XML Path Language (XPath) 2.0

2.1.1 Static Context

The static context of an expression is the information that is available during static analysis of the expression, prior to its evaluation. This information can be used to decide whether the expression contains a static error.

Any component of the static context that is not assigned a default value in the XPath specification, and is not assigned a value by the host language, may be assigned an implementation-defined initial value. If processing of an expression relies on some component of the static context that has not been assigned a value, a static error is raised.

Static context consists of the following components:

• XPath 1.0 compatibility mode. This value is true if rules for backward compatibility with XPath Version 1.0 are in effect; otherwise it is false.

• In-scope namespaces. This is a set of (prefix, URI) pairs. The in-scope namespaces are used for resolving prefixes used in QNames within the expression.

• Default element namespace. This is a namespace URI. This namespace is used for any unprefixed QName appearing in a position where an element or type name is expected.

• Default function namespace. This is a namespace URI. This namespace URI is used for any unprefixed QName appearing as the function name in a function call.

• In-scope schema definitions. This is a generic term for all the element, attribute, and type definitions that are in scope during processing of an expression. It includes the following three parts:

  • In-scope type definitions. The in-scope type definitions always include the built-in types of [XML Schema] and the predefined types in the namespace http://www.w3.org/2003/05/xpath-datatypes. Additional type definitions may be added to the in-scope type definitions by the host language.

    XML Schema distinguishes named types, which are given a QName by the schema designer, must be declared at the top level of a schema, and are uniquely identified by their QName, from anonymous types, which are not given a name by the schema designer, must be local, and are identified in an implementation-dependent way. Both named types and anonymous types can be present in the in-scope type definitions.

  • In-scope element declarations. Each element declaration is identified either by a QName (for a top-level element) or by an implementation-defined element identifier (for a local element). Element declarations may be provided by the language environment.An element declaration includes information about the substitution groups to which this element belongs.

  • In-scope attribute declarations. Each attribute declaration is identified either by a QName (for a top-level attribute) or by an implementation-defined attribute identifier (for a local attribute). Attribute declarations may be provided by the language environment.

• In-scope variables. This is a set of (QName, type) pairs. It defines the set of variables that have been declared and are available for reference within the expression. The QName represents the name of the variable, and the type represents its static data type.

  The static types of in-scope variables may be derived from static analysis of the expressions in which the variables are bound, or provided by the external environment.

• In-scope functions. This part of the static context defines the set of functions that are available to be called from within an expression. Each function is uniquely identified by its QName and its arity (number of parameters). The static context maps the QName and arity into a function signature and a function definition. The function signature specifies the static types of the function parameters and the function result.

  For each atomic type in the in-scope type definitions, there is a constructor function in the in-scope functions. Constructor functions are discussed in 3.10.4 Constructor Functions.

• In-scope collations. This is a set of (URI, collation) pairs. It defines the names of the collations that are available for use in function calls that take a collation name as an argument. A collation may be regarded as an object that supports two functions: a function that given a set of strings, returns a sequence containing those strings in sorted order; and a function that given two strings, returns true if they are considered equal, and false if not.

• Default collation. This is a collation. This collation is used by string comparison functions when no explicit collation is specified.

• Base URI. This is an absolute URI, used when necessary in the resolution of relative URIs (for example, by the fn:resolve-uri function.) The base URI is always provided by the external environment.

2.1.2 Evaluation Context

The evaluation context of an expression is defined as information that is available at the time the expression is evaluated.

Any component of the evaluation context that is not assigned a default value in the XPath specification, and is not assigned a value by the host language, may be assigned an implementation-defined initial value. If processing of an expression relies on some component of the evaluation context that has not been assigned a value, a dynamic error is raised.

The evaluation context consists of all the components of the static context, and the additional components listed below.

The first three components of the evaluation context (context item, context position, and context size) are called the focus of the expression. The focus enables the processor to keep track of which nodes are being processed by the expression.

The focus for the outermost expression may supplied by the environment in which the expression is evaluated--otherwise, the focus for the outermost expression is undefined. Any reference to a component of an undefined focus raises an error. Certain language constructs, notably the path expression E1/E2 and the predicate expression E1[E2], create a new focus for the evaluation of a sub-expression. In these constructs, E2 is evaluated once for each item in the sequence that results from evaluating E1. Each time E2 is evaluated, it is evaluated with a different focus. The focus for evaluating E2 is referred to below as the inner focus, while the focus for evaluating E1 is referred to as the outer focus. The inner focus exists only while E2 is being evaluated. When this evaluation is complete, evaluation of the containing expression continues with its original focus unchanged.

• The context item is the item currently being processed. An item is either an atomic value or a node. When the context item is a node, it can also be referred to as the context node. The context item is returned by the expression ".". When an expression E1/E2 or E1[E2] is evaluated, each item in the sequence obtained by evaluating E1 becomes the context item in the inner focus for an evaluation of E2.

• The context position is the position of the context item within the sequence of items currently being processed. It changes whenever the context item changes. Its value is always an integer greater than zero. The context position is returned by the expression fn:position(). When an expression E1/E2 or E1[E2] is evaluated, the context position in the inner focus for an evaluation of E2 is the position of the context item in the sequence obtained by evaluating E1. The position of the first item in a sequence is always 1 (one). The context position is always less than or equal to the context size.

• The context size is the number of items in the sequence of items currently being processed. Its value is always an integer greater than zero. The context size is returned by the expression last(). When an expression E1/E2 or E1[E2] is evaluated, the context size in the inner focus for an evaluation of E2 is the number of items in the sequence obtained by evaluating E1.

• Dynamic variables. This is a set of (QName, value) pairs. It contains the same QNames as the in-scope variables in the static context for the expression. Each variable name is associated with a typed value. The dynamic type associated with the value of a variable may be more specific than the static type associated with the same variable. The value of a variable is, in general, a sequence.

  The typed value of a variable may be set by execution of an expression that binds a value to the variable, or by the external environment.

• Current date and time. This information represents an implementation-dependent point in time during processing of a query or transformation. It can be retrieved by the fn:current-date, fn:current-time, and fn:current-dateTime functions. If invoked multiple times during the execution of a query or transformation, these functions always returns the same result.

• Implicit timezone. This is the timezone to be used when a date, time, or dateTime value that does not have a timezone is used in a comparison or in any other operation. This value is an instance of xdt:dayTimeDuration that is implementation-defined. See [ISO 8601] for the range of legal values of a timezone.

• Input sequence. An input sequence is a sequence of nodes that can be accessed by the input function. It might be thought of as an "implicit input". The content of the input sequence is determined by the host language.

2.3.1 Document Order

Document order defines a total ordering among all the nodes seen by the language processor. Informally, document order corresponds to a pre-order, depth-first, left-to-right traversal of the nodes in the Data Model.

