XQuery 1.0: An XML Query Language

2.1.1 Static Context

[Definition: 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. If analysis of an expression relies on some component of the static context that has not been assigned a value, a static error is raised.[err:XP0001]

The individual components of the static context are summarized below. Further rules governing the semantics of these components can be found in C.1 Static Context Components.

• [Definition: XPath 1.0 compatibility mode. This component must be set by all host languages that include XPath 2.0 as a subset, indicating whether rules for compatibility with XPath 1.0 are in effect. XQuery sets the value of this component to false. ]

• [Definition: 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.] Each in-scope namespace is classified as either an active namespace or a passive namespace. For details of this distinction, see 3.7.4 Namespace Nodes on Constructed Elements.

  Some namespaces are predefined; additional namespaces can be defined by Prologs, by namespace declaration attributes, and by computed namespace constructors.

• [Definition: Default element/type 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.] The initial default element/type namespace may be provided by the external environmentor by a declaration in the Prolog of a module.

• [Definition: 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. The initial default function namespace may be provided by the external environmentor by a declaration in the Prolog of a module.]

• [Definition: 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:

  • [Definition: In-scope type definitions. The in-scope type definitions always include the predefined types listed in 2.1.1.1 Predefined Types. If the Schema Import Feature is supported, in-scope type definitions also include all type definitions found in imported schemas. ]

    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.

  • [Definition: 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). If the Schema Import Feature is supported, in-scope element declarations include all element declarations found in imported schemas. An element declaration includes information about the substitution groups to which this element belongs.]

  • [Definition: 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). If the Schema Import Feature is supported, in-scope attribute declarations include all attribute declarations found in imported schemas.]

• [Definition: In-scope variables. This is a set of (QName, type) pairs. It defines the set of variables that are available for reference within an expression. The QName is the name of the variable, and the type is the static type of the variable.]

  Variable declarations in the Prolog of a module are added to the in-scope variables of the module. An expression that binds a variable (such as a let, for, some, or every expression) extends the in-scope variables of its subexpressions with the new bound variable and its type. Within a function declaration, the in-scope variables are extended by the names and types of the function parameters.

• [Definition: In-scope functions. This component defines the set of functions that are available to be called from within an expression. Each function is uniquely identified by its expanded QName and its arity (number of parameters). Each function in in-scope functions has a function signature and a function implementation.] [Definition: The function signature specifies the name of the function and the static types of its parameters and its result.] [Definition: The function implementation enables the function to map instances of its parameter types into an instance of its result type. For a user-defined function, the function implementation is an XQuery expression. For an external function, the function implementation is implementation dependent.]

  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 3.12.5 Constructor Functions.

• [Definition: 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.

• [Definition: Default collation. This collation is used by string comparison functions when no explicit collation is specified.]

• [Definition: Validation mode. The validation mode specifies the mode in which validation is performed by element constructors and by validate expressions. ] Its value is one of strict, lax, or skip. The initial validation mode may be provided by the environment external to a query or by the validation declaration in the Prolog of a module. If no validation mode is specified in either of these ways, the initial validation mode is lax.

  The validation mode for a subexpression is inherited from the containing expression. A validate expression that specifies a mode changes the validation mode of its subexpressions to the specified mode.

• [Definition: Validation context. An expression's validation context determines the context in which elements constructed by the expression are validated. ] Its value is either global or a context path that starts with a top-level element name or type name in the in-scope schema definitions. The default validation context of a module is global.

  The validation context for a subexpression is inherited from the containing expression. An element constructor extends the validation context of its subexpressions with the name of the constructed element, and a validate expression that specifies a context redefines the validation context of its subexpressions.

• [Definition: XMLSpace policy. This policy, declared in the Prolog, controls the processing of whitespace by element constructors.] Its value may be preserve or strip.

• [Definition: Base URI. This is an absolute URI, used when necessary in the resolution of relative URIs (for example, by the fn:resolve-uri function.)]

• [Definition: Statically-known documents. This is a mapping from strings onto types. The string represents the absolute URI of a resource that is potentially accessible using the fn:doc function. The type is the type of the document node that would result from calling the fn:doc function with this URI as its argument. ] If the argument to fn:doc is anthing other than a string literal that is present in statically-known documents, then the static type of fn:doc is document-node()?.

• [Definition: Statically-known collections. This is a mapping from strings onto types. The string represents the absolute URI of a resource that is potentially accessible using the fn:collection function. The type is the type of the sequence of nodes that would result from calling the fn:collection function with this URI as its argument.] If the argument to fn:collection is anthing other than a string literal that is present in statically-known collections, then the static type of fn:collection is node()?.

