net.sf.saxon.expr

Class LetExpression

public class LetExpression extends Assignation implements TailCallReturner, DivisibleInstruction

A LetExpression is modelled on the XQuery syntax let $x := expr return expr. This syntax is not available in the surface XPath language, but it is used internally in an optimized expression tree.
Field Summary
intevaluationMode
intrefCount
Constructor Summary
LetExpression()
Create a LetExpression
Method Summary
voidaddReference(boolean isLoopingReference)
Register a variable reference that refers to the variable bound in this let expression
voidcheckPermittedContents(SchemaType parentType, StaticContext env, boolean whole)
Check that any elements and attributes constructed or returned by this expression are acceptable in the content model of a given complex type.
intcomputeCardinality()
Determine the static cardinality of the expression
intcomputeSpecialProperties()
Get the static properties of this expression (other than its type).
Expressioncopy()
Copy an expression.
booleaneffectiveBooleanValue(XPathContext context)
Get the effective boolean value of the expression.
protected ValueRepresentationeval(XPathContext context)
Evaluate the variable.
ItemevaluateItem(XPathContext context)
Evaluate the expression as a singleton
voidevaluatePendingUpdates(XPathContext context, PendingUpdateList pul)
Evaluate an updating expression, adding the results to a Pending Update List.
voidexplain(ExpressionPresenter out)
Diagnostic print of expression structure.
ItemTypegetItemType(TypeHierarchy th)
Determine the data type of the items returned by the expression, if possible
intgetNominalReferenceCount()
Get the (nominal) count of the number of references to this variable
booleanimplementsStaticTypeCheck()
Determine whether this expression implements its own method for static type checking
booleanisIndexedVariable()
Test whether the variable bound by this let expression should be indexable
booleanisVacuousExpression()
Determine whether this is a vacuous expression as defined in the XQuery update specification
SequenceIteratoriterate(XPathContext context)
Iterate over the result of the expression to return a sequence of items
EventIteratoriterateEvents(XPathContext context)
Iterate over the result of the expression to return a sequence of events
intmarkTailFunctionCalls(StructuredQName qName, int arity)
Mark tail function calls
Expressionoptimize(ExpressionVisitor visitor, ItemType contextItemType)
Perform optimisation of an expression and its subexpressions.
voidprocess(XPathContext context)
Process this expression as an instruction, writing results to the current outputter
TailCallprocessLeavingTail(XPathContext context)
ProcessLeavingTail: called to do the real work of this instruction.
voidprocessLeft(Stack<XPathContext> contextStack, Stack state)
In streaming mode, process the first half of the instruction (for example, to start a new document or element)
voidprocessRight(Stack<XPathContext> contextStack, Stack state)
In streaming mode, process the second half of the instruction (for example, to end a new document or element)
Expressionpromote(PromotionOffer offer, Expression parent)
Promote this expression if possible
voidsetEvalBeforeChildren(boolean before)
When streaming, set whether the variable is evaluated before processing children or after
voidsetIndexedVariable()
Indicate that the variable bound by this let expression should be indexable (because it is used in an appropriate filter expression)
ExpressionstaticTypeCheck(SequenceType req, boolean backwardsCompatible, RoleLocator role, ExpressionVisitor visitor)
Static type checking for let expressions is delegated to the expression itself, and is performed on the "action" expression, to allow further delegation to the branches of a conditional
StringtoString()
The toString() method for an expression attempts to give a representation of the expression in an XPath-like form, but there is no guarantee that the syntax will actually be true XPath.
ExpressiontypeCheck(ExpressionVisitor visitor, ItemType contextItemType)
Type-check the expression.

Field Detail

evaluationMode

int evaluationMode

refCount

int refCount

Constructor Detail

LetExpression

public LetExpression()
Create a LetExpression

Method Detail

addReference

public void addReference(boolean isLoopingReference)
Register a variable reference that refers to the variable bound in this let expression

Parameters: isLoopingReference - true if the reference occurs within a loop, such as the predicate of a filter expression

checkPermittedContents

public void checkPermittedContents(SchemaType parentType, StaticContext env, boolean whole)
Check that any elements and attributes constructed or returned by this expression are acceptable in the content model of a given complex type. It's always OK to say yes, since the check will be repeated at run-time. The process of checking element and attribute constructors against the content model of a complex type also registers the type of content expected of those constructors, so the static validation can continue recursively.

computeCardinality

public int computeCardinality()
Determine the static cardinality of the expression

computeSpecialProperties

public int computeSpecialProperties()
Get the static properties of this expression (other than its type). The result is bit-signficant. These properties are used for optimizations. In general, if property bit is set, it is true, but if it is unset, the value is unknown.

copy

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

Returns: the copy of the original expression

effectiveBooleanValue

public boolean effectiveBooleanValue(XPathContext context)
Get the effective boolean value of the expression. This returns false if the value is the empty sequence, a zero-length string, a number equal to zero, or the boolean false. Otherwise it returns true.