Within a given document, the document node is the first node, followed by element nodes, text nodes, comment nodes, and processing instruction nodes in the order of their representation in the XML form of the document (after expansion of entities). Element nodes occur before their children, and the children of an element node occur before its following siblings. The namespace nodes of an element immediately follow the element node, in implementation-defined order. The attribute nodes of an element immediately follow its namespace nodes, and are also in implementation-defined order.

The relative order of nodes in distinct documents is implementation-defined but stable within a given query or transformation. Given two distinct documents A and B, if a node in document A is before a node in document B, then every node in document A is before every node in document B. The relative order among free-floating nodes (those not in a document) is also implementation-defined but stable.

2.3.2 Typed Value and String Value

Nodes have a typed value and a string value that can be extracted by calling the fn:data function and the fn:string function, respectively. The typed value of a node is a sequence of atomic values, and the string value of a node is a string. Element and attribute nodes also have a type annotation, which is a type identifier that is found in the in-scope type definitions. The type annotation represents the dynamic (run-time) type of the node. XPath does not provide a way to directly access the type annotation of an element or attribute node.

The typed value and string value for each kind of node are defined by the dm:typed-value and dm:string-value accessors in [XQuery 1.0 and XPath 2.0 Data Model]. The relationship between the typed value and the string value for various kinds of nodes is described and illustrated by examples below.

1. For text, document, comment, processing instruction, and namespace nodes, the typed value of the node is the same as its string value, as an instance of xdt:untypedAtomic. (The string value of a document node is formed by concatenating the string values of all its descendant text nodes, in document order.)

2. The typed value of an attribute node with the type annotation xdt:untypedAtomic is the same as its string value, as an instance of xdt:untypedAtomic. The typed value of an attribute node with any other type annotation is derived from its string value and type annotation in a way that is consistent with schema validation.

   Example: A1 is an attribute having string value "3.14E-2" and type annotation xs:double. The typed value of A1 is the xs:double value whose lexical representation is 3.14E-2.

   Example: A2 is an attribute with type annotation IDREFS, which is a list type derived from IDREF. Its string value is "bar baz faz". The typed value of A2 is a sequence of three atomic values ("bar", "baz", "faz"), each of type IDREF. The typed value of a node is never treated as an instance of a named list type. Instead, if the type annotation of a node is a list type (such as IDREFS), its typed value is treated as a sequence of the underlying base type (such as IDREF).

3. For an element node, the relationship between typed value and string value depends on the node's type annotation, as follows:

   1. If the type annotation is xs:anyType, or denotes a complex type with mixed content, then the typed value of the node is equal to its string value, as an instance of xdt:untypedAtomic.

      Example: E1 is an element node having type annotation xs:anyType and string value "1999-05-31". The typed value of E1 is "1999-05-31", as an instance of xdt:untypedAtomic.

      Example: E2 is an element node with the type annotation formula, which is a complex type with mixed content. The content of E2 consists of the character "H", a child element named subscript with string value "2", and the character "O". The typed value of E2 is "H2O" as an instance of xdt:untypedAtomic.

   2. If the type annotation denotes a simple type or a complex type with simple content, then the typed value of the node is derived from its string value and its type annotation in a way that is consistent with schema validation.

      Example: E3 is an element node with the type annotation cost, which is a complex type that has several attributes and a simple content type of xs:decimal. The string value of E3 is "74.95". The typed value of E3 is 74.95, as an instance of xs:decimal.

      Example: E4 is an element node with the type annotation hatsizelist, which is a simple type derived by list from the type hatsize, which in turn is derived from xs:integer. The string value of E4 is "7 8 9". The typed value of E4 is a sequence of three values (7, 8, 9), each of type hatsize.

   3. If the type annotation denotes a complex type with empty content, then the typed value of the node is the empty sequence.

   4. If the type annotation denotes a complex type with non-mixed complex content, then the typed value of the node is undefined. The fn:data function raises an error when applied to such a node.

      Example: E5 is an element node with the type annotation weather, which is a complex type whose content type specifies elementOnly. E5 has two child elements named temperature and precipitation. The typed value of E5 is undefined, and the fn:data function applied to E5 raises an error.

2.4.1 Predefined Types

All the built-in types of [XML Schema] are recognized by XPath. These built-in types are in the namespace http://www.w3.org/2001/XMLSchema, which is represented in this document by the prefix xs. Some examples of built-in schema types include xs:integer, xs:string, and xs:date.

In addition, XPath recognizes the predefined types listed below. All these predefined types are in the namespace http://www.w3.org/2003/05/xpath-datatypes, which is represented in this document by the prefix xdt.

1. xdt:anyAtomicType is an abstract type that includes all atomic values (and no values that are not atomic). It is a subtype of xs:anySimpleType, which is the base type for all simple types, including atomic, list, and union types. All specific atomic types such as xs:integer, xs:string, and xdt:untypedAtomic, are subtypes of xdt:anyAtomicType.

2. xdt:untypedAtomic is a specific atomic type used for untyped data, such as text that is not given a specific type by schema validation. It has no subtypes.

3. xdt:dayTimeDuration is a subtype of xs:duration whose lexical representation contains only day, hour, minute, and second components.

4. xdt:yearMonthDuration is a subtype of xs:duration whose lexical representation is restricted to contain only year and month components.

For more details about predefined types, see [XQuery 1.0 and XPath 2.0 Functions and Operators].

Additional type definitions may be added by the host language environment via the in-scope type definitions.

2.4.2 Type Checking

XPath defines two phases of processing called the analysis phase and the evaluation phase.

The analysis phase depends on the expression itself and on the static context. The analysis phase does not depend on any input data. The purpose of type-checking during the analysis phase is to provide early detection of type errors and to compute the type of a result.

During the analysis phase, each expression is assigned a static type. In some cases, the static type is derived from the lexical form of the expression; for example, the static type of the literal 5 is xs:integer. In other cases, the static type of an expression is inferred according to rules based on the static types of its operands; for example, the static type of the expression 5 + 1.2 is xs:decimal. The static type of an expression may be either a named type or a structural description--for example, xs:boolean? denotes an optional occurrence of the xs:boolean type. The rules for inferring the static types of various expressions are described in [XQuery 1.0 Formal Semantics]. During the analysis phase, if static type checking is in effect and an operand of an expression is found to have a static type that is not appropriate for that operand, a type error is raised. If static type checking raises no errors and assigns a static type T to an expression, then execution of the expression on valid input data is guaranteed either to produce a value of type T or to raise a dynamic error.

The evaluation phase is performed only after successful completion of the analysis phase. The evaluation phase depends on input data, on the expression being evaluated, and on the evaluation context. During the evaluation phase, a dynamic type is associated with each value as it is computed. The dynamic type of a value may be either a structural type (such as "sequence of integers") or a named type. The dynamic type of a value may be more specific than the static type of the expression that computed it (for example, the static type of an expression might be "zero or more integers or strings," but at run time its value may have the dynamic type "integer.") If an operand of an expression is found to have a dynamic type that is incompatible with the expected type for that operand, a type error is raised.

