net.sf.saxon.expr

Class LazyExpression

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.

Constructor Summary
LazyExpression(Expression operand)
Create a LazyExpression
Method Summary
Expressioncopy()
Copy an expression.
protected StringdisplayExpressionName()
ItemevaluateItem(XPathContext context)
Evaluate an expression as a single item.
SequenceIteratoriterate(XPathContext context)
Return an Iterator to iterate over the values of a sequence.
static ExpressionmakeLazyExpression(Expression operand)
Create a LazyExpression (factory method)
Expressionoptimize(ExpressionVisitor visitor, ItemType contextItemType)
Perform optimisation of an expression and its subexpressions.
voidprocess(XPathContext context)
Process the instruction, without returning any tail calls
ExpressiontypeCheck(ExpressionVisitor visitor, ItemType contextItemType)
The typeCheck method suppresses compile-time evaluation

Constructor Detail

LazyExpression

public LazyExpression(Expression operand)
Create a LazyExpression

Parameters: operand the expression to be evaluated lazily

Method Detail

copy

public Expression copy()
Copy an expression. This makes a deep copy.

Returns: the copy of the original expression

displayExpressionName

protected String displayExpressionName()

evaluateItem

public Item evaluateItem(XPathContext context)
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.

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: net.sf.saxon.trans.XPathException if any dynamic error occurs evaluating the expression

iterate

public SequenceIterator iterate(XPathContext context)
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.

Parameters: context supplies the context for evaluation

Returns: a SequenceIterator that can be used to iterate over the result of the expression

Throws: net.sf.saxon.trans.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)
Perform optimisation of an expression and its subexpressions. This method suppresses compile-time evaluation

process

public void process(XPathContext context)
Process the instruction, without returning any tail calls

Parameters: context The dynamic context, giving access to the current node, the current variables, etc.

typeCheck

public Expression typeCheck(ExpressionVisitor visitor, ItemType contextItemType)
The typeCheck method suppresses compile-time evaluation

Parameters: visitor an expression visitor contextItemType the static type of the context item

Returns: the expression after typechecking

Throws: XPathException