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: Statically known namespaces. This is a set of (prefix, URI) pairs that define all the namespaces that are known during static processing of a given expression.] Note the difference between in-scope namespaces, which is a dynamic property of an element node, and statically known namespaces, which is a static property of an expression.

  Some namespaces are predefined; additional namespaces can be added to the statically known namespaces by namespace declarations in a Prolog, by namespace declaration attributes in direct element constructors, and by local namespace declarations in computed element constructors.

• [Definition: Default element/type namespace. This is a namespace URI or "none". The namespace URI, if present, 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 a Prolog.

• [Definition: Default function namespace. This is a namespace URI that 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 a Prolog.]

• [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. Each named type definition is identified either by an expanded QName (for a named type) or by an implementation-dependent type identifier (for an anonymous type). The in-scope type definitions include the predefined types described in 2.4.1 Predefined Types. If the Schema Import Feature is supported, in-scope type definitions also include all type definitions found in imported schemas. ]

  • [Definition: In-scope element declarations. Each element declaration is identified either by an expanded QName (for a top-level element declaration) or by an implementation-dependent element identifier (for a local element declaration). 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 element's substitution group affiliation.

    [Definition: Substitution groups are defined in [XML Schema] Part 1, Section 2.2.2.2. Informally, the substitution group headed by a given element (called the head element) consists of the set of elements that can be substituted for the head element without affecting the outcome of schema validation.]

  • [Definition: In-scope attribute declarations. Each attribute declaration is identified either by an expanded QName (for a top-level attribute declaration) or by an implementation-dependent attribute identifier (for a local attribute declaration). 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 (expanded QName, type) pairs. It defines the set of variables that are available for reference within an expression. The expanded QName is the name of the variable, and the type is the static type of the variable.]

  Variable declarations in a Prolog are added to in-scope variables. 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.

  The static type of a variable may be either declared in a query or (if the Static Typing Feature is enabled) inferred by static type inference rules as described in [XQuery 1.0 and XPath 2.0 Formal Semantics].

• Context item static type. This component defines the static type of the context item within the scope of a given expression.

• [Definition: Function signatures. 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).] In addition to the name and arity, each function signature specifies the static types of the function parameters and result.

  The function signatures include the signatures of constructor functions, which are discussed in 3.12.5 Constructor Functions.

• [Definition: Statically known 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 identifies one of the collations in statically known collations as the collation to be used by string comparison functions and operators when no explicit collation is specified.]

• [Definition: Construction mode. The construction mode governs the behavior of element constructors. If construction mode is preserve, the type of a constructed element node is xs:anyType, and the attributes and descendants of the constructed element retain their original types. If construction mode is strip, the type of the constructed element node and all its descendants is xdt:untyped, and attributes of the constructed element have type xdt:untypedAtomic.]

• [Definition: Ordering mode. Ordering mode, which has the value ordered or unordered, affects the ordering of the result sequence returned by path expressions, union, intersect, and except expressions, and FLWOR expressions that have no order by clause.] Details are provided in the descriptions of these expressions.

• [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 available using the fn:doc function. The type is the static type of a call to fn:doc with the given URI as its literal argument. ] If the argument to fn:doc is not a string literal that is present in statically known documents, then the static type of fn:doc is document-node()?.

  Note:

  The purpose of the statically known documents is to provide static type information, not to determine which documents are available. A URI need not be found in the statically known documents to be accessed using fn:doc.

• [Definition: Statically known collections. This is a mapping from strings onto types. The string represents the absolute URI of a resource that is potentially available 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 not a string literal that is present in statically known collections, then the static type of fn:collection is node()*.

  Note:

  The purpose of the statically known collections is to provide static type information, not to determine which collections are available. A URI need not be found in the statically known collections to be accessed using fn:collection.

• XQuery Flagger status. This component has the value "on" if the XQuery Flagger is implemented and enabled; otherwise it has the value "off".

• XQuery Static Flagger status. This component has the value "on" if the XQuery Static Flagger is implemented and enabled; otherwise it has the value "off".

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 filter 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. 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.] 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.] Its value is always an integer greater than zero. The context size is returned by the expression fn: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: Variable values. This is a set of (expanded QName, value) pairs. It contains the same expanded 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: Function implementations. Each function in function signatures has a function implementation that 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.]

