net.sf.saxon.instruct

Class Choose

public class Choose extends Instruction

Compiled representation of an xsl:choose or xsl:if element in the stylesheet. Also used for typeswitch in XQuery.
Constructor Summary
Choose(Expression[] conditions, Expression[] actions)
Construct an xsl:choose instruction
Method Summary
PathMap.PathMapNodeSetaddToPathMap(PathMap pathMap, PathMap.PathMapNodeSet pathMapNodeSet)
Add a representation of this expression to a PathMap.
voidcheckForUpdatingSubexpressions()
Check to ensure that this expression does not contain any updating subexpressions.
voidcheckPermittedContents(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.
intcomputeCardinality()
Compute the cardinality of the sequence returned by evaluating this instruction
intcomputeSpecialProperties()
Get the static properties of this expression (other than its type).
Expressioncopy()
Copy an expression.
booleancreatesNewNodes()
Determine whether this instruction creates new nodes.
ItemevaluateItem(XPathContext context)
Evaluate an expression as a single item.
voidevaluatePendingUpdates(XPathContext context, PendingUpdateList pul)
Evaluate an updating expression, adding the results to a Pending Update List.
voidexplain(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
intgetImplementationMethod()
An implementation of Expression must provide at least one of the methods evaluateItem(), iterate(), or process().
intgetInstructionNameCode()
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.
ItemTypegetItemType(TypeHierarchy th)
Get the item type of the items returned by evaluating this instruction
booleanimplementsStaticTypeCheck()
Determine whether this expression implements its own method for static type checking
static booleanisSingleBranchChoice(Expression exp)
Test whether an expression is a single-branch choose, that is, an expression of the form if (condition) then exp else ()
booleanisUpdatingExpression()
Determine whether this is an updating expression as defined in the XQuery update specification
booleanisVacuousExpression()
Determine whether this is a vacuous expression as defined in the XQuery update specification
SequenceIteratoriterate(XPathContext context)
Return an Iterator to iterate over the values of a sequence.
EventIteratoriterateEvents(XPathContext context)
Deliver the result of the expression as a sequence of events.
Iterator<Expression>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 ExpressionmakeConditional(Expression condition, Expression thenExp, Expression elseExp)
Make a simple conditional expression (if (condition) then (thenExp) else (elseExp)
static ExpressionmakeConditional(Expression condition, Expression thenExp)
Make a simple conditional expression (if (condition) then (thenExp) else ()
intmarkTailFunctionCalls(StructuredQName qName, int arity)
Mark tail-recursive calls on functions.
Expressionoptimize(ExpressionVisitor visitor, ItemType contextItemType)
TailCallprocessLeavingTail(XPathContext context)
Process this instruction, that is, choose an xsl:when or xsl:otherwise child and process it.
protected voidpromoteInst(PromotionOffer offer)
Handle promotion offers, that is, non-local tree rewrites.
booleanreplaceSubExpression(Expression original, Expression replacement)
Replace one subexpression by a replacement subexpression
Expressionsimplify(ExpressionVisitor visitor)
Simplify an expression.
ExpressionstaticTypeCheck(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.
StringtoString()
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.
ExpressiontypeCheck(ExpressionVisitor visitor, ItemType contextItemType)

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

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.

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.

checkForUpdatingSubexpressions

public void checkForUpdatingSubexpressions()
Check to ensure that this expression does not contain any updating subexpressions. This check is overridden for those expressions that permit updating subexpressions.

Throws: net.sf.saxon.trans.XPathException if the expression has a non-permitted updateing subexpression

checkPermittedContents

public 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. 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.

computeCardinality

public int computeCardinality()
Compute 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

copy

public Expression copy()
Copy an expression. This makes a deep copy.

Returns: the copy of the original 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).

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

evaluatePendingUpdates

public void evaluatePendingUpdates(XPathContext context, PendingUpdateList pul)
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

Parameters: context the XPath dynamic evaluation context pul the pending update list to which the results should be written

explain

public void explain(ExpressionPresenter out)
Diagnostic print of expression structure. The abstract expression tree is written to the supplied output destination.

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

getConditions

public Expression[] getConditions()
Get the array of conditions to be tested

Returns: the array of condition expressions

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.

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.

getItemType

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

Parameters: th Type hierarchy cache

Returns: the static item type of the instruction

implementsStaticTypeCheck

public boolean implementsStaticTypeCheck()
Determine whether this expression implements its own method for static type checking

Returns: true - this expression has a non-trivial implementation of the staticTypeCheck() method

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

isUpdatingExpression

public boolean isUpdatingExpression()
Determine whether this is an updating expression as defined in the XQuery update specification

Returns: true if this is an updating expression

isVacuousExpression

public boolean isVacuousExpression()
Determine whether this is a vacuous expression as defined in the XQuery update specification

Returns: true if this expression is vacuous

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 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.

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)
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.

Parameters: context The dynamic evaluation context

Returns: the result of the expression as an iterator over a sequence of PullEvent objects

Throws: net.sf.saxon.trans.XPathException if a dynamic error occurs during expression evaluation

iterateSubExpressions

public Iterator<Expression> 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)

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

markTailFunctionCalls

public int markTailFunctionCalls(StructuredQName qName, int arity)
Mark tail-recursive calls on functions. For most expressions, this does nothing.

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.

optimize

public Expression optimize(ExpressionVisitor visitor, ItemType contextItemType)

processLeavingTail

public TailCall processLeavingTail(XPathContext context)
Process this instruction, that is, choose an xsl:when or xsl:otherwise child and process it.

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

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

replaceSubExpression

public boolean replaceSubExpression(Expression original, Expression replacement)
Replace one subexpression by a replacement subexpression

Parameters: original the original subexpression replacement the replacement subexpression

Returns: true if the original subexpression is found

simplify

public Expression simplify(ExpressionVisitor visitor)
Simplify an expression. This performs any static optimization (by rewriting the expression as a different expression).

Parameters: visitor expression visitor object

Returns: the simplified expression

Throws: XPathException if an error is discovered during expression rewriting

staticTypeCheck

public 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. 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.

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

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

Returns: a representation of the expression as a string

typeCheck

public Expression typeCheck(ExpressionVisitor visitor, ItemType contextItemType)