closure
        An equivalent sequence that can be repeatedly evaluated