net.sf.saxon.expr

Class LazyExpression

Implemented Interfaces:
EvaluableItem, InstructionInfoProvider, SequenceIterable, Serializable, SourceLocator

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.

Field Summary

Fields inherited from class net.sf.saxon.expr.UnaryExpression

operand

Fields inherited from class net.sf.saxon.expr.Expression

EVALUATE_METHOD, ITERATE_METHOD, PROCESS_METHOD, locationId, staticProperties

Constructor Summary

LazyExpression(Expression operand)
Create a LazyExpression

Method Summary

Expression
copy()
Copy an expression.
protected String
displayExpressionName()
Item
evaluateItem(XPathContext context)
Evaluate an expression as a single item.
SequenceIterator
iterate(XPathContext context)
Return an Iterator to iterate over the values of a sequence.
static Expression
makeLazyExpression(Expression operand)
Create a LazyExpression (factory method)
Expression
optimize(ExpressionVisitor visitor, ItemType contextItemType)
Perform optimisation of an expression and its subexpressions.
void
process(XPathContext context)
Process the instruction, without returning any tail calls
Expression
typeCheck(ExpressionVisitor visitor, ItemType contextItemType)
The typeCheck method suppresses compile-time evaluation

Methods inherited from class net.sf.saxon.expr.UnaryExpression

computeCardinality, computeSpecialProperties, displayExpressionName, displayOperator, equals, explain, getBaseExpression, getItemType, hashCode, iterateSubExpressions, optimize, promote, replaceSubExpression, simplify, toString, typeCheck

Methods inherited from class net.sf.saxon.expr.Expression

addToPathMap, adoptChildExpression, checkPermittedContents, computeCardinality, computeDependencies, computeSpecialProperties, computeStaticProperties, copy, display, doPromotion, dynamicError, effectiveBooleanValue, evaluateAsString, evaluateItem, explain, explain, findParentOf, getCardinality, getColumnNumber, getConstructType, getContainer, getDependencies, getExecutable, getHostLanguage, getImplementationMethod, getInstructionInfo, getIntrinsicDependencies, getItemType, getLineNumber, getLocationId, getLocationProvider, getPublicId, getSlotsUsed, getSpecialProperties, getSystemId, hasLoopingSubexpression, implementsStaticTypeCheck, iterate, iterateEvents, iterateSubExpressions, markTailFunctionCalls, optimize, process, promote, replaceSubExpression, resetLocalStaticProperties, setContainer, setFiltered, setFlattened, setLocationId, simplify, staticTypeCheck, suppressValidation, toString, typeCheck, typeError

Constructor Details

LazyExpression

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

Method Details

copy

public Expression copy()
Copy an expression. This makes a deep copy.
Overrides:
copy in interface Expression
Returns:
the copy of the original expression

displayExpressionName

protected String displayExpressionName()
Overrides:
displayExpressionName in interface UnaryExpression

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 interface 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 interface 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

makeLazyExpression

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

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 interface UnaryExpression

process

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

typeCheck

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