Even though static typing can catch many type errors before an expression is executed, it is possible for an expression to raise an error during evaluation that was not detected by static analysis. For example, an expression may contain a cast of a string into an integer, which is statically valid. However, if the actual value of the string at run time cannot be cast into an integer, a dynamic error will result. Similarly, an expression may apply an arithmetic operator to a value whose static type is xs:anySimpleType. This is not a static error, but at run time, if the value cannot be successfully cast to a numeric type, a dynamic error will be raised.

It is also possible for static analysis of an expression to raise a type error, even though execution of the expression on certain inputs would be successful. For example, an expression might contain a function that requires an element as its parameter, and the analysis phase might infer the static type of the function parameter to be an optional element. This case would be treated as a static type error, even though the function call would be successful for input data in which the optional element is present.

2.4.3 SequenceType

When it is necessary to refer to a type in an XPath expression, the syntax shown below is used. This syntax production is called "SequenceType", since it describes the type of an XPath value, which is a sequence.

SequenceType

QNames appearing in a SequenceType have their prefixes expanded to namespace URIs by means of the in-scope namespaces and the default element namespace. It is a static error to use a name in a SequenceType if that name is not found in the appropriate part of the in-scope schema definitions. If the name is used as an element name, it must appear in the in-scope element declarations; if it is used as an attribute name, it must appear in the in-scope attribute declarations; and if it is used as a type name, it must appear in the in-scope type definitions.

Here are some examples of SequenceTypes that might be used in XPath expressions:

• xs:date refers to the built-in Schema type date

• attribute()? refers to an optional attribute

• element() refers to any element

• element(po:shipto, po:address) refers to an element that has the name po:shipto (or is in the substitution group of that element), and has the type annotation po:address (or a subtype of that type)

• element(po:shipto, *) refers to an element named po:shipto (or in the substitution group of po:shipto), with no restrictions on its type

• element(*, po:address) refers to an element of any name that has the type annotation po:address (or a subtype of po:address). If the keyword nillable were used following po:address, that would indicate that the element may have empty content and the attribute xsi:nil="true", even though the declaration of the type po:address has required content.

• node()* refers to a sequence of zero or more nodes of any type

• item()+ refers to a sequence of one or more nodes or atomic values

2.4.4 Type Conversions

Some expressions do not require their operands to exactly match the expected type. For example, function parameters and returns expect a value of a particular type, but automatically perform certain type conversions, such as extraction of atomic values from nodes, promotion of numeric values, and implicit casting of untyped values. The conversion rules for function parameters and returns are discussed in 3.1.4 Function Calls. Other operators that provide special conversion rules include arithmetic operators, which are discussed in 3.4 Arithmetic Expressions, and value comparisons, which are discussed in 3.5.1 Value Comparisons.

2.5.1 Kinds of Errors

As described in 2.4.2 Type Checking, XPath defines an analysis phase, which does not depend on input data, and an evaluation phase, which does depend on input data.

The result of the analysis phase is either success or one or more type errors and/or static errors. Type errors reported by the analysis phase occur when the static type of an expression is not correct for the context in which it appears. Static errors are non-type-related errors such as syntax errors. The means by which errors are reported during the analysis phase is implementation-defined.

The result of the evaluation phase is either a result value, a type error, or a dynamic error. Type errors are raised during the evaluation phase when the dynamic type of an expression is not correct for the context in which it appears. Dynamic errors are non-type-related errors such as numeric overflow. If evaluation of an expression yields a value (that is, it does not raise an error), the value must be the value specified by the dynamic semantics defined in [XQuery 1.0 Formal Semantics].

If an implementation can determine by static analysis that an expression will necessarily raise a dynamic error (for example, because it attempts to construct a decimal value from a constant string that is not in the lexical space of xs:decimal), the implementation is allowed to report this error during the analysis phase (as well as during the evaluation phase).

[XQuery 1.0 Formal Semantics] defines the set of static, dynamic, and type errors. In addition to these errors, an XPath implementation may raise implementation-defined warnings, either during the analysis phase or the evaluation phase. The circumstances in which warnings are raised, and the ways in which warnings are handled, are implementation-defined.

In addition to the errors defined in this specification, an implementation may raise a dynamic error if insufficient resources are available for processing a given expression. For example, an implementation may specify limitations on the maximum numbers or sizes of various objects. These limitations, and the consequences of exceeding them, are implementation-defined.

2.5.2 Handling Dynamic Errors

Except as noted in this document, if any operand of an expression raises a dynamic error, the expression also raises a dynamic error. If an expression can validly return a value or raise a dynamic error, the implementation may choose to return the value or raise the dynamic error. For example, the logical expression expr1 and expr2 may return the value false if either operand returns false, or may raise a dynamic error if either operand raises a dynamic error.

If more than one operand of an expression raises an error, the implementation may choose which error is raised by the expression. For example, in this expression:

($x div $y) + xs:decimal($z)

both ($x div $y) and xs:decimal($z) may raise an error. The implementation may choose which error is raised by the "+" expression. Once one operand raises an error, the implementation is not required, but is permitted, to evaluate any other operands.

A dynamic error carries an error value, which may be a single item or an empty sequence. For example, an error value might be an integer, a string, a QName, or an element. An implementation may provide a mechanism whereby an application-defined error handler can process error values and produce diagnostics; in the absence of such an error handler, the string-value of the error value may be used directly as an error message.

A dynamic error may be raised by a built-in function or operator. For example, the input function raises an error if the input sequence is not defined in the evaluation context.

An error can be raised explicitly by calling the fn:error function, which only raises an error and never returns a value. The fn:error function takes an optional item as its parameter, which is used as the error value. For example, the following function call raises a dynamic error whose error value is a string:

fn:error(fn:concat("Unexpected value ", fn:string($v)))

2.5.3 Errors and Optimization

Because different implementations may choose to evaluate or optimize an expression in different ways, the detection and reporting of dynamic errors is implementation dependent.

When an implementation is able to evaluate an expression without evaluating some subexpression, the implementation is never required to evaluate that subexpression solely to determine whether it raises a dynamic error. For example, if a function parameter is never used in the body of the function, an implementation may choose whether to evaluate the expression bound to that parameter in a function call.

In some cases, an optimizer may be able to achieve substantial performance improvements by rearranging an expression so that the underlying operations such as projection, restriction, and sorting are performed in a different order than that specified in [XQuery 1.0 Formal Semantics]. In such cases, dynamic errors may occur that could not have occurred if the expression were evaluated as written. For example, consider the following expression:

$N[@x castable as xs:date]
      [xs:date(@x) gt xs:date("2000-01-01")]

This expression cannot fail with a casting error if it is evaluated exactly as written. An implementation is permitted, however, to reorder the predicates to achieve better performance (for example, by taking advantage of an index). This reordering could cause the above expression to fail. However, an expression must not be rearranged in a way that causes it to return a non-error result that is different from the result defined by [XQuery 1.0 Formal Semantics].