• [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 return 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: Available documents. This is a mapping of strings onto document nodes. The string represents the absolute URI of a resource. The document node is the root of a tree that represents that resource using the data model. The document node is returned by the fn:doc function when applied to that URI.] The set of available documents is not constrained by the set of statically known documents, and it may be empty.

• [Definition: Available 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 available collections is not constrained by 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. This process occurs outside the domain of XQuery, which is why Figure 1 represents it in the external processing domain. Here are some steps by which an XML document might 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 document in the data model might be constructed. A document or fragment 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 documents and instances in the data model are 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 that has not been validated (such as might occur in a schemaless document) is annotated with the type xdt:untyped. An element that has been validated and found to be partially valid is annotated with the type xs:anyType. If an element node is annotated xdt:untyped, all its descendant element nodes are also annotated xdt:untyped. However, if an element node is annotated xs:anyType, some of its descendant element nodes may have a more specific type annotation.

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). An implementation is free to use any strategy or algorithm whose result conforms to these specifications.

2.2.4 Serialization

[Definition: Serialization is the process of converting a sequence of nodes and atomic values from the 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 XQuery to be well defined, the data model, the static context, and the 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. This specification does not define the result of a query under any condition in which one or more of these constraints is not satisfied.

Some of the consistency constraints use the term data model schema. [Definition: For a given node in the data model, the data model schema is defined as the schema from which the type annotation of that node was derived.] For a node that was constructed by some process other than schema validation, the data model schema consists simply of the type definition that is represented by the type annotation of the node.

• For every data model node that has a type annotation, if that type annotation is found in the in-scope schema definitions (ISSD), then its definition in the ISSD must be the same as its definition in the data model schema. Furthermore, all types that are derived by extension from the given type in the data model schema must also be known by equivalent definitions in the ISSD.

• For every element name EN that is found both in a data model node and in the in-scope schema definitions (ISSD), all elements that are known in the data model schema to be in the substitution group headed by EN must also be known in the ISSD to be in the substitution group headed by EN.

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

• For each mapping of a string to a document node in available 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 2.4.4 SequenceType Matching.

• For each mapping of a string to a sequence of nodes in available 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 2.4.4 SequenceType Matching.

• For each (variable, type) pair in in-scope variables and the corresponding (variable, value) pair in variable values such that the variable names are equal, the value must match the type, using the matching rules in 2.4.4 SequenceType Matching.

• For each variable declared as external: If the variable declaration includes a declared type, the external environment must provide a value for the variable that matches the declared type, using the matching rules in 2.4.4 SequenceType Matching. If the variable declaration does not include a declared type, the external environment must provide a type and a matching value, using the same matching rules.

• For a given query, define a participating ISSD as the in-scope schema definitions of a module that is used in evaluating the query. If two participating ISSDs contain a definition for the same type name, element name, or attribute name, the definitions must be equivalent in both ISSDs. Furthermore, if two participating ISSDs each contain a definition of a type name T, the set of types derived by extension from T must be equivalent in both ISSDs. Also, if two participating ISSDs each contain a definition of an element name E, the substitution group headed by E must be equivalent in both ISSDs.

2.3.1 Document Order

An ordering called document order is defined among all the nodes used during a given query or transformation, which may consist of one or more trees (documents or fragments). Document order is defined in [XQuery 1.0 and XPath 2.0 Data Model], and its definition is repeated here for convenience. [Definition: The node ordering that is the reverse of document order is called reverse document order.]

Document order is a total ordering, although the relative order of some nodes is implementation-dependent. [Definition: Informally, document order is the order defined by a pre-order, depth-first traversal of the nodes in the data model.] Document order is stable, which means that the relative order of two nodes will not change during the processing of a given query or transformation, even if this order is implementation-dependent.

Within a tree, document order satisfies the following constraints:

1. The root node is the first node.

2. The relative order of siblings is determined by their order in the XML representation of the tree. A node N1 occurs before a node N2 in document order if and only if the start of N1 occurs before the start of N2 in the XML representation.

