Package net.sf.saxon.expr
Class LetExpression
- java.lang.Object
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- net.sf.saxon.expr.Expression
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- net.sf.saxon.expr.Assignation
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- net.sf.saxon.expr.LetExpression
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- All Implemented Interfaces:
java.io.Serializable
,javax.xml.transform.SourceLocator
,LocationProvider
,SaxonLocator
,Binding
,EvaluableItem
,SequenceIterable
,TailCallReturner
,InstructionInfo
,org.xml.sax.Locator
- 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
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Field Summary
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Fields inherited from class net.sf.saxon.expr.Assignation
action, requiredType, sequence, slotNumber, variableName
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Fields inherited from class net.sf.saxon.expr.Expression
EVALUATE_METHOD, ITERATE_METHOD, locationId, PROCESS_METHOD, staticProperties
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Constructor Summary
Constructors Constructor Description LetExpression()
Create a LetExpression
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Method Summary
All Methods Instance Methods Concrete Methods Modifier and Type Method Description void
addReference(VariableReference v)
Register a variable reference that refers to the variable bound in this let expressionvoid
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 expressionint
computeSpecialProperties()
Get the static properties of this expression (other than its type).Expression
copy()
Copy an expression.protected ValueRepresentation
eval(XPathContext context)
Evaluate the variable.Item
evaluateItem(XPathContext context)
Evaluate the expression as a singletonvoid
evaluatePendingUpdates(XPathContext context, PendingUpdateList pul)
Evaluate an updating expression, adding the results to a Pending Update List.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 possibleint
getNominalReferenceCount()
Get the (nominal) count of the number of references to this variableboolean
implementsStaticTypeCheck()
Determine whether this expression implements its own method for static type checkingboolean
isIndexedVariable()
Test whether the variable bound by this let expression should be indexableSequenceIterator
iterate(XPathContext context)
Iterate over the result of the expression to return a sequence of itemsEventIterator
iterateEvents(XPathContext context)
Iterate over the result of the expression to return a sequence of eventsint
markTailFunctionCalls(StructuredQName qName, int arity)
Mark tail function callsExpression
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 outputterTailCall
processLeavingTail(XPathContext context)
ProcessLeavingTail: called to do the real work of this instruction.Expression
promote(PromotionOffer offer)
Promote this expression if possiblevoid
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 conditionalExpression
typeCheck(ExpressionVisitor visitor, ItemType contextItemType)
Type-check the expression-
Methods inherited from class net.sf.saxon.expr.Assignation
addToPathMap, checkForUpdatingSubexpressions, evaluateVariable, extendBindingList, getAction, getLocalSlotNumber, getObjectName, getRequiredSlots, getRequiredType, getSequence, getVariableName, getVariableQName, isAssignable, isGlobal, isUpdatingExpression, iterateSubExpressions, promoteWhereClause, refineTypeInformation, replaceSubExpression, setAction, setRequiredType, setSequence, setSlotNumber, setVariableQName, simplify, suppressValidation
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Methods inherited from class net.sf.saxon.expr.Expression
adoptChildExpression, computeDependencies, computeStaticProperties, display, doPromotion, dynamicError, effectiveBooleanValue, evaluateAsString, explain, findParentOf, getCardinality, getColumnNumber, getColumnNumber, getConstructType, getContainer, getDependencies, getExecutable, getHostLanguage, getImplementationMethod, getIntrinsicDependencies, getLineNumber, getLineNumber, getLocationId, getLocationProvider, getProperties, getProperty, getPublicId, getSlotsUsed, getSpecialProperties, getSystemId, getSystemId, hasLoopingSubexpression, resetLocalStaticProperties, setContainer, setFiltered, setFlattened, setLocationId, toString, typeError
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Method Detail
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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)
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isIndexedVariable
public boolean isIndexedVariable()
Test whether the variable bound by this let expression should be indexable- Returns:
- true if the variable should be indexable
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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.
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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
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typeCheck
public Expression typeCheck(ExpressionVisitor visitor, ItemType contextItemType) throws XPathException
Type-check the expression- Overrides:
typeCheck
in classExpression
- Parameters:
visitor
- an expression visitorcontextItemType
- 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 toType.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)
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implementsStaticTypeCheck
public boolean implementsStaticTypeCheck()
Determine whether this expression implements its own method for static type checking- Overrides:
implementsStaticTypeCheck
in classExpression
- Returns:
- true - this expression has a non-trivial implementation of the staticTypeCheck() method
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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 classExpression
- Parameters:
req
- the required typebackwardsCompatible
- true if backwards compatibility mode appliesrole
- the role of the expression in relation to the required typevisitor
- 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
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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 classExpression
- Parameters:
visitor
- an expression visitorcontextItemType
- 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 toType.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)
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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 classExpression
- 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 typeenv
- the static contextwhole
- 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
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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 interfaceSequenceIterable
- Overrides:
iterate
in classExpression
- 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
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iterateEvents
public EventIterator iterateEvents(XPathContext context) throws XPathException
Iterate over the result of the expression to return a sequence of events- Overrides:
iterateEvents
in classExpression
- 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
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eval
protected ValueRepresentation 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
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evaluateItem
public Item evaluateItem(XPathContext context) throws XPathException
Evaluate the expression as a singleton- Specified by:
evaluateItem
in interfaceEvaluableItem
- Overrides:
evaluateItem
in classExpression
- 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
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process
public void process(XPathContext context) throws XPathException
Process this expression as an instruction, writing results to the current outputter- Overrides:
process
in classExpression
- Parameters:
context
- The dynamic context, giving access to the current node, the current variables, etc.- Throws:
XPathException
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getItemType
public ItemType getItemType(TypeHierarchy th)
Determine the data type of the items returned by the expression, if possible- Specified by:
getItemType
in classExpression
- 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)
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computeCardinality
public int computeCardinality()
Determine the static cardinality of the expression- Specified by:
computeCardinality
in classExpression
- 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
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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 classExpression
- Returns:
- the special properties, as a bit-significant integer
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markTailFunctionCalls
public int markTailFunctionCalls(StructuredQName qName, int arity)
Mark tail function calls- Overrides:
markTailFunctionCalls
in classExpression
- Parameters:
qName
- the name of the functionarity
- 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.
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promote
public Expression promote(PromotionOffer offer) throws XPathException
Promote this expression if possible- Overrides:
promote
in classAssignation
- 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
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copy
public Expression copy()
Copy an expression. This makes a deep copy.- Specified by:
copy
in classExpression
- Returns:
- the copy of the original expression
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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 interfaceTailCallReturner
- 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
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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 classExpression
- Parameters:
context
- the XPath dynamic evaluation contextpul
- the pending update list to which the results should be written- Throws:
XPathException
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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 classExpression
- Parameters:
out
- the expression presenter used to display the structure
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