net.sf.saxon.expr
Class LetExpression

java.lang.Object
  net.sf.saxon.expr.Expression
      net.sf.saxon.expr.Assignation
          net.sf.saxon.expr.LetExpression

All Implemented Interfaces:

Serializable, SourceLocator, Binding, EvaluableItem, SequenceIterable, TailCallReturner, InstructionInfoProvider

Direct Known Subclasses:

EagerLetExpression

public class LetExpression
extends Assignation
implements TailCallReturner

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.

See Also:

Serialized Form

Field Summary

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

EVALUATE_METHOD, ITERATE_METHOD, PROCESS_METHOD

Constructor Summary

LetExpression()
Create a LetExpression

Method Summary

void	addReference(VariableReference v) Register a variable reference that refers to the variable bound in this let expression
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.
int	computeCardinality() Determine the static cardinality of the expression
int	computeSpecialProperties() Get the static properties of this expression (other than its type).
Expression	copy() Copy an expression.
Item	evaluateItem(XPathContext context) Evaluate the expression as a singleton
void	explain(ExpressionPresenter out) Diagnostic print of expression structure.
ItemType	getItemType(TypeHierarchy th) Determine the data type of the items returned by the expression, if possible
int	getNominalReferenceCount() Get the (nominal) count of the number of references to this variable
boolean	implementsStaticTypeCheck() Determine whether this expression implements its own method for static type checking
boolean	isIndexedVariable() Test whether the variable bound by this let expression should be indexable
SequenceIterator	iterate(XPathContext context) Iterate over the result of the expression to return a sequence of items
EventIterator	iterateEvents(XPathContext context) Iterate over the result of the expression to return a sequence of events
int	markTailFunctionCalls(StructuredQName qName, int arity) Mark tail function calls
Expression	optimize(ExpressionVisitor visitor, ItemType contextItemType) Perform optimisation of an expression and its subexpressions.
void	process(XPathContext context) Process this expression as an instruction, writing results to the current outputter
TailCall	processLeavingTail(XPathContext context) ProcessLeavingTail: called to do the real work of this instruction.
Expression	promote(PromotionOffer offer) Promote this expression if possible
void	setIndexedVariable() Indicate that the variable bound by this let expression should be indexable (because it is used in an appropriate filter expression)
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
Expression	typeCheck(ExpressionVisitor visitor, ItemType contextItemType) Type-check the expression

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

addToPathMap, evaluateVariable, getAction, getLocalSlotNumber, getRequiredSlots, getRequiredType, getSequence, getVariableName, getVariableQName, isAssignable, isGlobal, iterateSubExpressions, refineTypeInformation, replaceSubExpression, setAction, setRequiredType, setSequence, setSlotNumber, setVariableQName, simplify, suppressValidation

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

adoptChildExpression, computeDependencies, computeStaticProperties, display, doPromotion, effectiveBooleanValue, evaluateAsString, explain, findParentOf, getCardinality, getColumnNumber, getContainer, getDependencies, getExecutable, getHostLanguage, getImplementationMethod, getInstructionInfo, getIntrinsicDependencies, getLineNumber, getLocationId, getLocationProvider, getPublicId, getSlotsUsed, getSpecialProperties, getSystemId, hasLoopingSubexpression, setContainer, setFiltered, setFlattened, setLocationId, toString

Methods inherited from class java.lang.Object

equals, getClass, hashCode, notify, notifyAll, wait, wait, wait

Constructor Detail

LetExpression

public LetExpression()

Create a LetExpression

Method Detail

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)

isIndexedVariable

public boolean isIndexedVariable()

Test whether the variable bound by this let expression should be indexable

Returns:

true if the variable should be indexable

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.

addReference

public void addReference(VariableReference v)

Register a variable reference that refers to the variable bound in this let expression

Parameters:

v - the variable reference

typeCheck

public Expression typeCheck(ExpressionVisitor visitor,
                            ItemType contextItemType)
                     throws XPathException

Type-check the expression

Overrides:

typeCheck in class Expression

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 to perform necessary run-time type checks, and to perform other type-related optimizations

