Package net.sf.saxon.expr.instruct

## Class Block

- java.lang.Object
- - net.sf.saxon.expr.Expression
  - - net.sf.saxon.expr.instruct.Instruction
    - - net.sf.saxon.expr.instruct.Block

- All Implemented Interfaces:
    `ExportAgent`, `TailCallReturner`, `Locatable`, `IdentityComparable`, `Traceable`

      public class Block
      extends Instruction

  An expression that delivers the concatenation of the results of its subexpressions. This may represent an XSLT sequence constructor, or an XPath/XQuery expression of the form (a,b,c,d).

- - ### Field Summary

    - ### Fields inherited from class net.sf.saxon.expr.Expression

      `EFFECTIVE_BOOLEAN_VALUE, EVALUATE_METHOD, ITEM_FEED_METHOD, ITERATE_METHOD, MAX_COST, MAX_SEQUENCE_LENGTH, MAX_STRING_LENGTH, PROCESS_METHOD, staticProperties, UNBOUNDED_LOWER, UNBOUNDED_UPPER, UPDATE_METHOD, WATCH_METHOD`

  - ### Constructor Summary

    Constructors
    Constructor | Description
    --- | ---
    `Block​(Expression[] children)` | Create a block, supplying its child expressions

  - ### Method Summary

    All Methods Static Methods Instance Methods Concrete Methods
    Modifier and Type | Method | Description
    --- | --- | ---
    `void` | `checkForUpdatingSubexpressions()` | Check to ensure that this expression does not contain any updating subexpressions.
    `void` | `checkPermittedContents​(SchemaType parentType, 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 cardinality of the expression
    `int` | `computeSpecialProperties()` | Get the static properties of this expression (other than its type).
    `Expression` | `copy​(RebindingMap rebindings)` | Copy an expression.
    `void` | `evaluatePendingUpdates​(XPathContext context, PendingUpdateList pul)` | Evaluate an updating expression, adding the results to a Pending Update List.
    `void` | `export​(ExpressionPresenter out)` | Diagnostic print of expression structure.
    `java.lang.String` | `getExpressionName()` | Get a name identifying the kind of expression, in terms meaningful to a user.
    `int` | `getImplementationMethod()` | An implementation of Expression must provide at least one of the methods evaluateItem(), iterate(), or process().
    `ItemType` | `getItemType()` | Determine the data type of the items returned by this expression
    `Operand[]` | `getOperanda()` | Get the children of this instruction
    `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.
    `java.lang.String` | `getStreamerName()` | Get the (partial) name of a class that supports streaming of this kind of expression
    `boolean` | `hasVariableBinding​(Binding binding)` | Ask whether this expression is, or contains, the binding of a given variable
    `boolean` | `implementsStaticTypeCheck()` | Determine whether this expression implements its own method for static type checking
    `boolean` | `isCandidateForSharedAppend()` | Determine whether the block is a candidate for evaluation using a "shared append expression" where the result of the evaluation is a sequence implemented as a list of subsequences
    `boolean` | `isInstruction()` | Ask whether this expression is an instruction.
    `boolean` | `isVacuousExpression()` | Determine whether this is a vacuous expression as defined in the XQuery update specification
    `SequenceIterator` | `iterate​(XPathContext context)` | Iterate over the results of all the child expressions
    `static Expression` | `makeBlock​(java.util.List<Expression> list)` | Static factory method to create a block from a list of expressions
    `static Expression` | `makeBlock​(Expression e1, Expression e2)` | Static factory method to create a block.
    `boolean` | `mayCreateNewNodes()` | Determine whether this instruction creates new nodes.
    `Expression` | `mergeAdjacentTextInstructions()` | Merge any adjacent instructions that create literal text nodes
    `static boolean` | `neverReturnsTypedNodes​(Instruction insn, TypeHierarchy th)` | Determine whether the block includes any instructions that might return nodes with a type annotation
    `java.lang.Iterable<Operand>` | `operands()` | Get the immediate sub-expressions of this expression, with information about the relationship of each expression to its parent expression.
    `Expression` | `optimize​(ExpressionVisitor visitor, ContextItemStaticInfo contextInfo)` | Perform optimisation of an expression and its subexpressions.
    `TailCall` | `processLeavingTail​(Outputter output, XPathContext context)` | ProcessLeavingTail: called to do the real work of this instruction.
    `Expression` | `simplify()` | Simplify an expression.
    `Expression` | `staticTypeCheck​(SequenceType req, boolean backwardsCompatible, RoleDiagnostic role, ExpressionVisitor visitor)` | Static type checking for let expressions is delegated to the child expressions, to allow further delegation to the branches of a conditional
    `java.lang.String` | `toShortString()` | Produce a short string identifying the expression for use in error messages
    `Expression` | `typeCheck​(ExpressionVisitor visitor, ContextItemStaticInfo contextInfo)` | Perform type checking of an expression and its subexpressions.

