net.sf.saxon.instruct

Class Instruction

public abstract class Instruction extends ComputedExpression implements SourceLocator, TailCallReturner

Abstract superclass for all instructions in the compiled stylesheet. This represents a compiled instruction, and as such, the minimum information is retained from the original stylesheet.
Note: this class implements SourceLocator: that is, it can identify where in the stylesheet the source instruction was located.
Constructor Summary
Instruction()
Constructor
Method Summary
static voidappendItem(Item it, SequenceReceiver out, int locationId)
Static method to append an item that results from evaluating an expression to the current result receiver.
protected static ParameterSetassembleParams(XPathContext context, WithParam[] actualParams)
Assemble a ParameterSet.
protected static ParameterSetassembleTunnelParams(XPathContext context, WithParam[] actualParams)
Assemble a ParameterSet.
intcomputeCardinality()
Get the cardinality of the sequence returned by evaluating this instruction
intcomputeSpecialProperties()
Get the static properties of this expression (other than its type).
booleancreatesNewNodes()
Determine whether this instruction creates new nodes.
protected static XPathExceptiondynamicError(SourceLocator loc, XPathException error, XPathContext context)
Construct an exception with diagnostic information.
StringevaluateAsString(XPathContext context)
Evaluate an expression as a String.
ItemevaluateItem(XPathContext context)
Evaluate an expression as a single item.
intgetImplementationMethod()
An implementation of Expression must provide at least one of the methods evaluateItem(), iterate(), or process().
InstructionInfogetInstructionInfo()
intgetInstructionNameCode()
Get the namecode of the instruction for use in diagnostics
ItemTypegetItemType(TypeHierarchy th)
Get the item type of the items returned by evaluating this instruction
SourceLocatorgetSourceLocator()
Get a SourceLocator identifying the location of this instruction
booleanisXSLT(XPathContext context)
Establish whether this is an XSLT instruction or an XQuery instruction (used to produce appropriate diagnostics)
SequenceIteratoriterate(XPathContext context)
Return an Iterator to iterate over the values of a sequence.
voidprocess(XPathContext context)
Process the instruction, without returning any tail calls
abstract TailCallprocessLeavingTail(XPathContext context)
ProcessLeavingTail: called to do the real work of this instruction.
Expressionpromote(PromotionOffer offer)
Offer promotion for this subexpression.
protected voidpromoteInst(PromotionOffer offer)
Handle promotion offers, that is, non-local tree rewrites.
abstract Expressionsimplify(StaticContext env)
Simplify an expression.

Constructor Detail

Instruction

public Instruction()
Constructor

Method Detail

appendItem

public static void appendItem(Item it, SequenceReceiver out, int locationId)
Static method to append an item that results from evaluating an expression to the current result receiver. The method checks to see whether the item returned from the expression is actually a function-call-package representing the result of a tail call optimization; if so, the tail function calls are complete and the final result is passed on.

Parameters: it the item: possibly a FunctionCallPackage out the Receiver

Throws: XPathException

assembleParams

protected static ParameterSet assembleParams(XPathContext context, WithParam[] actualParams)
Assemble a ParameterSet. Method used by instructions that have xsl:with-param children. This method is used for the non-tunnel parameters.

assembleTunnelParams

protected static ParameterSet assembleTunnelParams(XPathContext context, WithParam[] actualParams)
Assemble a ParameterSet. Method used by instructions that have xsl:with-param children. This method is used for the tunnel parameters.

computeCardinality

public int computeCardinality()
Get the cardinality of the sequence returned by evaluating this instruction

Returns: the static cardinality

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.

Returns: a set of flags indicating static properties of this expression

createsNewNodes

public boolean createsNewNodes()
Determine whether this instruction creates new nodes. This implementation returns a default value of false

dynamicError

protected static XPathException dynamicError(SourceLocator loc, XPathException error, XPathContext context)
Construct an exception with diagnostic information. Note that this method returns the exception, it does not throw it: that is up to the caller.

Parameters: error The exception containing information about the error context The controller of the transformation

Returns: an exception based on the supplied exception, but with location information added relating to this instruction

evaluateAsString

public final String evaluateAsString(XPathContext context)
Evaluate an expression as a String. This function must only be called in contexts where it is known that the expression will return a single string (or where an empty sequence is to be treated as a zero-length string). Implementations should not attempt to convert the result to a string, other than converting () to "". This method is used mainly to evaluate expressions produced by compiling an attribute value template.

Parameters: context The context in which the expression is to be evaluated

Returns: the value of the expression, evaluated in the current context. The expression must return a string or (); if the value of the expression is (), this method returns "".

Throws: net.sf.saxon.trans.XPathException if any dynamic error occurs evaluating the expression java.lang.ClassCastException if the result type of the expression is not xs:string?

evaluateItem

public Item evaluateItem(XPathContext context)
Evaluate an expression as a single item. This always returns either a single Item or null (denoting the empty sequence). No conversion is done. This method should not be used unless the static type of the expression is a subtype of "item" or "item?": that is, it should not be called if the expression may return a sequence. There is no guarantee that this condition will be detected.

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

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 prefered. For instructions this is the process() method.

getInstructionInfo

public InstructionInfo getInstructionInfo()

getInstructionNameCode

public int getInstructionNameCode()
Get the namecode of the instruction for use in diagnostics

getItemType

public ItemType getItemType(TypeHierarchy th)
Get the item type of the items returned by evaluating this instruction

Parameters: th

Returns: the static item type of the instruction

getSourceLocator

public SourceLocator getSourceLocator()
Get a SourceLocator identifying the location of this instruction

isXSLT

public boolean isXSLT(XPathContext context)
Establish whether this is an XSLT instruction or an XQuery instruction (used to produce appropriate diagnostics)

Parameters: context

Returns: true for XSLT, false for XQuery

iterate

public SequenceIterator iterate(XPathContext context)
Return an Iterator to iterate over the values of a sequence. The value of every expression can be regarded as a sequence, so this method is supported for all expressions. This default implementation handles iteration for expressions that return singleton values: for non-singleton expressions, the subclass must provide its own implementation.

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

process

public void process(XPathContext context)
Process the instruction, without returning any tail calls

Parameters: context The dynamic context, giving access to the current node, the current variables, etc.

processLeavingTail

public abstract TailCall processLeavingTail(XPathContext context)
ProcessLeavingTail: called to do the real work of this instruction. This method must be implemented in each subclass. The results of the instruction are written to the current Receiver, which can be obtained via the Controller.

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.

promote

public Expression promote(PromotionOffer offer)
Offer promotion for this subexpression. The offer will be accepted if the subexpression is not dependent on the factors (e.g. the context item) identified in the PromotionOffer. This method is always called at compile time.

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: net.sf.saxon.trans.XPathException if any error is detected

promoteInst

protected void promoteInst(PromotionOffer offer)
Handle promotion offers, that is, non-local tree rewrites.

Parameters: offer The type of rewrite being offered

Throws: XPathException

simplify

public abstract Expression simplify(StaticContext env)
Simplify an expression. This performs any static optimization (by rewriting the expression as a different expression). The default implementation does nothing.

Returns: the simplified expression

Throws: net.sf.saxon.trans.XPathException if an error is discovered during expression rewriting