Throws:

XPathException - if an error is discovered during this phase (typically a type error)

implementsStaticTypeCheck

public boolean implementsStaticTypeCheck()

Determine whether this expression implements its own method for static type checking

Overrides:

implementsStaticTypeCheck in class Expression

Returns:

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

staticTypeCheck

public Expression staticTypeCheck(SequenceType req,
                                  boolean backwardsCompatible,
                                  RoleLocator role,
                                  ExpressionVisitor visitor)
                           throws XPathException

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

Overrides:

staticTypeCheck in class Expression

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

optimize

public Expression optimize(ExpressionVisitor visitor,
                           ItemType contextItemType)
                    throws XPathException

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.

Overrides:

optimize in class Expression

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)

checkPermittedContents

public void checkPermittedContents(SchemaType parentType,
                                   StaticContext env,
                                   boolean whole)
                            throws XPathException

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.

Overrides:

checkPermittedContents in class Expression

Parameters:

parentType - the "given complex type": the method is checking that the nodes returned by this expression are acceptable members of the content model of this type

env - the static context

whole - if true, we want to check that the value of this expression satisfies the content model as a whole; if false we want to check that the value of the expression is acceptable as one part of the content

Throws:

XPathException - if the value delivered by this expression cannot be part of the content model of the given type

iterate

public SequenceIterator iterate(XPathContext context)
                         throws XPathException

Iterate over the result of the expression to return a sequence of items

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

iterateEvents

public EventIterator iterateEvents(XPathContext context)
                            throws XPathException

Iterate over the result of the expression to return a sequence of events

Overrides:

iterateEvents in class Expression

Parameters:

context - The dynamic evaluation context

Returns:

the result of the expression as an iterator over a sequence of PullEvent objects

Throws:

XPathException - if a dynamic error occurs during expression evaluation

evaluateItem

public Item evaluateItem(XPathContext context)
                  throws XPathException

Evaluate the expression as a singleton

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

process

public void process(XPathContext context)
             throws XPathException

Process this expression as an instruction, writing results to the current outputter

Overrides:

process in class Expression

Parameters:

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

Throws:

XPathException

getItemType

public ItemType getItemType(TypeHierarchy th)

Determine the data type of the items returned by the expression, if possible

Specified by:

getItemType in class Expression

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)

computeCardinality

public int computeCardinality()

Determine the static cardinality of the expression

Returns:

the computed cardinality, as one of the values StaticProperty.ALLOWS_ZERO_OR_ONE, StaticProperty.EXACTLY_ONE, StaticProperty.ALLOWS_ONE_OR_MORE, StaticProperty.ALLOWS_ZERO_OR_MORE

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.

Returns:

the special properties, as a bit-significant integer

markTailFunctionCalls

public int markTailFunctionCalls(StructuredQName qName,
                                 int arity)

Mark tail function calls

Overrides:

markTailFunctionCalls in class Expression

Parameters:

qName - the name of the function

arity - the arity (number of parameters) of the function

Returns:

0 if no tail call was found; 1 if a tail call on a different function was found; 2 if a tail recursive call was found and if this call accounts for the whole of the value.

promote

public Expression promote(PromotionOffer offer)
                   throws XPathException

Promote this expression if possible

Overrides:

promote in class Assignation

Parameters:

offer - details of the offer, for example the offer to move expressions that don't depend on the context to an outer level in the containing expression

Returns:

if the offer is not accepted, return this expression unchanged. Otherwise return the result of rewriting the expression to promote this subexpression

Throws:

XPathException - if any error is detected

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

processLeavingTail

public TailCall processLeavingTail(XPathContext context)
                            throws XPathException

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.

Specified by:

processLeavingTail in interface TailCallReturner

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.

Throws:

XPathException

explain

public void explain(ExpressionPresenter out)

Diagnostic print of expression structure. The abstract expression tree is written to the supplied output destination.

Specified by:

explain in class Expression

Parameters:

out - the expression presenter used to display the structure