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:

Binding, ExportAgent, TailCallReturner, LocalBinding, Locatable, IdentityComparable

Direct Known Subclasses:

EagerLetExpression

public class LetExpression
extends Assignation
implements TailCallReturner

A LetExpression represents the XQuery construct let $x := expr return expr. It is used also for XSLT local variables.

Field Summary

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

hasLoopingReference, isIndexedVariable, references, requiredType, slotNumber, variableName

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

EFFECTIVE_BOOLEAN_VALUE, EVALUATE_METHOD, ITEM_FEED_METHOD, ITERATE_METHOD, MAX_COST, MAX_SEQUENCE_LENGTH, MAX_STRING_LENGTH, PROCESS_METHOD, staticProperties, UNBOUNDED_LOWER, UNBOUNDED_UPPER, UPDATE_METHOD, WATCH_METHOD

Constructor Summary

Constructors

Constructor and Description
LetExpression() Create a LetExpression

Method Summary

Modifier and Type	Method and Description
void	checkPermittedContents(SchemaType parentType, 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(RebindingMap rebindings) Copy an expression.
boolean	effectiveBooleanValue(XPathContext context) Get the effective boolean value of the expression.
Sequence<?>	eval(XPathContext context) Evaluate the variable.
Item	evaluateItem(XPathContext context) Evaluate the expression as a singleton
void	evaluatePendingUpdates(XPathContext context, PendingUpdateList pul) Evaluate an updating expression, adding the results to a Pending Update List.
void	export(ExpressionPresenter out) Diagnostic print of expression structure.
double	getCost() Return the estimated cost of evaluating an expression.
Evaluator	getEvaluator()
java.lang.String	getExpressionName() Get a name identifying the kind of expression, in terms meaningful to a user.
int	getImplementationMethod() An implementation of Expression must provide at least one of the methods evaluateItem(), iterate(), or process().
IntegerValue[]	getIntegerBounds() For an expression that returns an integer or a sequence of integers, get a lower and upper bound on the values of the integers that may be returned, from static analysis.
ItemType	getItemType() Determine the data type of the items returned by the expression, if possible
UType	getStaticUType(UType contextItemType) Get the static type of the expression as a UType, following precisely the type inference rules defined in the XSLT 3.0 specification.
boolean	implementsStaticTypeCheck() Determine whether this expression implements its own method for static type checking
boolean	isInstruction() Ask whether this expression is an instruction.
boolean	isLiftable(boolean forStreaming) Ask whether the expression can be lifted out of a loop, assuming it has no dependencies on the controlling variable/focus of the loop
boolean	isNeedsLazyEvaluation()
boolean	isVacuousExpression() Determine whether this is a vacuous expression as defined in the XQuery update specification
SequenceIterator<?>	iterate(XPathContext context) Iterate over the result of the expression to return a sequence of items
int	markTailFunctionCalls(StructuredQName qName, int arity) Mark tail function calls
Expression	optimize(ExpressionVisitor visitor, ContextItemStaticInfo 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.
void	resetLocalStaticProperties() Reset the static properties of the expression to -1, so that they have to be recomputed next time they are used.
void	setEvaluationMode(int mode)
void	setEvaluator()
void	setEvaluator(Evaluator evaluator)
void	setInstruction(boolean inst) Say whether this expression originates as an XSLT instruction
void	setNeedsEagerEvaluation(boolean req) Say that this variable must be evaluated eagerly.
void	setNeedsLazyEvaluation(boolean req)
Expression	staticTypeCheck(SequenceType req, boolean backwardsCompatible, RoleDiagnostic 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
java.lang.String	toShortString() Produce a short string identifying the expression for use in error messages
java.lang.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.
Expression	typeCheck(ExpressionVisitor visitor, ContextItemStaticInfo contextInfo) Type-check the expression.

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

addReference, addToPathMap, checkForUpdatingSubexpressions, computeDependencies, evaluateVariable, getAction, getActionOp, getIntegerBoundsForVariable, getLocalSlotNumber, getNominalReferenceCount, getObjectName, getRequiredSlots, getRequiredType, getSequence, getSequenceOp, getVariableEQName, getVariableName, getVariableQName, hasVariableBinding, isAssignable, isGlobal, isIndexedVariable, isUpdatingExpression, operands, rebuildReferenceList, refineTypeInformation, removeDeadReferences, replaceVariable, setAction, setIndexedVariable, setRequiredType, setSequence, setSlotNumber, setVariableQName, suppressValidation, unordered, verifyReferences

