net.sf.saxon.expr
Class LetExpression

java.lang.Object
  extended by net.sf.saxon.expr.Expression
      extended by net.sf.saxon.expr.Assignation
          extended by net.sf.saxon.expr.LetExpression
All Implemented Interfaces:
Serializable, SourceLocator, LocationProvider, SaxonLocator, Binding, EvaluableItem, TailCallReturner, SequenceIterable, InstructionInfo, 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

Field Summary
 
Fields inherited from class net.sf.saxon.expr.Assignation
action, requiredType, sequence, slotNumber, variableName
 
Fields inherited from class net.sf.saxon.expr.Expression
EVALUATE_METHOD, EVENT_FEED_METHOD, ITEM_FEED_METHOD, ITERATE_METHOD, locationId, PROCESS_METHOD, PUSH_SELECTION, staticProperties, WATCH_METHOD
 
Constructor Summary
LetExpression()
          Create a LetExpression
 
Method Summary
 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.
 boolean effectiveBooleanValue(XPathContext context)
          Get the effective boolean value of the expression.
protected  ValueRepresentation 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 explain(ExpressionPresenter out)
          Diagnostic print of expression structure.
 String getExpressionName()
          Get a name identifying the kind of expression, in terms meaningful to a user.
 ItemType getItemType(TypeHierarchy th)
          Determine the data type of the items returned by the expression, if possible
 boolean implementsStaticTypeCheck()
          Determine whether this expression implements its own method for static type checking
 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
 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, Expression parent)
          Promote this expression if possible
 Expression staticTypeCheck(SequenceType req, boolean backwardsCompatible, RoleLocator role, TypeCheckerEnvironment 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
 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, ItemType contextItemType)
          Type-check the expression.
 
Methods inherited from class net.sf.saxon.expr.Assignation
addReference, addToPathMap, checkForUpdatingSubexpressions, evaluateVariable, extendBindingList, getAction, getLocalSlotNumber, getNominalReferenceCount, getObjectName, getRequiredSlots, getRequiredType, getSequence, getVariableName, getVariableQName, isAssignable, isGlobal, isIndexedVariable, isUpdatingExpression, iterateSubExpressions, promoteWhereClause, refineTypeInformation, replaceSubExpression, replaceVariable, setAction, setIndexedVariable, setRequiredType, setSequence, setSlotNumber, setVariableQName, simplify, suppressValidation
 
Methods inherited from class net.sf.saxon.expr.Expression
adoptChildExpression, computeDependencies, computeStaticProperties, doPromotion, dynamicError, evaluateAsString, explain, getCardinality, getColumnNumber, getColumnNumber, getConstructType, getContainer, getDependencies, getEvaluationMethod, getExecutable, getHostLanguage, getImplementationMethod, getIntrinsicDependencies, getLineNumber, getLineNumber, getLocationId, getLocationProvider, getProperties, getProperty, getPublicId, getSlotsUsed, getSpecialProperties, getSystemId, getSystemId, hasLoopingSubexpression, isSubtreeExpression, iterateSameFocusSubExpressions, resetLocalStaticProperties, setContainer, setEvaluationMethod, setFiltered, setFlattened, setLocationId, typeError
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait
 

Constructor Detail

LetExpression

public LetExpression()
Create a LetExpression

Method Detail

getExpressionName

public 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.

typeCheck

public Expression typeCheck(ExpressionVisitor visitor,
                            ItemType contextItemType)
                     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
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,
                                  TypeCheckerEnvironment 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)

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,
                                   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

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

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

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

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

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

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:
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,
                          Expression parent)
                   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

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

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

Overrides:
toString in class Expression
Returns:
a representation of the expression as a string

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


Copyright (c) 2004-2010 Saxonica Limited. All rights reserved.