net.sf.saxon.instruct
Class Choose

java.lang.Object
  net.sf.saxon.expr.Expression
      net.sf.saxon.instruct.Instruction
          net.sf.saxon.instruct.Choose

All Implemented Interfaces:

Serializable, SourceLocator, EvaluableItem, SequenceIterable, TailCallReturner, InstructionInfoProvider

public class Choose
extends Instruction

Compiled representation of an xsl:choose or xsl:if element in the stylesheet. Also used for typeswitch in XQuery.

See Also:

Serialized Form

Field Summary

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

EVALUATE_METHOD, ITERATE_METHOD, PROCESS_METHOD

Constructor Summary

Choose(Expression[] conditions, Expression[] actions)
Construct an xsl:choose instruction

Method Summary

PathMap.PathMapNodeSet	addToPathMap(PathMap pathMap, PathMap.PathMapNodeSet pathMapNodeSet) Add a representation of this expression to a PathMap.
void	checkPermittedContents(SchemaType parentType, StaticContext env, 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() Compute the cardinality of the sequence returned by evaluating this instruction
int	computeSpecialProperties() Get the static properties of this expression (other than its type).
Expression	copy() Copy an expression.
boolean	createsNewNodes() Determine whether this instruction creates new nodes.
Item	evaluateItem(XPathContext context) Evaluate an expression as a single item.
void	explain(ExpressionPresenter out) Diagnostic print of expression structure.
Expression[]	getActions() Get the array of actions to be performed
Expression[]	getConditions() Get the array of conditions to be tested
int	getImplementationMethod() An implementation of Expression must provide at least one of the methods evaluateItem(), iterate(), or process().
int	getInstructionNameCode() Get the name of this instruction for diagnostic and tracing purposes We assume that if there was only one condition then it was an xsl:if; this is not necessarily so, but it's adequate for tracing purposes.
ItemType	getItemType(TypeHierarchy th) Get the item type of the items returned by evaluating this instruction
boolean	implementsStaticTypeCheck() Determine whether this expression implements its own method for static type checking
static boolean	isSingleBranchChoice(Expression exp) Test whether an expression is a single-branch choose, that is, an expression of the form if (condition) then exp else ()
SequenceIterator	iterate(XPathContext context) Return an Iterator to iterate over the values of a sequence.
EventIterator	iterateEvents(XPathContext context) Deliver the result of the expression as a sequence of events.
Iterator	iterateSubExpressions() Get all the XPath expressions associated with this instruction (in XSLT terms, the expression present on attributes of the instruction, as distinct from the child instructions in a sequence construction)
static Expression	makeConditional(Expression condition, Expression thenExp) Make a simple conditional expression (if (condition) then (thenExp) else ()
static Expression	makeConditional(Expression condition, Expression thenExp, Expression elseExp) Make a simple conditional expression (if (condition) then (thenExp) else (elseExp)
int	markTailFunctionCalls(StructuredQName qName, int arity) Mark tail-recursive calls on functions.
Expression	optimize(ExpressionVisitor visitor, ItemType contextItemType) Perform optimisation of an expression and its subexpressions.
TailCall	processLeavingTail(XPathContext context) Process this instruction, that is, choose an xsl:when or xsl:otherwise child and process it.
boolean	replaceSubExpression(Expression original, Expression replacement) Replace one subexpression by a replacement subexpression
Expression	simplify(ExpressionVisitor visitor) Simplify an expression.
Expression	staticTypeCheck(SequenceType req, boolean backwardsCompatible, RoleLocator role, ExpressionVisitor visitor) Static type checking for conditional expressions is delegated to the expression itself, and is performed separately on each branch of the conditional, so that dynamic checks are added only on those branches where the check is actually required.
Expression	typeCheck(ExpressionVisitor visitor, ItemType contextItemType) Perform type checking of an expression and its subexpressions.

