# Package net.sf.saxon.expr.instruct

This package provides classes for the compiled representation of the various elements and other instructions found in an XSLT stylesheet. The same constructs are also used for evaluating similar constructs in XQuery, in particular, expressions that construct new nodes.

Instances of these classes are constructed when the stylesheet or query is compiled. In the case of XSLT, the objects representing the compile-time stylesheet (in package `net.sf.saxon.style`) can then be discarded and garbage-collected.

The most important class is `Instruction`, which represents an XSLT Instruction. In most cases these instructions have a one-to-one relationship with instructions in the original source XSLT stylesheet, and the names of the subclasses (for example ApplyImports, ApplyTemplates, Choose) reflect this.

In XSLT 1.0, XSLT instructions and XPath expressions were quite distinct, and were evaluated in different ways: XSLT instructions in "push" mode (they were described as "writing to the result tree"), and XPath expressions in "pull" mode (reading from the source tree). This distinction is no longer present in the XSLT 2.0 processing model, and the boundary between the `Instruction` and `Expression` classes is therefore a rather fuzzy one. Both instructions and expressions can now be evaluated in either push or pull mode. Flow-of-control constructs such as conditional expressions and FOR expressions are evaluated in either mode depending on their parent expression.

- Interface Summary
  Interface | Description
  --- | ---
  ConditionalInstruction | This is a marker interface for Choose and Switch instructions
  Debugger | This interface may be implemented by an external debugging tool
  InstructionWithComplexContent | Interface implemented by instructions that contain a sequence constructor evaluated using the rules for "constructing complex content": for example, xsl:element, xsl:copy, xsl:document, xsl:result-document
  ITemplateCall | An interface satisfied by all instructions that invoke templates: apply-templates, call-template.
  TailCall | Interface representing a Tail Call.
  TailCallReturner | This interface represents an expression that is capable of being processed leaving tail calls for the calling instruction to deal with.
  ValidatingInstruction | Interface implemented by instructions that have validation and type attributes