To avoid unexpected errors caused by reordering of expressions, tests that are designed to prevent dynamic errors should be expressed using conditional expressions, as in the following example:

$N[if (@x castable as xs:date)
   then xs:date(@x) gt xs:date("2000-01-01")
   else false()]

In the case of a conditional expression, the implementation is required not to evaluate the then branch if the condition is false, and not to evaluate the else branch if the condition is true. Conditional expressions are the only kinds of expressions that provide guaranteed conditions under which a particular subexpression will not be evaluated.

2.6.1 Basic XPath

A Basic XPath implementation must implement the full XPath language as described in this specification, subject to the following limitations:

1. In a Basic XPath implementation, the in-scope type definitions consist only of the built-in types defined in [XML Schema] and the additional predefined types in the http://www.w3.org/2003/05/xpath-datatypes namespace.

2. A mapping from a Post-Schema Validation Infoset (PSVI) to the Data Model is specified in [XQuery 1.0 and XPath 2.0 Data Model]. In a Basic XPath implementation, this mapping maps each datatype that is not one of the predefined types listed above into its nearest supertype that belongs to this list. As a result of this mapping, all complex types are mapped into xs:anyType. (Of course, mapping from a PSVI is only one way in which a Data Model instance might be constructed--other ways are also possible.)

3. If any SequenceType contains a typename that is not one of the predefined types listed above, a Basic XPath implementation raises a static error.

4. If any SequenceType contains an ElementTest or AttributeTest that contains a TypeName or a SchemaContextPath, a Basic XPath implementation raises a static error.

5. If the processing of an expression depends on the type of some value, and that type is not one of the predefined types listed above, a Basic XPath implementation raises a dynamic error.

6. A Basic XPath implementation is not required to raise type errors during the analysis phase. If an expression contains one or more non-type-related static errors, then a Basic XPath implementation must raise at least one of these static errors during the analysis phase. If the analysis phase is successful but one or more dynamic errors are encountered during the evaluation phase, then a Basic XPath implementation must raise at least one of these dynamic errors.

2.6.2 Static Typing Feature

The Static Typing Feature removes the limitation specified by Rule 6 of Basic XPath. An implementation that includes this feature is required to detect type errors during the analysis phase. If an expression contains one or more static errors or type errors, then a Static Typing implementation must raise at least one of these errors during the analysis phase.

3.1.1 Literals

A literal is a direct syntactic representation of an atomic value. XPath supports two kinds of literals: numeric literals and string literals.

The value of a numeric literal containing no "." and no e or E character is an atomic value whose type is xs:integer and whose value is obtained by parsing the numeric literal according to the rules of the xs:integer datatype. The value of a numeric literal containing "." but no e or E character is an atomic value whose type is xs:decimal and whose value is obtained by parsing the numeric literal according to the rules of the xs:decimal datatype. The value of a numeric literal containing an e or E character is an atomic value whose type is xs:double and whose value is obtained by parsing the numeric literal according to the rules of the xs:double datatype.

The value of a string literal is an atomic value whose type is xs:string and whose value is the string denoted by the characters between the delimiting apostrophes or quotation marks. If the literal is delimited by apostrophes, two adjacent apostrophes within the literal are interpreted as a single apostrophe. Similarly, if the literal is delimited by quotation marks, two adjacent quotation marks within the literal are interpreted as one quotation mark.

If a string literal is used in an XPath expression contained within the value of an XML attribute, the characters used to delimit the literal should be different from the characters that are used to delimit the attribute.

Here are some examples of literal expressions:

• "12.5" denotes the string containing the characters '1', '2', '.', and '5'.

• 12 denotes the integer value twelve.

• 12.5 denotes the decimal value twelve and one half.

• 125E2 denotes the double value twelve thousand, five hundred.

• "He said, ""I don't like it.""" denotes a string containing two quotation marks and one apostrophe.

The boolean values true and false can be represented by calls to the built-in functions fn:true() and fn:false(), respectively.

Values of other XML Schema built-in types can be constructed by calling the constructor for the given type. The constructors for XML Schema built-in types are defined in [XQuery 1.0 and XPath 2.0 Functions and Operators]. In general, the name of a constructor function for a given type is the same as the name of the type (including its namespace). For example:

• xs:integer("12") returns the integer value twelve.

• xs:date("2001-08-25") returns an item whose type is xs:date and whose value represents the date 25th August 2001.

• xdt:dayTimeDuration("PT5H") returns an item whose type is xdt:dayTimeDuration and whose value represents a duration of five hours.

It is also possible to construct values of various types by using a cast expression. For example:

• 9 cast as hatsize returns the atomic value 9 whose type is hatsize.

3.1.2 Variables

A variable reference is a QName preceded by a $-sign. Two variable references are equivalent if their local names are the same and their namespace prefixes are bound to the same namespace URI in the in-scope namespaces. An unprefixed variable reference is in no namespace.

Every variable reference must match a name in the in-scope variables, which include variables from the following sources:

1. A variable may be added to the in-scope variables by the host language environment.

2. A variable may be bound by an XPath expression. The kinds of expressions that can bind variables are for expressions (3.7 For Expressions) and quantified expressions (3.9 Quantified Expressions).

Every variable binding has a static scope. The scope defines where references to the variable can validly occur. It is a static error to reference a variable that is not in scope. If a variable is bound in the static context for an expression, that variable is in scope for the entire expression.

If a variable reference matches two or more bindings that are in scope, then the reference is taken as referring to the inner binding, that is, the one whose scope is smaller. At evaluation time, the value of a variable reference is the value of the expression to which the relevant variable is bound. The scope of a variable binding is defined separately for each kind of expression that can bind variables.

3.1.3 Parenthesized Expressions

Parentheses may be used to enforce a particular evaluation order in expressions that contain multiple operators. For example, the expression (2 + 4) * 5 evaluates to thirty, since the parenthesized expression (2 + 4) is evaluated first and its result is multiplied by five. Without parentheses, the expression 2 + 4 * 5 evaluates to twenty-two, because the multiplication operator has higher precedence than the addition operator.

Empty parentheses are used to denote an empty sequence, as described in 3.3.1 Constructing Sequences.

3.1.4 Function Calls

A function call consists of a QName followed by a parenthesized list of zero or more expressions, called arguments. If the QName in the function call has no namespace prefix, it is considered to be in the default function namespace.

If the expanded QName and number of arguments in a function call do not match the name and arity of an in-scope function in the static context, an error is raised (the host language environment may define this error as either a static or a dynamic error.)

