Class Closure

  • All Implemented Interfaces:
    java.io.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:
    Serialized Form
    • Constructor Detail

      • Closure

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

      • 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
      • 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