- Class Summary
  Class | Description
  --- | ---
  Actor | This object represents the compiled form of a user-written function, template, attribute-set, etc (the source can be either an XSLT stylesheet function or an XQuery function).
  AnalyzeString | An xsl:analyze-string element in the stylesheet.
  ApplyImports | An xsl:apply-imports element in the stylesheet.
  ApplyNextMatchingTemplate | An xsl:apply-imports or xsl:next-match element in the stylesheet.
  ApplyTemplates | An instruction representing an xsl:apply-templates element in the stylesheet
  ApplyTemplates.ApplyTemplatesPackage | An ApplyTemplatesPackage is an object that encapsulates the sequence of nodes to be processed, the mode, the parameters to be supplied, and the execution context.
  AttributeCreator | Abstract class for fixed and computed attribute constructor expressions
  AttributeSet | The compiled form of an xsl:attribute-set element in the stylesheet.
  Bindery | The Bindery class holds information about variables and their values.
  Bindery.FailureValue | A value that can be saved in a global variable to indicate that evaluation failed, and that subsequent attempts at evaluation should also fail
  Block | An expression that delivers the concatenation of the results of its subexpressions.
  BlockIterator | Iterate over the instructions in a sequence of instructions (or an XPath comma expression), concatenating the result of each instruction into a single combined sequence.
  BreakInstr | A compiled xsl:break instruction.
  CallTemplate | Instruction representing an xsl:call-template element in the stylesheet.
  CallTemplate.CallTemplatePackage | A CallTemplatePackage is an object that encapsulates the name of a template to be called, the parameters to be supplied, and the execution context.
  Choose | Compiled representation of an xsl:choose or xsl:if element in the stylesheet.
  Comment | An instruction representing an xsl:comment element in the stylesheet.
  ComponentTracer | A wrapper expression used to trace execution of components such as functions, templates, and global variables in XSLT and XQuery.
  ComputedAttribute | An instruction derived from an xsl:attribute element in stylesheet, or from an attribute constructor in XQuery, in cases where the attribute name is not known statically
  ComputedElement | An instruction representing an xsl:element element in an XSLT stylesheet, or a computed element constructor in XQuery.
  ConditionalBlock | An XSLT 3.0 sequence constructor containing xsl:on-empty and/or xsl:on-non-empty instructions
  Copy | Handler for xsl:copy elements in stylesheet.
  CopyOf | An xsl:copy-of element in the stylesheet.
  Doctype | A saxon:doctype element in the stylesheet.
  DocumentInstr | An instruction to create a document node.
  DummyNamespaceResolver | A dummy namespace resolver used when validating QName-valued attributes written to the result tree.
  ElementCreator | An instruction that creates an element node.
  EvaluateInstr | An EvaluateInstr is the compiled form of an xsl:evaluate instruction
  Executable | A compiled stylesheet or a query in executable form.
  FixedAttribute | An instruction derived from an xsl:attribute element in stylesheet, or from an attribute constructor in XQuery.
  FixedElement | An instruction that creates an element node whose name is known statically.
  ForEach | Handler for xsl:for-each elements in a stylesheet.
  ForEachGroup | Handler for xsl:for-each-group elements in stylesheet.
  Fork | This class implements an xsl:fork expression.
  GlobalContextRequirement | Information about the requirements placed by a query or stylesheet on the global context item: whether it is mandatory or optional, what its type must be, and whether it has a default value.
  GlobalParam | The compiled form of a global xsl:param element in an XSLT stylesheet or an external variable declared in the prolog of a Query.
  GlobalParameterSet | A GlobalParameterSet is a set of parameters supplied when invoking a stylesheet or query.
  GlobalVariable | A compiled global variable in a stylesheet or query.
  Instruction | Abstract superclass for all instructions in the compiled stylesheet.
  IterateInstr | An IterateInstr is the compiled form of an xsl:iterate instruction
  LocalParam | The compiled form of an xsl:param element within a template in an XSLT stylesheet.
  LocalParamBlock | Represents the set of xsl:param elements at the start of an xsl:iterate instruction
  MemoFunction | A user-defined function that is declared as a memo function, meaning that it remembers results of previous calls.
  Message | An xsl:message element in the stylesheet.
  NamedTemplate | The runtime object corresponding to a named xsl:template element in the stylesheet.
  NamedTemplate.LocalParamInfo |
  NamespaceConstructor | A namespace constructor instruction.
  NextIteration | Implements a xsl:next-iteration instruction within the body of xsl:iterate
  NextMatch | An xsl:next-match element in the stylesheet
  NumberInstruction | This instruction performs the node-counting function of the xsl:number instruction.
  OnEmptyExpr | A compiled xsl:on-empty instruction.
  OnNonEmptyExpr | A compiled xsl:on-non-empty instruction.
  OriginalFunction | This class represents a function invoked using xsl:original from within an xs:override element.
  ParameterSet | A ParameterSet is a set of parameters supplied when calling a template.
  ParentNodeConstructor | An abstract class to act as a common parent for instructions that create element nodes and document nodes.
  ProcessingInstruction | An xsl:processing-instruction element in the stylesheet, or a processing-instruction constructor in a query
  ResultDocument | The compiled form of an xsl:result-document element in the stylesheet.
  SequenceInstr | A compiled xsl:sequence instruction.
  SimpleNodeConstructor | Common superclass for XSLT instructions whose content template produces a text value: xsl:attribute, xsl:comment, xsl:processing-instruction, xsl:namespace, and xsl:text, and their XQuery equivalents
  SlotManager | A SlotManager supports functions, templates, etc: specifically, any executable code that requires a stack frame containing local variables.
  SourceDocument | Non-streamable implementation of the xsl:source-document instruction
  TemplateRule | The runtime object corresponding to an xsl:template element in the stylesheet.
  TraceExpression | A wrapper expression used to trace expressions in XSLT and XQuery.
  UseAttributeSet | This instruction corresponds to a single QName appearing within a use-attribute-sets attribute on a literal result element, xsl:element, or xsl:copy instruction.
  UserFunction | This object represents the compiled form of a user-written function (the source can be either an XSLT stylesheet function or an XQuery function).
  UserFunctionParameter | Run-time object representing a formal argument to a user-defined function
  ValueOf | An xsl:value-of element in the stylesheet.
  WherePopulated | A compiled xsl:where-populated instruction (formerly xsl:conditional-content).
  WithParam | An object derived from a xsl:with-param element in the stylesheet.

- Enum Summary
  Enum | Description
  --- | ---
  UserFunction.Determinism |

- Exception Summary
  Exception | Description
  --- | ---
  TerminationException | An exception thrown by xsl:message terminate="yes".