A function call is evaluated as follows:

1. Each argument expression is evaluated, producing an argument value. The order of argument evaluation is implementation-dependent and a function need not evaluate an argument if the function can evaluate its body without evaluating that argument.

2. Each argument value is converted by applying the function conversion rules listed below.

3. The function is executed using the converted argument values. The result is a value of the function's declared return type.

The function conversion rules are used to convert an argument value to its expected type; that is, to the declared type of the function parameter. The expected type is expressed as a SequenceType. The function conversion rules are applied to a given value as follows:

• If XPath 1.0 compatibility mode is true, then one of the following conversions is applied:

  1. If the expected type is xs:string or xs:string?, then the given value V is effectively replaced by fn:string(fn:subsequence(V, 1, 1)).

  2. If the expected type is xs:double or xs:double?, then the given value V is effectively replaced by fn:number(fn:subsequence(V, 1, 1)).

  3. If the expected type is a (possibly optional) node or item, then the given value V is effectively replaced by fn:subsequence(V, 1, 1).

  4. Otherwise, the given value is unchanged.

• If the expected type is a sequence of an atomic type (possibly with an occurrence indicator *, +, or ?), the following conversions are applied:

  1. Atomization is applied to the given value, resulting in a sequence of atomic values.

  2. Each item in the atomic sequence that is of type xdt:untypedAtomic is cast to the expected atomic type.

  3. For each numeric item in the atomic sequence that can be promoted to the expected atomic type using the promotion rules in B.1 Type Promotion, the promotion is done.

• If, after the above conversions, the resulting value does not match the expected type according to the rules for SequenceType Matching, a type error is raised. Note that the rules for SequenceType Matching permit a value of a derived type to be substituted for a value of its base type.

A core library of functions is defined in [XQuery 1.0 and XPath 2.0 Functions and Operators]. Additional functions may be provided in the static context.

Since the arguments of a function call are separated by commas, any argument expression that contains a top-level comma operator must be enclosed in parentheses. Here are some illustrative examples of function calls:

• three-argument-function(1, 2, 3) denotes a function call with three arguments.

• two-argument-function((1, 2), 3) denotes a function call with two arguments, the first of which is a sequence of two values.

• two-argument-function(1, ()) denotes a function call with two arguments, the second of which is an empty sequence.

• one-argument-function((1, 2, 3)) denotes a function call with one argument that is a sequence of three values.

• one-argument-function(( )) denotes a function call with one argument that is an empty sequence.

• zero-argument-function( ) denotes a function call with zero arguments.

3.1.5 XPath Comments

XPath comments can be used to provide informative annotation. These comments are lexical constructs only, and do not affect the processing of an expression. Comments are delimited by the symbols (: and :). Comments may be nested.

Comments may be used anywhere that ignorable whitespace is allowed. See A.2 Lexical structure for the exact lexical states where comments are recognized.

The following is an example of a comment:

(: Houston, we have a problem :)

3.2.1 Steps

A step generates a sequence of items and then filters the sequence by zero or more predicates. The value of the step consists of those items that satisfy the predicates. Predicates are described in 3.2.2 Predicates. XPath provides two kinds of step, called a filter step and an axis step.

A filter step consists simply of a primary expression followed by zero or more predicates. The result of the filter expression consists of all the items returned by the primary expression for which all the predicates are true. If no predicates are specified, the result is simply the result of the primary expression. This result may contain nodes, atomic values, or any combination of these. The ordering of the items returned by a filter step is the same as their order in the result of the primary expression.

The result of an axis step is always a sequence of zero or more nodes, and these nodes are always returned in document order. An axis step may be either a forward step or a reverse step, followed by zero or more predicates. An axis step might be thought of as beginning at the context node and navigating to those nodes that are reachable from the context node via a specified axis. Such a step has two parts: an axis, which defines the "direction of movement" for the step, and a node test, which selects nodes based on their kind, name, and/or type.

In the abbreviated syntax for a step, the axis can be omitted and other shorthand notations can be used as described in 3.2.4 Abbreviated Syntax.

The unabbreviated syntax for an axis step consists of the axis name and node test separated by a double colon. The result of the step consists of the nodes reachable from the context node via the specified axis that have the node kind, name, and/or type specified by the node test. For example, the step child::para selects the para element children of the context node: child is the name of the axis, and para is the name of the element nodes to be selected on this axis. The available axes are described in 3.2.1.1 Axes. The available node tests are described in 3.2.1.2 Node Tests. Examples of steps are provided in 3.2.3 Unabbreviated Syntax and 3.2.4 Abbreviated Syntax.

3.2.2 Predicates

A predicate consists of an expression, called a predicate expression, enclosed in square brackets. A predicate serves to filter a sequence, retaining some items and discarding others. For each item in the sequence to be filtered, the predicate expression is evaluated using an inner focus derived from that item, as described in 2.1.2 Evaluation Context. The result of the predicate expression is coerced to a Boolean value, called the predicate truth value, as described below. Those items for which the predicate truth value is true are retained, and those for which the predicate truth value is false are discarded.

The predicate truth value is derived by applying the following rules, in order:

1. If the value of the predicate expression is an atomic value of a numeric type, the predicate truth value is true if the value of the predicate expression is equal to the context position, and is false otherwise.

2. Otherwise, the predicate truth value is the Effective Boolean Value of the predicate expression.

Here are some examples of axis steps that contain predicates:

• This example selects the second chapter element that is a child of the context node:

  child::chapter[2]
  

• This example selects all the descendants of the context node whose name is "toy" and whose color attribute has the value "red":

  descendant::toy[attribute::color = "red"]
  

• This example selects all the employee children of the context node that have a secretary subelement:

  child::employee[secretary]
  

Here are some examples of filter steps that contain predicates:

• List all the integers from 1 to 100 that are divisible by 5. (See 3.3.1 Constructing Sequences for an explanation of the to operator.)

  (1 to 100)[. mod 5 eq 0]
  

• The result of the following expression is the integer 95:

  (99 to 0)[5]
  

3.2.3 Unabbreviated Syntax

This section provides a number of examples of path expressions in which the axis is explicitly specified in each step. The syntax used in these examples is called the unabbreviated syntax. In many common cases, it is possible to write path expressions more concisely using an abbreviated syntax, as explained in 3.2.4 Abbreviated Syntax.

• child::para selects the para element children of the context node

• child::* selects all element children of the context node

• child::text() selects all text node children of the context node

• child::node() selects all the children of the context node, whatever their node type

• attribute::name selects the name attribute of the context node

• attribute::* selects all the attributes of the context node

• parent::* selects the parent of the context node. If the context node is an attribute node, this expression returns the element node (if any) to which the attribute node is attached.

