Class Closure<T extends Item>

  • All Implemented Interfaces:
    ValueRepresentation<T>
    Direct Known Subclasses:
    MemoClosure, SingletonClosure

    public class Closure<T extends Item>
    extends Value<T>
    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

      • makeIteratorClosure

        public static <T extends ItemValueRepresentation<T> makeIteratorClosure​(SequenceIterator<T> iterator)
                                                                           throws XPathException
        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
        Throws:
        XPathException - if a dynamic error occurs
      • 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 - if a dynamic error occurs
      • getItemType

        public ItemType getItemType​(TypeHierarchy th)
        Get the static item type
        Overrides:
        getItemType in class Value<T extends Item>
        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<T extends Item>
        Returns:
        the cardinality
      • getExpression

        public Expression getExpression()
      • setExpression

        public void setExpression​(Expression expression)
      • setSavedXPathContext

        public void setSavedXPathContext​(XPathContextMajor savedXPathContext)
      • iterate

        public SequenceIterator<T> iterate()
                                    throws XPathException
        Evaluate the expression in a given context to return an iterator over a sequence
        Specified by:
        iterate in class Value<T extends Item>
        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<T extends Item>
        Parameters:
        context - The dynamic context, giving access to the current node, the current variables, etc.
        Throws:
        XPathException - if an error occurs (for example if the value is a closure that needs to be evaluated)
      • reduce

        public Value<T> 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<T extends Item>
        Returns:
        the value in simplified form
        Throws:
        XPathException - if an error occurs