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:
Binding, ExportAgent, LocalBinding, Locatable, IdentityComparable, Traceable
Direct Known Subclasses:
EagerLetExpression
public class LetExpression extends Assignation
A LetExpression represents the XQuery construct let $x := expr return expr. It is used also for XSLT local variables.

  • Constructor Details

    • LetExpression

      public LetExpression()
      Create a LetExpression

  • Method Details

    • 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 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()

    • isNeedsEagerEvaluation

      public boolean isNeedsEagerEvaluation()

    • supportsLazyEvaluation

      public boolean supportsLazyEvaluation()
      Description copied from class: Expression
      Ask whether the expression supports lazy evaluation.
      Overrides:
      supportsLazyEvaluation in class Expression
      Returns:
      false either if the expression cannot be evaluated lazily because it has dependencies that cannot be saved in the context, or because lazy evaluation is pointless (for example, for literals and variable references).

    • 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, Supplier<RoleDiagnostic> roleSupplier, 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
      roleSupplier - 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:
      a rough estimate of the evaluation cost

    • 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

    • gatherProperties

      public void gatherProperties(BiConsumer<String,Object> consumer)
      Get the properties of this object to be included in trace messages, by supplying the property values to a supplied consumer function
      Parameters:
      consumer - the function to which the properties should be supplied, as (property name, value) pairs.

    • 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 - if evaluation of the variable fails

    • 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(Outputter output, XPathContext context) throws XPathException
      Process this expression as an instruction, writing results to the current outputter
      Overrides:
      process in class Expression
      Parameters:
      output - the destination for the result
      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

      protected 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

      protected 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

    • 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

    • toShortString

      public 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(SequenceEvaluator evaluator)

    • getEvaluator

      public SequenceEvaluator getEvaluator()

    • getElaborator

      public Elaborator getElaborator()
      Make an elaborator for this expression
      Overrides:
      getElaborator in class Expression
      Returns:
      a suitable elaborator