2.1.2 Dynamic Context

[Definition: The dynamic context of an expression is defined as information that is available at the time the expression is evaluated.] If evaluation of an expression relies on some part of the dynamic context that has not been assigned a value, a dynamic error is raised.[err:XP0002]

The individual components of the dynamic context are summarized below. Further rules governing the semantics of these components can be found in C.2 Dynamic Context Components.

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

[Definition: The first three components of the dynamic 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.

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.

• [Definition: The context item is the item currently being processed in a path expression. An item is either an atomic value or a node.][Definition: 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.

• [Definition: The context position is the position of the context item within the sequence of items currently being processed in a path expression. ]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.

• [Definition: The context size is the number of items in the sequence of items currently being processed in a path expression.] 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.

• [Definition: 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. The QName is the name of the variable and the value is the dynamic value of the variable.]

• [Definition: 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.]

• [Definition: 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.]

• [Definition: Accessible documents. This is a mapping of strings onto document nodes. The string represents the absolute URI of a resource. The document node is the representation of that resource as an instance of the data model, as returned by the fn:doc function when applied to that URI. ]The set of accessible documents may be the same as, or a subset or superset of, the set of statically-known documents, and it may be empty.

• [Definition: Accessible collections. This is a mapping of strings onto sequences of nodes. The string represents the absolute URI of a resource. The sequence of nodes represents the result of the fn:collection function when that URI is supplied as the argument. ] The set of accessible collections may be the same as, or a subset or superset of, the set of statically-known collections, and it may be empty.

2.2.1 Data Model Generation

Before an expression can be processed, the input documents to be accessed by the expression must be represented in the data model. Figure 1 depicts the steps by which an XML document may be converted to the data model:

1. A document may be parsed using an XML parser that generates an XML Information Set (see [XML Infoset]). The parsed document may then be validated against one or more schemas. This process, which is described in [XML Schema], results in an abstract information structure called the Post-Schema Validation Infoset (PSVI). If a document has no associated schema, its Information Set is preserved. (See DM1 in Fig. 1.)

2. The Information Set or PSVI may be transformed into the data model by a process described in [XQuery 1.0 and XPath 2.0 Data Model]. (See DM2 in Fig. 1.)

The above steps provide an example of how a data model instance might be constructed. A data model instance might also be synthesized directly from a relational database, or constructed in some other way (see DM3 in Fig. 1.) XQuery is defined in terms of operations on the data model, but it does not place any constraints on how the input data model instance is constructed.

Each atomic value, element node, and attribute node in the data model is annotated with its dynamic type. The dynamic type specifies a range of values -- for example, an attribute named version might have the dynamic type xs:decimal, indicating that it contains a decimal value. For example, if the data model was derived from an input XML document, the dynamic types of the elements and attributes are derived from schema validation.

The value of an attribute is represented directly within the attribute node. An attribute node whose type is unknown (such as might occur in a schemaless document) is annotated with the dynamic type xdt:untypedAtomic.

The value of an element is represented by the children of the element node, which may include text nodes and other element nodes. The dynamic type of an element node indicates how the values in its child text nodes are to be interpreted. An element whose type is unknown (such as might occur in a schemaless document) is annotated with the type xdt:untypedAny.

An atomic value of unknown type is annotated with the type xdt:untypedAtomic.

2.2.2 Schema Import Processing

2.2.3 Expression Processing

XQuery defines two phases of processing called the static analysis phase and the dynamic evaluation phase (see Fig. 1).

2.2.4 Serialization

[Definition: Serialization is the process of converting an instance of the [XQuery 1.0 and XPath 2.0 Data Model] into a sequence of octets (step DM4 in Figure 1.) ] The general framework for serialization of the data model is described in [XSLT 2.0 and XQuery 1.0 Serialization].

2.2.5 Consistency Constraints

In order for an expression to be well defined, the expression, its static context, and its dynamic context must be mutually consistent. The consistency constraints listed below are prerequisites for correct functioning of an XQuery implementation. Enforcement of these consistency constraints is beyond the scope of this specification.

• For each item type (i.e., element, attribute, or type name) referenced in an instance of the data model whose expanded name matches a name in the in-scope schema definitions (ISSD), the corresponding element, attribute, or type definition in the ISSD must be equivalent to the definition originally provided in the PSVI from which the data model instance was created.

• Every item type (i.e., every element, attribute, or type name) referenced in in-scope variables or in-scope functions must be in the in-scope schema definitions.

• Every name used in a SequenceType must be in the in-scope schema definitions.

• The element declaration for every element name referenced in a SequenceType or KindTest must be in the in-scope element declarations.

• The attribute declaration for every attribute name referenced in a SequenceType or KindTest must be in the in-scope attribute declarations.

• For each mapping of a string to a document node in accessible documents, if there exists a mapping of the same string to a document type in statically-known documents, the document node must match the document type, using the matching rules in 2.4.1.1 SequenceType Matching.