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

adoptChildExpression, allowExtractingCommonSubexpressions, checkedOperands, computeHashCode, computeStaticProperties, dynamicError, equals, evaluateAsString, explain, getCardinality, getConfiguration, getConstructType, getDependencies, getEvaluationMethod, getExtraProperty, getInterpretedExpression, getIntrinsicDependencies, getLocalRetainedStaticContext, getLocation, getNetCost, getPackageData, getParentExpression, getProperties, getProperty, getRetainedStaticContext, getScopingExpression, getSlotsUsed, getSpecialProperties, getStaticBaseURI, getStaticBaseURIString, getStaticType, getStreamerName, hasCompatibleStaticContext, hashCode, hasSpecialProperty, identityHashCode, isCallOn, isEqual, isIdentical, isMultiThreaded, isStaticPropertiesKnown, isSubtreeExpression, operandList, operandSparseList, optimizeChildren, prepareForStreaming, restoreParentPointers, setEvaluationMethod, setExtraProperty, setFiltered, setFlattened, setLocation, setParentExpression, setRetainedStaticContext, setRetainedStaticContextLocally, setRetainedStaticContextThoroughly, setStaticProperty, simplify, simplifyChildren, toPattern, typeCheckChildren, typeError, verifyParentPointers

Methods inherited from class java.lang.Object

clone, finalize, getClass, notify, notifyAll, wait, wait, wait

Constructor Detail

LetExpression

public LetExpression()

Create a LetExpression

Method Detail

setInstruction

public void setInstruction(boolean inst)

Say whether this expression originates as an XSLT instruction

Parameters:

inst - true if this is an xsl:variable instruction

isInstruction

public boolean isInstruction()

Ask whether this expression is an instruction. In XSLT streamability analysis this is used to distinguish constructs corresponding to XSLT instructions from other constructs.

Overrides:

isInstruction in class Expression

Returns:

true if this construct originates as an XSLT instruction

getExpressionName

public java.lang.String getExpressionName()

Get a name identifying the kind of expression, in terms meaningful to a user.

Overrides:

getExpressionName in class Expression

Returns:

a name identifying the kind of expression, in terms meaningful to a user. The name will always be in the form of a lexical XML QName, and should match the name used in explain() output displaying the expression.

setNeedsEagerEvaluation

public void setNeedsEagerEvaluation(boolean req)

Say that this variable must be evaluated eagerly. This is a requirement for example for variables appearing within a try/catch, or for variables referenced from within a construct that uses multi-threading, for example xsl:result-document or a multi-threaded xsl:for-each.

Parameters:

req - set to true if eager evaluation is required

setNeedsLazyEvaluation

public void setNeedsLazyEvaluation(boolean req)

isNeedsLazyEvaluation

public boolean isNeedsLazyEvaluation()

isLiftable

public boolean isLiftable(boolean forStreaming)

Ask whether the expression can be lifted out of a loop, assuming it has no dependencies on the controlling variable/focus of the loop

Overrides:

isLiftable in class Expression

Parameters:

forStreaming - true if we are handling streaming code (where rewrites need to be more cautious)

Returns:

true if the expression can be loop lifted

resetLocalStaticProperties

public void resetLocalStaticProperties()

Description copied from class: Expression

Reset the static properties of the expression to -1, so that they have to be recomputed next time they are used.

Overrides:

resetLocalStaticProperties in class Expression

typeCheck

public Expression typeCheck(ExpressionVisitor visitor,
                            ContextItemStaticInfo contextInfo)
                     throws XPathException

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)

Overrides:

typeCheck in class Expression

Parameters:

visitor - an expression visitor

contextInfo - Information available statically about the context item: whether it is (possibly) absent; its static type; its streaming posture.

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,
                                  RoleDiagnostic 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,
                           ContextItemStaticInfo 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)

setEvaluator

public void setEvaluator()

getCost

public double getCost()

Return the estimated cost of evaluating an expression. This is a very crude measure based on the syntactic form of the expression (we have no knowledge of data values). We take the cost of evaluating a simple scalar comparison or arithmetic expression as 1 (one), and we assume that a sequence has length 5. The resulting estimates may be used, for example, to reorder the predicates in a filter expression so cheaper predicates are evaluated first.