• descendant::para selects the para element descendants of the context node

• ancestor::div selects all div ancestors of the context node

• ancestor-or-self::div selects the div ancestors of the context node and, if the context node is a div element, the context node as well

• descendant-or-self::para selects the para element descendants of the context node and, if the context node is a para element, the context node as well

• self::para selects the context node if it is a para element, and otherwise selects nothing

• child::chapter/descendant::para selects the para element descendants of the chapter element children of the context node

• child::*/child::para selects all para grandchildren of the context node

• / selects the root of the node hierarchy that contains the context node

• /descendant::para selects all the para elements in the same document as the context node

• /descendant::list/child::member selects all the member elements that have a list parent and that are in the same document as the context node

• child::para[fn:position() = 1] selects the first para child of the context node

• child::para[fn:position() = fn:last()] selects the last para child of the context node

• child::para[fn:position() = fn:last()-1] selects the last but one para child of the context node

• child::para[fn:position() > 1] selects all the para children of the context node other than the first para child of the context node

• following-sibling::chapter[fn:position() = 1]selects the next chapter sibling of the context node

• preceding-sibling::chapter[fn:position() = 1]selects the previous chapter sibling of the context node

• /descendant::figure[fn:position() = 42] selects the forty-second figure element in the document

• /child::doc/child::chapter[fn:position() = 5]/child::section[fn:position() = 2]selects the second section of the fifth chapter of the doc document element

• child::para[attribute::type="warning"]selects all para children of the context node that have a type attribute with value warning

• child::para[attribute::type='warning'][fn:position() = 5]selects the fifth para child of the context node that has a type attribute with value warning

• child::para[fn:position() = 5][attribute::type="warning"]selects the fifth para child of the context node if that child has a type attribute with value warning

• child::chapter[child::title='Introduction']selects the chapter children of the context node that have one or more title children with string-value equal to Introduction

• child::chapter[child::title] selects the chapter children of the context node that have one or more title children

• child::*[self::chapter or self::appendix] selects the chapter and appendix children of the context node

• child::*[self::chapter or self::appendix][fn:position() = fn:last()] selects the last chapter or appendix child of the context node

3.2.4 Abbreviated Syntax

The abbreviated syntax permits the following abbreviations:

1. The most important abbreviation is that the axis name can be omitted from an axis step. If the axis name is omitted from an axis step, the default axis is child unless the axis step contains an AttributeTest; in that case, the default axis is attribute. For example, a path expression section/para is an abbreviation for child::section/child::para. Similarly, section/attribute(@id) is an abbreviation for child::section/attribute::attribute(@id).

2. There is also an abbreviation for attributes: attribute:: can be abbreviated by @. For example, a path expression para[@type="warning"] is short for child::para[attribute::type="warning"] and so selects para children with a type attribute with value equal to warning.

3. // is effectively replaced by /descendant-or-self::node()/ during processing of a path expression. For example, //para is an abbreviation for /descendant-or-self::node()/child::para and so will select any para element in the document (even a para element that is a document element will be selected by //para since the document element node is a child of the root node); div1//para is short for div1/descendant-or-self::node()/child::para and so will select all para descendants of div1 children.

   Note that the path expression //para[1] does not mean the same as the path expression /descendant::para[1]. The latter selects the first descendant para element; the former selects all descendant para elements that are the first para children of their parents.

4. A step consisting of . returns the context item. This is particularly useful in conjunction with the // operator. For example, the path expression .//para returns all para descendant elements of the context node.

5. A step consisting of .. is short for parent::node(). For example, ../title is short for parent::node()/child::title and so will select the title children of the parent of the context node.

Here are some examples of path expressions that use the abbreviated syntax:

• para selects the para element children of the context node

• * selects all element children of the context node

• text() selects all text node children of the context node

• @name selects the name attribute of the context node

• @* selects all the attributes of the context node

• para[1] selects the first para child of the context node

• para[fn:last()] selects the last para child of the context node

• */para selects all para grandchildren of the context node

• /doc/chapter[5]/section[2] selects the second section of the fifth chapter of the doc

• chapter//para selects the para element descendants of the chapter element children of the context node

• //para selects all the para descendants of the document root and thus selects all para elements in the same document as the context node

• //list/member selects all the member elements in the same document as the context node that have a list parent

• . selects the context item

• .//para selects the para element descendants of the context node

• .. selects the parent of the context node

• ../@lang selects the lang attribute of the parent of the context node

• para[@type="warning"] selects all para children of the context node that have a type attribute with value warning

• para[@type="warning"][5] selects the fifth para child of the context node that has a typeattribute with value warning

• para[5][@type="warning"] selects the fifth para child of the context node if that child has a type attribute with value warning

• chapter[title="Introduction"] selects the chapter children of the context node that have one or more title children with string-value equal to Introduction

• chapter[title] selects the chapter children of the context node that have one or more title children

• employee[@secretary and @assistant] selects all the employee children of the context node that have both a secretary attribute and an assistant attribute

• book/(chapter|appendix)/section selects every section element that has a parent that is either a chapter or an appendix element, that in turn is a child of a book element that is a child of the context node.

• book/fn:id(publisher)/name returns the same result as fn:id(book/publisher)/name.

• If E is any expression that returns a sequence of nodes, then the expression E/. returns the same nodes in document order, with duplicates eliminated based on node identity.

3.3.1 Constructing Sequences

One way to construct a sequence is by using the comma operator, which evaluates each of its operands and concatenates the resulting values, in order, into a single result sequence. Empty parentheses can be used to denote an empty sequence. In places where the grammar calls for ExprSingle, such as the arguments of a function call, any expression that contains a top-level comma operator must be enclosed in parentheses.

A sequence may contain duplicate values or nodes, but a sequence is never an item in another sequence. When a new sequence is created by concatenating two or more input sequences, the new sequence contains all the items of the input sequences and its length is the sum of the lengths of the input sequences.

Here are some examples of expressions that construct sequences:

• This expression is a sequence of five integers:

  (10, 1, 2, 3, 4)
  

• This expression constructs one sequence from the sequences 10, (1, 2), the empty sequence (), and (3, 4):

  (10, (1, 2), (), (3, 4))
  

  It evaluates to the sequence:

  10, 1, 2, 3, 4

• This expression contains all salary children of the context node followed by all bonus children:

  (salary, bonus)
  

• Assuming that $price is bound to the value 10.50, this expression:

  ($price, $price)
  

  evaluates to the sequence

  10.50, 10.50

A RangeExpr can be used to construct a sequence of consecutive integers. Each of the operands of the to operator is converted as though it was an argument of a function with the expected parameter type xs:integer (this process raises an error if the operand cannot be converted to a single integer). A sequence is constructed containing the two integer operands and every integer between the two operands. If the first operand is less than the second, the sequence is in increasing order, otherwise it is in decreasing order.