Parameters: context The context in which the expression is to be evaluated

Returns: the effective boolean value

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

eval

protected ValueRepresentation eval(XPathContext context)
Evaluate the variable.

Parameters: context the dynamic evaluation context

Returns: the result of evaluating the expression that is bound to the variable

evaluateItem

public Item evaluateItem(XPathContext context)
Evaluate the expression as a singleton

evaluatePendingUpdates

public void evaluatePendingUpdates(XPathContext context, PendingUpdateList pul)
Evaluate an updating expression, adding the results to a Pending Update List. The default implementation of this method, which is used for non-updating expressions, throws an UnsupportedOperationException

Parameters: context the XPath dynamic evaluation context pul the pending update list to which the results should be written

explain

public void explain(ExpressionPresenter out)
Diagnostic print of expression structure. The abstract expression tree is written to the supplied output destination.

getItemType

public ItemType getItemType(TypeHierarchy th)
Determine the data type of the items returned by the expression, if possible

Parameters: th the type hierarchy cache

Returns: one of the values Type.STRING, Type.BOOLEAN, Type.NUMBER, Type.NODE, or Type.ITEM (meaning not known in advance)

getNominalReferenceCount

public int getNominalReferenceCount()
Get the (nominal) count of the number of references to this variable

Returns: zero if there are no references, one if there is a single reference that is not in a loop, some higher number if there are multiple references (or a single reference in a loop), or the special value @link RangeVariable#FILTERED} if there are any references in filter expressions that require searching.

implementsStaticTypeCheck

public boolean implementsStaticTypeCheck()
Determine whether this expression implements its own method for static type checking

Returns: true - this expression has a non-trivial implementation of the staticTypeCheck() method

isIndexedVariable

public boolean isIndexedVariable()
Test whether the variable bound by this let expression should be indexable

Returns: true if the variable should be indexable

isVacuousExpression

public boolean isVacuousExpression()
Determine whether this is a vacuous expression as defined in the XQuery update specification

Returns: true if this expression is vacuous

iterate

public SequenceIterator iterate(XPathContext context)
Iterate over the result of the expression to return a sequence of items

iterateEvents

public EventIterator iterateEvents(XPathContext context)
Iterate over the result of the expression to return a sequence of events

markTailFunctionCalls

public int markTailFunctionCalls(StructuredQName qName, int arity)
Mark tail function calls

optimize

public Expression optimize(ExpressionVisitor visitor, ItemType contextItemType)
Perform optimisation of an expression and its subexpressions.

This method is called after all references to functions and variables have been resolved to the declaration of the function or variable, and after all type checking has been done.

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 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)

process

public void process(XPathContext context)
Process this expression as an instruction, writing results to the current outputter

processLeavingTail

public TailCall processLeavingTail(XPathContext context)
ProcessLeavingTail: called to do the real work of this instruction. The results of the instruction are written to the current Receiver, which can be obtained via the Controller.

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

Returns: null if the instruction has completed execution; or a TailCall indicating a function call or template call that is delegated to the caller, to be made after the stack has been unwound so as to save stack space.

processLeft

public void processLeft(Stack<XPathContext> contextStack, Stack state)
In streaming mode, process the first half of the instruction (for example, to start a new document or element)

Parameters: contextStack state a stack on which the instruction can save state information during the call on processLeft()

processRight

public void processRight(Stack<XPathContext> contextStack, Stack state)
In streaming mode, process the second half of the instruction (for example, to end a new document or element)

Parameters: contextStack state a stack on which the instruction can save state information during the call on processLeft()

promote

public Expression promote(PromotionOffer offer, Expression parent)
Promote this expression if possible

setEvalBeforeChildren

public void setEvalBeforeChildren(boolean before)
When streaming, set whether the variable is evaluated before processing children or after

Parameters: before true if the initializer should be evaluated during the processLeft() call

setIndexedVariable

public void setIndexedVariable()
Indicate that the variable bound by this let expression should be indexable (because it is used in an appropriate filter expression)

staticTypeCheck

public Expression staticTypeCheck(SequenceType req, boolean backwardsCompatible, RoleLocator role, ExpressionVisitor visitor)
Static type checking for let expressions is delegated to the expression itself, and is performed on the "action" expression, to allow further delegation to the branches of a conditional

Parameters: req the required type backwardsCompatible true if backwards compatibility mode applies role the role of the expression in relation to the required type visitor an expression visitor

Returns: the expression after type checking (perhaps augmented with dynamic type checking code)

Throws: XPathException if failures occur, for example if the static type of one branch of the conditional is incompatible with the required type

toString

public String toString()
The toString() method for an expression attempts to give a representation of the expression in an XPath-like form, but there is no guarantee that the syntax will actually be true XPath. In the case of XSLT instructions, the toString() method gives an abstracted view of the syntax

Returns: a representation of the expression as a string

typeCheck

public Expression typeCheck(ExpressionVisitor visitor, ItemType contextItemType)
Type-check the expression. This also has the side-effect of counting the number of references to the variable (treating references that occur within a loop specially)