Class LazyExpression

All Implemented Interfaces:
Serializable, SourceLocator, LocationProvider, SaxonLocator, EvaluableItem, SequenceIterable, InstructionInfo, Locator

public class LazyExpression extends UnaryExpression
A LazyExpression is an expression that forces lazy evaluation: it must not be evaluated eagerly, because a failure must not be reported unless the value is actually referenced. This is used for an expression that has been moved out of a loop. If the loop iterates zero times, the expression will not be evaluated, and in particular, it will not cause a dynamic error.

Note that the LazyExpression class does not itself implement any kind of delayed evaluation: calling its evaluateItem() and iterate() methods produces an immediate result. Instead, the existence of a LazyExpression on the expression tree acts as a signal to other classes that evaluation should be delayed, typically by holding the result of the iterate() method in a Closure object.

See Also:
  • Constructor Details

    • LazyExpression

      public LazyExpression(Expression operand)
      Create a LazyExpression
      Parameters:
      operand - the expression to be evaluated lazily
  • Method Details

    • makeLazyExpression

      public static Expression makeLazyExpression(Expression operand)
      Create a LazyExpression (factory method)
      Parameters:
      operand - the expression to be evaluated lazily
      Returns:
      the LazyExpression
    • typeCheck

      public Expression typeCheck(ExpressionVisitor visitor, ItemType contextItemType) throws XPathException
      The typeCheck method suppresses compile-time evaluation
      Overrides:
      typeCheck in class UnaryExpression
      Parameters:
      visitor - an expression visitor
      contextItemType - the static type of the context item
      Returns:
      the expression after typechecking
      Throws:
      XPathException
    • optimize

      public Expression optimize(ExpressionVisitor visitor, ItemType contextItemType) throws XPathException
      Perform optimisation of an expression and its subexpressions. This method suppresses compile-time evaluation
      Overrides:
      optimize in class UnaryExpression
      Parameters:
      visitor - an expression visitor
      contextItemType - the static type of "." at the point where this expression is invoked. The parameter is set to null if it is known statically that the context item will be undefined. If the type of the context item is not known statically, the argument is set to Type.ITEM_TYPE
      Returns:
      the original expression, rewritten if appropriate to optimize execution
      Throws:
      XPathException - if an error is discovered during this phase (typically a type error)
    • evaluateItem

      public Item evaluateItem(XPathContext context) throws XPathException
      Evaluate an expression as a single item. This always returns either a single Item or null (denoting the empty sequence). No conversion is done. This method should not be used unless the static type of the expression is a subtype of "item" or "item?": that is, it should not be called if the expression may return a sequence. There is no guarantee that this condition will be detected.
      Specified by:
      evaluateItem in interface EvaluableItem
      Overrides:
      evaluateItem in class Expression
      Parameters:
      context - The context in which the expression is to be evaluated
      Returns:
      the node or atomic value that results from evaluating the expression; or null to indicate that the result is an empty sequence
      Throws:
      XPathException - if any dynamic error occurs evaluating the expression
    • iterate

      public SequenceIterator iterate(XPathContext context) throws XPathException
      Return an Iterator to iterate over the values of a sequence. The value of every expression can be regarded as a sequence, so this method is supported for all expressions. This default implementation handles iteration for expressions that return singleton values: for non-singleton expressions, the subclass must provide its own implementation.
      Specified by:
      iterate in interface SequenceIterable
      Overrides:
      iterate in class Expression
      Parameters:
      context - supplies the context for evaluation
      Returns:
      a SequenceIterator that can be used to iterate over the result of the expression
      Throws:
      XPathException - if any dynamic error occurs evaluating the expression
    • process

      public void process(XPathContext context) throws XPathException
      Process the instruction, without returning any tail calls
      Overrides:
      process in class Expression
      Parameters:
      context - The dynamic context, giving access to the current node, the current variables, etc.
      Throws:
      XPathException
    • copy

      public Expression copy()
      Copy an expression. This makes a deep copy.
      Specified by:
      copy in class Expression
      Returns:
      the copy of the original expression
    • displayExpressionName

      protected String displayExpressionName()
      Description copied from class: UnaryExpression
      Return the element name to use in the expression tree
      Overrides:
      displayExpressionName in class UnaryExpression