• This example uses a range expression as one operand in constructing a sequence:

  (10, 1 to 4)
  

  It evaluates to the sequence:

  10, 1, 2, 3, 4

• This example constructs a sequence of length one:

  10 to 10
  

  It evaluates to a sequence consisting of the single integer 10.

3.3.2 Combining Sequences

XPath provides several operators for combining sequences of nodes. The union and | operators are equivalent. They take two node sequences as operands and return a sequence containing all the nodes that occur in either of the operands. The intersect operator takes two node sequences as operands and returns a sequence containing all the nodes that occur in both operands. The except operator takes two node sequences as operands and returns a sequence containing all the nodes that occur in the first operand but not in the second operand. All of these operators return their result sequences in document order without duplicates based on node identity. If an operand of union, intersect, or except contains an item that is not a node, a type error is raised.

Here are some examples of expressions that combine sequences. Assume the existence of three element nodes that we will refer to by symbolic names A, B, and C. Assume that $seq1 is bound to a sequence containing A and B, $seq2 is also bound to a sequence containing A and B, and $seq3 is bound to a sequence containing B and C. Then:

• $seq1 union $seq1 evaluates to a sequence containing A and B.

• $seq2 union $seq3 evaluates to a sequence containing A, B, and C.

• $seq1 intersect $seq1 evaluates to a sequence containing A and B.

• $seq2 intersect $seq3 evaluates to a sequence containing B only.

• $seq1 except $seq2 evaluates to the empty sequence.

• $seq2 except $seq3 evaluates to a sequence containing A only.

In addition to the sequence operators described here,[XQuery 1.0 and XPath 2.0 Functions and Operators] includes functions for indexed access to items or sub-sequences of a sequence, for indexed insertion or removal of items in a sequence, and for removing duplicate values or nodes from a sequence.

3.5.1 Value Comparisons

Value comparisons are intended for comparing single values. The result of a value comparison is defined by applying the following rules, in order:

1. Atomization is applied to each operand. If the result, called an atomized operand, does not contain exactly one atomic value, a type error is raised.

2. Any atomized operand that has the dynamic type xdt:untypedAtomic is cast to the type xs:string.

3. The result of the comparison is true if the value of the first operand is (equal, not equal, less than, less than or equal, greater than, greater than or equal) to the value of the second operand; otherwise the result of the comparison is false. B.2 Operator Mapping describes which combinations of atomic types are comparable, and how comparisons are performed on values of various types. If the value of the first atomized operand is not comparable with the value of the second atomized operand, a type error is raised.

Here are some examples of value comparisons:

• The following comparison is true only if $book1 has a single author subelement and its value is "Kennedy":

  $book1/author eq "Kennedy"
  

• The following comparison is true if hatsize and shoesize are both user-defined types that are derived by restriction from a primitive numeric type:

  hatsize(5) eq shoesize(5)
  

3.5.2 General Comparisons

General comparisons are existentially quantified comparisons that may be applied to operand sequences of any length. The result of a general comparison that does not raise an error is always true or false.

Atomization is applied to each operand of a general comparison. The result of the comparison is true if and only if there is a pair of atomic values, one belonging to the result of atomization of the first operand and the other belonging to the result of atomization of the second operand, that have the required magnitude relationship. Otherwise the result of the general comparison is false. The magnitude relationship between two atomic values is determined as follows:

1. If either atomic value has the dynamic type xdt:untypedAtomic, that value is cast to a required type, which is determined as follows:

   1. If the dynamic type of the other atomic value is a numeric type, the required type is xs:double.

   2. If the dynamic type of the other atomic value is xdt:untypedAtomic, the required type is xs:string.

   3. Otherwise, the required type is the dynamic type of the other atomic value.

   If the cast to the required type fails, a dynamic error is raised.

2. If XPath 1.0 compatibility mode is true, and at least one of the atomic values has a numeric type, then both atomic values are cast to to the type xs:double.

3. After any necessary casting, the atomic values are compared using one of the value comparison operators eq, ne, lt, le, gt, or ge, depending on whether the general comparison operator was =, !=, <, <=, >, or >=. The values have the required magnitude relationship if the result of this value comparison is true.

When evaluating a general comparison in which either operand is a sequence of items, an implementation may return true as soon as it finds an item in the first operand and an item in the second operand for which the underlying value comparison is true. Similarly, a general comparison may raise a dynamic error as soon as it encounters an error in evaluating either operand, or in comparing a pair of items from the two operands. As a result of these rules, the result of a general comparison is not deterministic in the presence of errors.

Here are some examples of general comparisons:

• The following comparison is true if the value of any author subelement of $book1 has the string value "Kennedy":

  $book1/author = "Kennedy"
  

• The following example contains three general comparisons. The value of the first two comparisons is true, and the value of the third comparison is false. This example illustrates the fact that general comparisons are not transitive.

  (1, 2) = (2, 3)
  (2, 3) = (3, 4)
  (1, 2) = (3, 4)
  

• Suppose that $a, $b, and $c are bound to element nodes with type annotation xdt:untypedAtomic, with string values "1", "2", and "2.0" respectively. Then ($a, $b) = ($c, 3.0) returns false, because $b and $c are compared as strings. However, ($a, $b) = ($c, 2.0) returns true, because $b and 2.0 are compared as numbers.

3.5.3 Node Comparisons

The result of a node comparison is defined by applying the following rules, in order:

1. Each operand must be either a single node or an empty sequence; otherwise a type error is raised.

2. If either operand is an empty sequence, the result of the comparison is an empty sequence.

3. A comparison with the is operator is true if the two operands are nodes that have the same identity; otherwise it is false. A comparison with the isnot operator is true if the two operands are nodes that have different identities; otherwise it is false. See [XQuery 1.0 and XPath 2.0 Data Model] for a discussion of node identity.

Use of the is operator is illustrated below.

• The following comparison is true only if the left and right sides each evaluate to exactly the same single node:

  //book[isbn="1558604820"] is //book[call="QA76.9 C3845"]
  

3.5.4 Order Comparisons

The result of an order comparison is defined by applying the following rules, in order:

1. Both operands must be either a single node or an empty sequence; otherwise a type error is raised.

2. If either operand is an empty sequence, the result of the comparison is an empty sequence.

3. A comparison with the << operator returns true if the first operand node is earlier than the second operand node in document order; otherwise it returns false.

4. A comparison with the >> operator returns true if the first operand node is later than the second operand node in document order; otherwise it returns false.

Here is an example of an order comparison:

• The following comparison is true only if the node identified by the left side occurs before the node identified by the right side in document order:

  //purchase[parcel="28-451"] << //sale[parcel="33-870"]
  

3.10.1 Instance Of

The boolean operator instance of returns true if the value of its first operand matches the type named in its second operand, according to the rules for SequenceType Matching; otherwise it returns false. For example:

• 5 instance of xs:integer

  This example returns true because the given value is an instance of the given type.