Overrides:

getCost in class Assignation

Returns:

isVacuousExpression

public boolean isVacuousExpression()

Determine whether this is a vacuous expression as defined in the XQuery update specification

Overrides:

isVacuousExpression in class Expression

Returns:

true if this expression is vacuous

checkPermittedContents

public void checkPermittedContents(SchemaType parentType,
                                   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

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

getIntegerBounds

public IntegerValue[] getIntegerBounds()

For an expression that returns an integer or a sequence of integers, get a lower and upper bound on the values of the integers that may be returned, from static analysis. The default implementation returns null, meaning "unknown" or "not applicable". Other implementations return an array of two IntegerValue objects, representing the lower and upper bounds respectively. The values UNBOUNDED_LOWER and UNBOUNDED_UPPER are used by convention to indicate that the value may be arbitrarily large. The values MAX_STRING_LENGTH and MAX_SEQUENCE_LENGTH are used to indicate values limited by the size of a string or the size of a sequence.

Overrides:

getIntegerBounds in class Expression

Returns:

the lower and upper bounds of integer values in the result, or null to indicate unknown or not applicable.

getImplementationMethod

public int getImplementationMethod()

An implementation of Expression must provide at least one of the methods evaluateItem(), iterate(), or process(). This method indicates which of these methods is provided directly. The other methods will always be available indirectly, using an implementation that relies on one of the other methods.

Specified by:

getImplementationMethod in class Expression

Returns:

the implementation method, for example Expression.ITERATE_METHOD or Expression.EVALUATE_METHOD or Expression.PROCESS_METHOD

iterate

public SequenceIterator<?> iterate(XPathContext context)
                            throws XPathException

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

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

eval

public Sequence<?> eval(XPathContext context)
                 throws XPathException

Evaluate the variable.

Parameters:

context - the dynamic evaluation context

Returns:

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

Throws:

XPathException

evaluateItem

public Item evaluateItem(XPathContext context)
                  throws XPathException

Evaluate the expression as a singleton

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

effectiveBooleanValue

public boolean effectiveBooleanValue(XPathContext context)
                              throws XPathException

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.

Overrides:

effectiveBooleanValue in class Expression

Parameters:

context - The context in which the expression is to be evaluated

Returns:

the effective boolean value

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 - if a dynamic error occurs

getItemType

public ItemType getItemType()

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

Specified by:

getItemType in class Expression

Returns:

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

getStaticUType

public UType getStaticUType(UType contextItemType)

Get the static type of the expression as a UType, following precisely the type inference rules defined in the XSLT 3.0 specification.

Overrides:

getStaticUType in class Expression

Parameters:

contextItemType - the type of the context item

Returns:

the static item type of the expression according to the XSLT 3.0 defined rules

computeCardinality

public int computeCardinality()

Determine the static cardinality of the expression

Specified by:

computeCardinality in class 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. May also return StaticProperty.ALLOWS_ZERO if the result is known to be an empty sequence, or StaticProperty.ALLOWS_MANY if if is known to return a sequence of length two 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.

Overrides:

computeSpecialProperties in class Expression

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:

UserFunctionCall.NOT_TAIL_CALL if no tail call was found; UserFunctionCall.FOREIGN_TAIL_CALL if a tail call on a different function was found; UserFunctionCall.SELF_TAIL_CALL if a tail recursive call was found and if this call accounts for the whole of the value.

copy

public Expression copy(RebindingMap rebindings)

Copy an expression. This makes a deep copy.

Specified by:

copy in class Expression

Parameters:

rebindings - variables whose binding needs to change

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 - if a dynamic error occurs during the evaluation of the instruction

evaluatePendingUpdates

public void evaluatePendingUpdates(XPathContext context,
                                   PendingUpdateList pul)
                            throws XPathException

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

Overrides:

evaluatePendingUpdates in class Expression

Parameters:

context - the XPath dynamic evaluation context

pul - the pending update list to which the results should be written

Throws:

XPathException - if evaluation fails

toString

public java.lang.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

Overrides:

toString in class Expression

Returns:

a representation of the expression as a string

toShortString

public java.lang.String toShortString()

Produce a short string identifying the expression for use in error messages

Overrides:

toShortString in class Expression

Returns:

a short string, sufficient to identify the expression

export

public void export(ExpressionPresenter out)
            throws XPathException

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

Specified by:

export in interface ExportAgent

Specified by:

export in class Expression

Parameters:

out - the expression presenter used to display the structure

Throws:

XPathException - if the export fails, for example if an expression is found that won't work in the target environment.

setEvaluator

public void setEvaluator(Evaluator evaluator)

setEvaluationMode

public void setEvaluationMode(int mode)

getEvaluator

public Evaluator getEvaluator()