Methods inherited from class net.sf.saxon.instruct.Instruction

evaluateAsString, getInstructionInfo, getSourceLocator, isXSLT, process, promote

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

adoptChildExpression, computeDependencies, computeStaticProperties, display, doPromotion, effectiveBooleanValue, explain, findParentOf, getCardinality, getColumnNumber, getContainer, getDependencies, getExecutable, getHostLanguage, getIntrinsicDependencies, getLineNumber, getLocationId, getLocationProvider, getPublicId, getSlotsUsed, getSpecialProperties, getSystemId, hasLoopingSubexpression, setContainer, setFiltered, setFlattened, setLocationId, suppressValidation, toString

Methods inherited from class java.lang.Object

equals, getClass, hashCode, notify, notifyAll, wait, wait, wait

Methods inherited from interface javax.xml.transform.SourceLocator

getColumnNumber, getLineNumber, getPublicId, getSystemId

Constructor Detail

Choose

public Choose(Expression[] conditions,
              Expression[] actions)

Construct an xsl:choose instruction

Parameters:

conditions - the conditions to be tested, in order

actions - the actions to be taken when the corresponding condition is true

Method Detail

makeConditional

public static Expression makeConditional(Expression condition,
                                         Expression thenExp,
                                         Expression elseExp)

Make a simple conditional expression (if (condition) then (thenExp) else (elseExp)

Parameters:

condition - the condition to be tested

thenExp - the expression to be evaluated if the condition is true

elseExp - the expression to be evaluated if the condition is false

Returns:

the expression

makeConditional

public static Expression makeConditional(Expression condition,
                                         Expression thenExp)

Make a simple conditional expression (if (condition) then (thenExp) else ()

Parameters:

condition - the condition to be tested

thenExp - the expression to be evaluated if the condition is true

Returns:

the expression

isSingleBranchChoice

public static boolean isSingleBranchChoice(Expression exp)

Test whether an expression is a single-branch choose, that is, an expression of the form if (condition) then exp else ()

Parameters:

exp - the expression to be tested

Returns:

true if the expression is a choose expression and there is only one condition, so that the expression returns () if this condition is false

getConditions

public Expression[] getConditions()

Get the array of conditions to be tested

Returns:

the array of condition expressions

getActions

public Expression[] getActions()

Get the array of actions to be performed

Returns:

the array of expressions to be evaluated when the corresponding condition is true

getInstructionNameCode

public int getInstructionNameCode()

Get the name of this instruction for diagnostic and tracing purposes We assume that if there was only one condition then it was an xsl:if; this is not necessarily so, but it's adequate for tracing purposes.

Overrides:

getInstructionNameCode in class Instruction

Returns:

a code identifying the instruction: typically but not always the fingerprint of a name in the XSLT namespace

simplify

public Expression simplify(ExpressionVisitor visitor)
                    throws XPathException

Simplify an expression. This performs any static optimization (by rewriting the expression as a different expression).

Specified by:

simplify in class Instruction

Parameters:

visitor - expression visitor object

Returns:

the simplified expression

Throws:

XPathException - if an error is discovered during expression rewriting

typeCheck

public Expression typeCheck(ExpressionVisitor visitor,
                            ItemType contextItemType)
                     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.

If the implementation returns a value other than "this", then it is required to ensure that the parent pointer and location information in the returned expression have been set up correctly. It should not rely on the caller to do this, although for historical reasons many callers do so.

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,
                                  ExpressionVisitor visitor)
                           throws XPathException

Static type checking for conditional expressions is delegated to the expression itself, and is performed separately on each branch of the conditional, so that dynamic checks are added only on those branches where the check is actually required. This also results in a static type error if any branch is incapable of delivering a value of the required type. One reason for this approach is to avoid doing dynamic type checking on a recursive function call as this prevents tail-call optimization being used.

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,
                           ItemType contextItemType)
                    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

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)

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

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.

Overrides:

getImplementationMethod in class Instruction

Returns:

the implementation method, for example Expression.ITERATE_METHOD or Expression.EVALUATE_METHOD or Expression.PROCESS_METHOD

markTailFunctionCalls

public int markTailFunctionCalls(StructuredQName qName,
                                 int arity)

Mark tail-recursive calls on functions. For most expressions, this does nothing.

Overrides:

markTailFunctionCalls in class Expression

Parameters:

qName - the name of the function

arity - the arity (number of parameters) of the function

Returns:

0 if no tail call was found; 1 if a tail call on a different function was found; 2 if a tail recursive call was found and if this call accounts for the whole of the value.

getItemType

public ItemType getItemType(TypeHierarchy th)

Get the item type of the items returned by evaluating this instruction

Overrides:

getItemType in class Instruction

Parameters:

th - Type hierarchy cache

Returns:

the static item type of the instruction

computeCardinality

public int computeCardinality()

Compute the cardinality of the sequence returned by evaluating this instruction

Overrides:

computeCardinality in class 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.