    - ### Methods inherited from class net.sf.saxon.expr.instruct.Instruction

      `alwaysCreatesNewNodes, assembleParams, assembleTunnelParams, dynamicError, evaluateAsString, evaluateItem, getInstructionNameCode, getNetCost, getSourceLocator, isXSLT, process, someOperandCreatesNewNodes`

    - ### Methods inherited from class net.sf.saxon.expr.Expression

      `addToPathMap, adoptChildExpression, allowExtractingCommonSubexpressions, checkedOperands, computeDependencies, computeHashCode, computeStaticProperties, dynamicError, effectiveBooleanValue, equals, explain, getCardinality, getConfiguration, getCost, getDependencies, getEvaluationMethod, getExtraProperty, getIntegerBounds, getInterpretedExpression, getIntrinsicDependencies, getLocalRetainedStaticContext, getLocation, getObjectName, getPackageData, getParentExpression, getProperties, getProperty, getRetainedStaticContext, getScopingExpression, getSlotsUsed, getSpecialProperties, getStaticBaseURI, getStaticBaseURIString, getStaticType, getTracingTag, hasCompatibleStaticContext, hashCode, hasSpecialProperty, identityHashCode, isCallOn, isEqual, isIdentical, isLiftable, isMultiThreaded, isStaticPropertiesKnown, isSubtreeExpression, isUpdatingExpression, markTailFunctionCalls, operandList, operandSparseList, optimizeChildren, prepareForStreaming, resetLocalStaticProperties, restoreParentPointers, setEvaluationMethod, setExtraProperty, setFiltered, setFlattened, setLocation, setParentExpression, setRetainedStaticContext, setRetainedStaticContextLocally, setRetainedStaticContextThoroughly, setStaticProperty, simplifyChildren, suppressValidation, toPattern, toString, typeCheckChildren, typeError, unordered, verifyParentPointers`

    - ### Methods inherited from class java.lang.Object

      `clone, finalize, getClass, notify, notifyAll, wait, wait, wait`

    - ### Methods inherited from interface net.sf.saxon.trace.Traceable

      `gatherProperties`

- - ### Constructor Detail

    - #### Block

          public Block​(Expression[] children)

      Create a block, supplying its child expressions
      Parameters:
        `children` - the child expressions in order

  - ### Method Detail

    - #### isInstruction

          public boolean isInstruction()

      Description copied from class: `Expression`
      Ask whether this expression is an instruction. In XSLT streamability analysis this is used to distinguish constructs corresponding to XSLT instructions from other constructs, typically XPath expressions (but also things like attribute constructors on a literal result element, references to attribute sets, etc. However, an XPath expression within a text value template in a text node of an XSLT stylesheet is treated as an instruction. In a non-XSLT environment (such as XQuery) this property has no meaning and its setting is undefined.
      Overrides:
        `isInstruction` in class `Instruction`
      Returns:
        true if this construct originates as an XSLT instruction

    - #### operands

          public java.lang.Iterable<Operand> operands()

      Description copied from class: `Expression`
      Get the immediate sub-expressions of this expression, with information about the relationship of each expression to its parent expression. Default implementation works off the results of iterateSubExpressions()

      If the expression is a Callable, then it is required that the order of the operands returned by this function is the same as the order of arguments supplied to the corresponding call() method.