• 5 instance of xs:decimal

  This example returns true because the given value is an integer literal, and xs:integer is derived by restriction from xs:decimal.

• . instance of element()

  This example returns true if the context item is an element node.

3.10.2 Cast

Occasionally it is necessary to convert a value to a specific datatype. For this purpose, XPath provides a cast expression that creates a new value of a specific type based on an existing value. A cast expression takes two operands: an input expression and a target type. The type of the input expression is called the input type. The target type must be a named atomic type, represented by a QName, optionally followed by the occurrence indicator ? if an empty sequence is permitted. If the target type has no namespace prefix, it is considered to be in the default element namespace. The semantics of the cast expression are as follows:

1. Atomization is performed on the input expression.

2. If the result of atomization is a sequence of more than one atomic value, a type error is raised.

3. If the result of atomization is an empty sequence:

   1. If ? is specified after the target type, the result of the cast expression is an empty sequence.

   2. If ? is not specified after the target type, a type error is raised.

4. If the result of atomization is a single atomic value, the result of the cast expression depends on the input type and the target type. In general, the cast expression attempts to create a new value of the target type based on the input value. Only certain combinations of input type and target type are supported. The rules are listed below. For the purpose of these rules, we use the terms subtype and supertype in the following sense: if type B is derived from type A by restriction, then B is a subtype of A, and A is a supertype of B.

   1. cast is supported for the combinations of input type and target type listed in [XQuery 1.0 and XPath 2.0 Functions and Operators]. For each of these combinations, both the input type and the target type are built-in schema types. For example, a value of type xs:string can be cast into the type xs:decimal. For each of these built-in combinations, the semantics of casting are specified in [XQuery 1.0 and XPath 2.0 Functions and Operators].

   2. cast is supported if the input type is a derived atomic type and the target type is a supertype of the input type. In this case, the input value is mapped into the value space of the target type, unchanged except for its type. For example, if shoesize is derived by restriction from xs:integer, a value of type shoesize can be cast into the type xs:integer.

   3. cast is supported if the target type is a derived atomic type and the input type is xs:string or xdt:untypedAtomic. The input value is first converted to a value in the lexical space of the target type by applying the whitespace normalization rules for the target type; a dynamic error is raised if the resulting lexical value does not satisfy the pattern facet of the target type. The lexical value is then converted to the value space of the target type using the schema-defined rules for the target type; a dynamic error is raised if the resulting value does not satisfy all the facets of the target type.

   4. cast is supported if the target type is a derived atomic type and the input type is a supertype of the target type. The input value must satisfy all the facets of the target type (in the case of the pattern facet, this is checked by generating a string representation of the input value, using the rules for casting to xs:string). The resulting value is the same as the input value, but with a different dynamic type.

   5. If a primitive type P1 can be cast into a primitive type P2, then any subtype of P1 can be cast into any subtype of P2, provided that the facets of the target type are satisfied. First the input value is cast to P1 using rule (b) above. Next, the value of type P1 is cast to the type P2, using rule (a) above. Finally, the value of type P2 is cast to the target type, using rule (d) above.

   6. For any combination of input type and target type that is not in the above list, a cast expression raises a type error.

If casting from the input type to the target type is supported but nevertheless it is not possible to cast the input value into the value space of the target type, a dynamic error is raised. This includes the case when any facet of the target type is not satisfied. For example, the expression "2003-02-31" cast as xs:date would raise a dynamic error.

3.10.3 Castable

XPath provides a form of Boolean expression that tests whether a given value is castable into a given target type. The expression V castable as T returns true if the value V can be successfully cast into the target type T by using a cast expression; otherwise it returns false. The castable predicate can be used to avoid errors at evaluation time. It can also be used to select an appropriate type for processing of a given value, as illustrated in the following example:

if ($x castable as hatsize)
then $x cast as hatsize
else if ($x castable as IQ)
then $x cast as IQ
else $x cast as xs:string

3.10.4 Constructor Functions

Constructor functions provide an alternative syntax for casting.

For every built-in atomic type T that is defined in [XML Schema], as well as the predefined types xdt:dayTimeDuration, xdt:yearMonthDuration, and xdt:untypedAtomic, a built-in constructor function is provided. The signature of the built-in constructor function for type T is as follows:

T($x as item) as T

The constructor function for type T accepts any single item (either a node or an atomic value) as input, and returns a value of type T (or raises a dynamic error). Its semantics are exactly the same as a cast expression with target type T. The built-in constructor functions are described in more detail in [XQuery 1.0 and XPath 2.0 Functions and Operators]. The following are examples of built-in constructor functions:

• This example is equivalent to "2000-01-01" cast as xs:date.

  xs:date("2000-01-01")
  

• This example is equivalent to ($floatvalue * 0.2E-5) cast as xs:decimal.

  xs:decimal($floatvalue * 0.2E-5)
  

• This example returns a dayTimeDuration value equal to 21 days. It is equivalent to "P21D" cast as xdt:dayTimeDuration.

  xdt:dayTimeDuration("P21D")
  

For each user-defined top-level atomic type T in the in-scope type definitions, a constructor function is effectively defined. Like the built-in constructor functions, the constructor functions for user-defined types have the same name (including namespace or lack of namespace) as the type, accept any item as input, and have semantics identical to a cast expression with the user-defined type as target type. For example, if usa:zipcode is a user-defined top-level atomic type in the in-scope type definitions, then the expression usa:zipcode("12345") is equivalent to the expression "12345" cast as usa:zipcode.

If the argument to any constructor function is a literal value, the result of the function may be computed statically, and an error encountered in this process may be reported as a static error.

3.10.5 Treat

XPath provides an expression called treat that can be used to modify the static type of its operand.

Like cast, the treat expression takes two operands: an expression and a SequenceType. Unlike cast, however, treat does not change the dynamic type or value of its operand. Instead, the purpose of treat is to ensure that an expression has an expected type at evaluation time.

The semantics of expr1 treat as type1 are as follows:

• During static analysis (if the Static Typing Feature is implemented):

  type1 must be derived by restriction from the static type of expr1--otherwise, a type error is raised. The static type of the treat expression is type1. This enables the expression to be used as an argument of a function that requires a parameter of type1.

• During expression evaluation (at "run-time"):

  If expr1 matches type1, using the SequenceType Matching rules in 2.4.3 SequenceType, the treat expression returns the value of expr1; otherwise, it raises a dynamic error. If the value of expr1 is returned, its identity is preserved. The treat expression ensures that the value of its expression operand conforms to the expected type at run-time.

• Example:

  $myaddress treat as element(*, USAddress)
  

  The static type of $myaddress may be element(*, Address), a less specific type than element(*, USAddress). However, at run-time, the value of $myaddress must match the type element(*, USAddress) using SequenceType Matching rules; otherwise a dynamic error is raised.