Overrides:

computeSpecialProperties in class Instruction

Returns:

a set of flags indicating static properties of this expression

createsNewNodes

public final boolean createsNewNodes()

Determine whether this instruction creates new nodes. This implementation returns true if any of the "actions" creates new nodes. (Nodes created by the conditions can't contribute to the result).

Overrides:

createsNewNodes in class Instruction

Returns:

true if the instruction creates new nodes (or if it can't be proved that it doesn't)

iterateSubExpressions

public Iterator iterateSubExpressions()

Get all the XPath expressions associated with this instruction (in XSLT terms, the expression present on attributes of the instruction, as distinct from the child instructions in a sequence construction)

Overrides:

iterateSubExpressions in class Expression

Returns:

an iterator containing the sub-expressions of this expression

replaceSubExpression

public boolean replaceSubExpression(Expression original,
                                    Expression replacement)

Replace one subexpression by a replacement subexpression

Overrides:

replaceSubExpression in class Expression

Parameters:

original - the original subexpression

replacement - the replacement subexpression

Returns:

true if the original subexpression is found

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

addToPathMap

public PathMap.PathMapNodeSet addToPathMap(PathMap pathMap,
                                           PathMap.PathMapNodeSet pathMapNodeSet)

Add a representation of this expression to a PathMap. The PathMap captures a map of the nodes visited by an expression in a source tree.

The default implementation of this method assumes that an expression does no navigation other than the navigation done by evaluating its subexpressions, and that the subexpressions are evaluated in the same context as the containing expression. The method must be overridden for any expression where these assumptions do not hold. For example, implementations exist for AxisExpression, ParentExpression, and RootExpression (because they perform navigation), and for the doc(), document(), and collection() functions because they create a new navigation root. Implementations also exist for PathExpression and FilterExpression because they have subexpressions that are evaluated in a different context from the calling expression.

Overrides:

addToPathMap in class Expression

Parameters:

pathMap - the PathMap to which the expression should be added

pathMapNodeSet - the set of PathMap nodes to which the paths from this expression should be appended

Returns:

the pathMapNode representing the focus established by this expression, in the case where this expression is the first operand of a path expression or filter expression. For an expression that does navigation, it represents the end of the arc in the path map that describes the navigation route. For other expressions, it is the same as the input pathMapNode.

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

processLeavingTail

public TailCall processLeavingTail(XPathContext context)
                            throws XPathException

Process this instruction, that is, choose an xsl:when or xsl:otherwise child and process it.

Specified by:

processLeavingTail in interface TailCallReturner

Specified by:

processLeavingTail in class Instruction

Parameters:

context - the dynamic context of this transformation

Returns:

a TailCall, if the chosen branch ends with a call of call-template or apply-templates. It is the caller's responsibility to execute such a TailCall. If there is no TailCall, returns null.

Throws:

XPathException - if any non-recoverable dynamic error occurs

evaluateItem

public Item evaluateItem(XPathContext context)
                  throws XPathException

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.

Specified by:

evaluateItem in interface EvaluableItem

Overrides:

evaluateItem in class Instruction

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

iterate

public SequenceIterator iterate(XPathContext context)
                         throws XPathException

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 relies on the process() method: it "pushes" the results of the instruction to a sequence in memory, and then iterates over this in-memory sequence. In principle instructions should implement a pipelined iterate() method that avoids the overhead of intermediate storage.

Specified by:

iterate in interface SequenceIterable

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

iterateEvents

public EventIterator iterateEvents(XPathContext context)
                            throws XPathException

Deliver the result of the expression as a sequence of events.

The events (of class PullEvent) are either complete items, or one of startElement, endElement, startDocument, or endDocument, known as semi-nodes. The stream of events may also include a nested EventIterator. If a start-end pair exists in the sequence, then the events between this pair represent the content of the document or element. The content sequence will have been processed to the extent that any attribute and namespace nodes in the content sequence will have been merged into the startElement event. Namespace fixup will have been performed: that is, unique prefixes will have been allocated to element and attribute nodes, and all namespaces will be declared by means of a namespace node in the startElement event or in an outer startElement forming part of the sequence. However, duplicate namespaces may appear in the sequence.

The content of an element or document may include adjacent or zero-length text nodes, atomic values, and nodes represented as nodes rather than broken down into 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