Package 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, LocationProvider, SaxonLocator, Binding, TailCallReturner, 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:

  • Constructor Details

    • LetExpression

      public LetExpression()
      Create a LetExpression

  • Method Details

    • 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.

    • setRefCount

      public void setRefCount(int refCount)

    • typeCheck

      public Expression typeCheck(ExpressionVisitor visitor, ExpressionVisitor.ContextItemType 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, ExpressionVisitor.ContextItemType 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)

    • replaceSubExpression

      public boolean replaceSubExpression(Expression original, Expression replacement)
      Replace one subexpression by a replacement subexpression
      Overrides:
      replaceSubExpression in class Assignation
      Parameters:
      original - the original subexpression
      replacement - the replacement subexpression
      Returns:
      true if the original subexpression is found

    • 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

    • 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.

    • 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

    • 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

      public 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
      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(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:
      UserFunctionCall.NOT_TAIL_CALL if no tail call was found; UserFunctionCall.FOREIGN_TAIL_CALL if a tail call on a different function was found;

    • 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
      parent - the containing expression in the expression tree
      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 - 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 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

    • setEvaluationMode

      public void setEvaluationMode(int evaluationMode)

    • getEvaluationMode

      public int getEvaluationMode()