• For each mapping of a string to a sequence of nodes in accessible collections, if there exists a mapping of the same string to a type in statically-known collections, the sequence of nodes must match the type, using the matching rules in 2.4.1.1 SequenceType Matching.

• The dynamic variables in the dynamic context and the in-scope variables in the static context must correspond as follows:

  • All variables defined in in-scope variables must be defined in dynamic variables.

  • For each (variable, type) pair in in-scope variables and the corresponding (variable, value) pair in dynamic variables such that the variable names are equal, the value must match the type, using the matching rules in 2.4.1.1 SequenceType Matching.

2.3.1 Document Order

[Definition: Document order defines a total ordering among all the nodes seen by the language processor and is defined formally in the data model.] 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 dependent 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 dependent but stable.

2.3.2 Typed Value and String Value

Nodes have a typed value and a string value. [Definition: The typed value of a node is a sequence of atomic values and can be extracted by applying the fn:data function to the node. The typed value for each kind of node is defined by the dm:typed-value accessor in [XQuery 1.0 and XPath 2.0 Data Model]. ] [Definition: The string value of a node is a string and can be extracted by applying the the fn:string function to the node. The string value for each kind of node is defined by the dm:string-value accessor in [XQuery 1.0 and XPath 2.0 Data Model].] [Definition: Element and attribute nodes have a type annotation, which represents (in an implementation-dependent way) the dynamic (run-time) type of the node.] XQuery does not provide a way to directly access the type annotation of an element or attribute node.

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 a type error [err:XP0007] 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.

2.3.3 Input Sources

XQuery has a set of functions that provide access to input data. These functions are of particular importance because they provide a way in which an expression can reference a document or a collection of documents. The input functions are described informally here, and in more detail in [XQuery 1.0 and XPath 2.0 Functions and Operators].

An expression can access input documents either by calling one of the input functions or by referencing some part of the expression context that is initialized by the external environment, such as a variable or a pre-initialized context item.

The input functions supported by XQuery are as follows:

• The fn:doc function takes a string containing a URI that refers to an XML document, and returns a document node whose content is the data model representation of the given document.

• The fn:collection function returns the nodes found in a collection. A collection may be any sequence of nodes. A collection is identified by a string, which must be a valid URI. For example, the expression fn:collection("http://example.org")//customer identifies all the customer elements that are descendants of nodes found in the collection whose URI is http://example.org.

If a given input function is invoked repeatedly with the same arguments during the scope of a single query or transformation, each invocation returns the same result.

2.4.1 SequenceType

[Definition: When it is necessary to refer to a type in an XQuery expression, the syntax shown below is used. This syntax production is called SequenceType, since it describes the type of an XQuery 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/type namespace. It is a static error [err:XP0008] 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 XQuery 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

2.4.2 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 3.1.5 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 2.2.3 Expression Processing, XQuery defines an analysis phase, which does not depend on input data, and an evaluation phase, which does depend on input data. Errors may be raised during each phase.

[Definition: A static error is an error that must be detected during the analysis phase. A syntax error is an example of a static error. The means by which static errors are reported during the analysis phase is implementation defined. ]

[Definition: A dynamic error is an error that must be detected during the evaluation phase and may be detected during the analysis phase. Numeric overflow is an example of a dynamic error. ]

[Definition: A type error may be raised during the analysis or evaluation phase. During the analysis phase, a type error occurs when the static type of an expression does not match the expected type of the context in which the expression occurs. During the evaluation phase, a type error occurs when the dynamic type of a value does not match the expected type of the context in which the value occurs. ]

The result of the analysis phase is either success or one or more type errors and/or static errors.

The result of the evaluation phase is either a result value, a type error, or a dynamic error. 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 and XPath 2.0 Formal Semantics].

If any expression (at any level) can be evaluated during the analysis phase (because all its explicit operands are known and it has no dependencies on the dynamic context), then any error in performing this evaluation may be reported as a static error. However, the fn:error() function must not be evaluated during the analysis phase. For example, an implementation is allowed (but not required) to treat the following expression as a static error, because it calls a constructor function with a constant string that is not in the lexical space of the target type:

xs:date("Next Tuesday")

[XQuery 1.0 and XPath 2.0 Formal Semantics] defines the set of static, dynamic, and type errors. In addition to these errors, an XQuery 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 the sub-expressions ($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. [Definition: An error value is a single item or the 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 div operator raises an error if its second operand equals zero.

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 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 and XPath 2.0 Formal Semantics]. In such cases, dynamic errors may be raised that would not have been raised 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 raise a casting error if it is evaluated exactly as written (i.e., left to right). 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 raise an error. However, an expression must not be rearranged in a way that causes it to return a result value that is different from the result value defined by [XQuery 1.0 and XPath 2.0 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 and typeswitch expressions are the only expressions that provide guaranteed conditions under which a particular subexpression will not be evaluated.