      Specified by:
        `operands` in class `Instruction`
      Returns:
        an iterator containing the sub-expressions of this expression

    - #### hasVariableBinding

          public boolean hasVariableBinding​(Binding binding)

      Ask whether this expression is, or contains, the binding of a given variable
      Overrides:
        `hasVariableBinding` in class `Expression`
      Parameters:
        `binding` - the variable binding
      Returns:
        true if this expression is the variable binding (for example a ForExpression or LetExpression) or if it is a FLWOR expression that binds the variable in one of its clauses.

    - #### makeBlock

          public static Expression makeBlock​(Expression e1,
                                             Expression e2)

      Static factory method to create a block. If one of the arguments is already a block, the contents will be merged into a new composite block
      Parameters:
        `e1` - the first subexpression (child) of the block
        `e2` - the second subexpression (child) of the block
      Returns:
        a Block containing the two subexpressions, and if either of them is a block, it will have been collapsed to create a flattened sequence

    - #### makeBlock

          public static Expression makeBlock​(java.util.List<Expression> list)

      Static factory method to create a block from a list of expressions
      Parameters:
        `list` - the list of expressions making up this block. The members of the List must be instances of Expression. The list is effectively copied; subsequent changes to the contents of the list have no effect.
      Returns:
        a Block containing the two subexpressions, and if either of them is a block, it will have been collapsed to create a flattened sequence

    - #### getExpressionName

          public java.lang.String getExpressionName()

      Description copied from class: `Instruction`
      Get a name identifying the kind of expression, in terms meaningful to a user.
      Overrides:
        `getExpressionName` in class `Instruction`
      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.

    - #### getOperanda

          public Operand[] getOperanda()

      Get the children of this instruction
      Returns:
        the children of this instruction, as an array of Operand objects. May return a zero-length array if there are no children.

    - #### computeSpecialProperties

          public int computeSpecialProperties()

      Description copied from class: `Instruction`
      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 `Instruction`
      Returns:
        a set of flags indicating static properties of this expression

    - #### implementsStaticTypeCheck

          public boolean implementsStaticTypeCheck()

      Description copied from class: `Expression`
      Determine whether this expression implements its own method for static type checking
      Overrides:
        `implementsStaticTypeCheck` in class `Expression`
      Returns:
        true if this expression has a non-trivial implementation of the staticTypeCheck() method

    - #### staticTypeCheck

          public Expression staticTypeCheck​(SequenceType req,
                                            boolean backwardsCompatible,
                                            RoleDiagnostic role,
                                            ExpressionVisitor visitor)
                                     throws XPathException

      Static type checking for let expressions is delegated to the child expressions, 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

    - #### neverReturnsTypedNodes

          public static boolean neverReturnsTypedNodes​(Instruction insn,
                                                       TypeHierarchy th)

      Determine whether the block includes any instructions that might return nodes with a type annotation
      Parameters:
        `insn` - the instruction (for example this block)
        `th` - the type hierarchy cache
      Returns:
        true if any expression in the block can return type-annotated nodes

    - #### mergeAdjacentTextInstructions

          public Expression mergeAdjacentTextInstructions()

      Merge any adjacent instructions that create literal text nodes
      Returns:
        the expression after merging literal text instructions

    - #### copy

          public Expression copy​(RebindingMap rebindings)

      Copy an expression. This makes a deep copy.
      Specified by:
        `copy` in class `Expression`
      Parameters:
        `rebindings` - Variables that need to be re-bound
      Returns:
        the copy of the original expression

    - #### getItemType

          public final ItemType getItemType()

      Determine the data type of the items returned by this expression
      Overrides:
        `getItemType` in class `Instruction`
      Returns:
        the data type

    - #### 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 static type of the context item
      Returns:
        the static item type of the expression according to the XSLT 3.0 defined rules