3.1.1 Literals

[Definition: A literal is a direct syntactic representation of an atomic value.] XQuery 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.

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.

• Ben & Jerry's denotes the string "Ben & Jerry's".

• €99.50 denotes the string "€99.50".

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 declared in a Prolog, in the current module or an imported module. See 4 Modules and Prologs for a discussion of modules and Prologs.

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

3. A variable may be bound by an XQuery expression. The kinds of expressions that can bind variables are FLWOR expressions (3.8 FLWOR Expressions), quantified expressions (3.11 Quantified Expressions), and typeswitch expressions (3.12.2 Typeswitch). Function calls also bind values to the formal parameters of functions before executing the function body.

Every variable binding has a static scope. The scope defines where references to the variable can validly occur. It is a static error [err:XP0016] 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 Context Item Expression

A context item expression evaluates to the context item, which may be either a node (as in the expression fn:doc("bib.xml")//book[count(./author)>1]) or an atomic value (as in the expression (1 to 100)[. mod 5 eq 0]).

3.1.5 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, a static error is raised.[err:XP0017]

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. If the function is a built-in function, it is executed using the converted argument values. The result is a value of the function's declared return type.

4. If the function is a user-declared function, the converted argument values are bound to the formal parameters of the function, and the function body is evaluated. The value returned by the function body is then converted to the declared return type of the function by applying the function conversion rules.

   When a converted argument value is bound to a function parameter, the argument value retains its most specific dynamic type, even though this may be a subtype of the type of the formal parameter. For example, a function with a parameter $p of type xs:decimal can be invoked with an argument of type xs:integer, which is derived from xs:decimal. During the processing of this function invocation, the dynamic type of $p inside the body of the function is considered to be xs:integer. Similarly, the value returned by a function retains its most specific type, which may be a subtype of the declared return type of the function. For example, a function that has a declared return type of xs:decimal may in fact return a value of dynamic type xs:integer.

   A function does not inherit a focus (context item, context position, and context size) from the environment of the function call. During evaluation of a function body, the focus is undefined, except where it is defined by the action of some expression inside the function body. It is a static error [err:XP0018] for an expression to depend on the focus when the focus is undefined.

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

• 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.[err:XP0006] 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 declared in a Prolog, imported from a library module, or provided by the external environment as part of the static context. For details on processing function names that have no namespace prefix, see 4.4 Namespace Declaration.

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.

3.1.6 XQuery Comments

XQuery 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. XQuery 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. If the context item is not a node, a type error is raised.[err:XP0020]

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 Dynamic 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, the path expression section/para is an abbreviation for child::section/child::para, and the path expression section/@id is an abbreviation for child::section/attribute::id. Similarly, section/attribute(@id) is an abbreviation for child::section/attribute::attribute(@id). Note that the latter expression contains both an axis specification and a node test.

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 .. 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

• .//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. A type error [err:XP0006] is raised 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

XQuery 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.[err:XP0006]

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.[err:XQ0004][err:XP0006]

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.[err:XQ0004][err:XP0006]

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 because the two constructed nodes have the same value after atomization, even though they have different identities:

  <a>5</a> eq <a>5</a>
  

• 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.[err:XP0021]

2. 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.[err:XQ0004][err:XP0006]

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"]
  

• The following comparison is false because each constructed node has its own identity:

  <a>5</a> is <a>5</a>
  

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.[err:XQ0004][err:XP0006]

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"]
  

3.12.1 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.

• <a>{5}</a> instance of xs:integer

  This example returns false because the given value is not an integer; instead, it is an element containing an integer.

• <a>{5}</a> instance of element(*, xs:integer)

  This example returns true if the validation process on the constructed element is successful and the schema definition for element a calls for content of type xs:integer.

• . instance of element()

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

3.12.3 Cast

Occasionally it is necessary to convert a value to a specific datatype. For this purpose, XQuery 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/type 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.[err:XQ0004][err:XP0006]

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.[err:XQ0004][err:XP0006]

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 [err:XP0029] 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[err:XP0029] 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.[err:XQ0004][err:XP0006]

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.[err:XP0021] 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.

3.12.4 Castable

XQuery 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

3.12.5 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 that is in a namespace, 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) 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.

User-defined atomic types that are not in a namespace do not have implicit constructor functions. To construct an instance of such a type, it is necessary to use a cast expression. For example, if the user-defined type apple is derived from xs:integer but is not in a namespace, an instance of this type can be constructed as follows:

17 cast as apple

3.12.6 Treat

XQuery 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:

  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:

  If expr1 matches type1, using the SequenceType Matching rules in 2.4.1 SequenceType, the treat expression returns the value of expr1; otherwise, it raises a dynamic error.[err:XP0006] 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.[err:XP0050]