    - #### computeCardinality

          public int computeCardinality()

      Determine the cardinality of the expression
      Overrides:
        `computeCardinality` in class `Instruction`
      Returns:
        the static cardinality

    - #### mayCreateNewNodes

          public final boolean mayCreateNewNodes()

      Determine whether this instruction creates new nodes. This implementation returns true if any child instruction returns true.
      Overrides:
        `mayCreateNewNodes` in class `Instruction`
      Returns:
        true if the instruction creates new nodes under some input conditions (or if it can't be proved that it doesn't)

    - #### checkForUpdatingSubexpressions

          public void checkForUpdatingSubexpressions()
                                              throws XPathException

      Check to ensure that this expression does not contain any updating subexpressions. This check is overridden for those expressions that permit updating subexpressions.
      Overrides:
        `checkForUpdatingSubexpressions` in class `Expression`
      Throws:
        `XPathException` - if the expression has a non-permitted updateing subexpression

    - #### 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

    - #### simplify

          public Expression simplify()
                              throws XPathException

      Simplify an expression. This performs any static optimization (by rewriting the expression as a different expression). The default implementation does nothing.
      Overrides:
        `simplify` in class `Expression`
      Returns:
        the simplified expression (or the original if unchanged, or if modified in-situ)
      Throws:
        `XPathException` - if an error is discovered during expression rewriting

    - #### isCandidateForSharedAppend

          public boolean isCandidateForSharedAppend()

      Determine whether the block is a candidate for evaluation using a "shared append expression" where the result of the evaluation is a sequence implemented as a list of subsequences
      Returns:
        true if the block is a candidate for "shared append" processing

    - #### typeCheck

          public Expression typeCheck​(ExpressionVisitor visitor,
                                      ContextItemStaticInfo contextInfo)
                               throws XPathException

      Description copied from class: `Expression`
      Perform type checking of an expression and its subexpressions. This is the second phase of static optimization.

      This checks statically that the operands of the expression have the correct type; if necessary it generates code to do run-time type checking or type conversion. A static type error is reported only if execution cannot possibly succeed, that is, if a run-time type error is inevitable. The call may return a modified form of the expression.

      This method is called after all references to functions and variables have been resolved to the declaration of the function or variable. However, the types of such functions and variables may not be accurately known if they have not been explicitly declared.

      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)

    - #### optimize

          public Expression optimize​(ExpressionVisitor visitor,
                                     ContextItemStaticInfo contextInfo)
                              throws XPathException

      Description copied from class: `Expression`
      Perform optimisation of an expression and its subexpressions. This is the third and final phase of static optimization.

      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
        `contextInfo` - 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)

    - #### 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

    - #### 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.

    - #### toShortString

          public java.lang.String toShortString()

      Description copied from class: `Expression`
      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

    - #### processLeavingTail

          public TailCall processLeavingTail​(Outputter output,
                                             XPathContext context)
                                      throws XPathException

      Description copied from class: `Instruction`
      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.
      Specified by:
        `processLeavingTail` in interface `TailCallReturner`
      Specified by:
        `processLeavingTail` in class `Instruction`
      Parameters:
        `output` - the destination for the result
        `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

    - #### 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. This implementation provides both iterate() and process() methods natively.
      Overrides:
        `getImplementationMethod` in class `Instruction`
      Returns:
        the implementation method, for example `Expression.ITERATE_METHOD` or `Expression.EVALUATE_METHOD` or `Expression.PROCESS_METHOD`

    - #### iterate

          public SequenceIterator iterate​(XPathContext context)
                                   throws XPathException

      Iterate over the results of all the child expressions
      Overrides:
        `iterate` in class `Instruction`
      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

    - #### 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

    - #### getStreamerName

          public java.lang.String getStreamerName()

      Description copied from class: `Expression`
      Get the (partial) name of a class that supports streaming of this kind of expression
      Overrides:
        `getStreamerName` in class `Expression`
      Returns:
        the partial name of a class that can be instantiated to provide streaming support in Saxon-EE, or null if there is no such class