3. Attribute nodes immediately follow the element node with which they are associated. The relative order of attribute nodes is stable but implementation-dependent.

4. Element nodes occur before their children; children occur before following-siblings.

The relative order of nodes in distinct trees is stable but implementation-dependent, subject to the following constraint: If any node in tree T1 is before any node in tree T2, then all nodes in tree T1 are before all nodes in tree T2.

2.3.2 Atomization

The semantics of some XQuery operators depend on a process called atomization. [Definition: Atomization is applied to a value when the value is used in a context in which a sequence of atomic values is required. The result of atomization is either a sequence of atomic values or a type error. Atomization of a sequence is defined as the result of invoking the fn:data function on the sequence, as defined in [XQuery 1.0 and XPath 2.0 Functions and Operators].]

The semantics of fn:data are repeated here for convenience. The result of fn:data is the sequence of atomic values produced by applying the following rules to each item in the input sequence:

• If the item is an atomic value, it is returned.

• If the item is a node, its typed value is returned.

Atomization is used in processing the following types of expressions:

• Arithmetic expressions

• Comparison expressions

• Function calls and returns

• Cast expressions

• Computed element and attribute constructors.

2.3.3 Effective Boolean Value

Under certain circumstances (listed below), it is necessary to find the effective boolean value of a value. [Definition: The effective boolean value of a value is defined as the result of applying the fn:boolean function to the value, as defined in [XQuery 1.0 and XPath 2.0 Functions and Operators].]

The semantics of fn:boolean are repeated here for convenience. fn:boolean returns false if its operand is any of the following:

• An empty sequence

• The boolean value false

• A zero-length value of type xs:string or xdt:untypedAtomic

• A numeric value that is equal to zero

• The xs:double or xs:float value NaN

Otherwise, fn:boolean returns true.

The effective boolean value of a sequence is computed implicitly during processing of the following types of expressions:

• Logical expressions (and, or)

• The fn:not function

• The where clause of a FLWOR expression

• Certain types of predicates, such as a[b]

• Conditional expressions (if)

• Quantified expressions (some, every)

2.3.4 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; they are defined in [XQuery 1.0 and XPath 2.0 Functions and Operators].

An expression can access input data 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 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 takes a string containing a URI, and returns the data model representation of the collection identified by the URI. A collection may be any sequence of nodes. 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 arguments that resolve to the same absolute URI during the scope of a single query or transformation, each invocation returns the same result.

2.4.1 Predefined Types

1. [Definition: xdt:untyped is used to denote the dynamic type of an element node that has not been validated, or has been validated in skip mode.] It has no subtypes.

2. [Definition: xdt:untypedAtomic is used to denote untyped atomic data, such as text that has not been assigned a more specific type.] It has no subtypes. An attribute that has been validated in skip mode, or that has a PSVI property of xs:anySimpleType, is represented in the Data Model by an attribute node with the type xdt:untypedAtomic.

3. [Definition: xdt:dayTimeDuration is a subtype of xs:duration whose lexical representation is restricted to contain only day, hour, minute, and second components.]

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

5. [Definition: xdt:anyAtomicType 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.

   Note:

   xdt:anyAtomicType will not appear as the type of an actual value in the Data Model.

The relationships among the types in the xs and xdt namespaces are illustrated in Figure 2. A more complete description of the XQuery type hierarchy can be found in [XQuery 1.0 and XPath 2.0 Functions and Operators].

Figure 2: Summary of XQuery Type Hierarchy

2.4.2 Typed Value and String Value

In the data model, every node has a typed value and a string value. 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]. The string value of a node is a string and can be extracted by applying 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]. Element and attribute nodes have a type annotation, which represents (in an implementation-dependent way) the dynamic (run-time) type of the node. In the [XQuery 1.0 and XPath 2.0 Data Model], type annotation is defined by the dm:type-name accessor; however, 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 and document nodes, the typed value of the node is the same as its string value, as an instance of the type 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 a comment or processing instruction node is the same as its string value. It is an instance of the type xs:string.

3. 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 using the lexical-to-value-space mapping defined in [XML Schema] Part 2 for the relevant type.

   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 xs:IDREFS, which is a list datatype derived from the atomic datatype xs: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 xs: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 xs:IDREFS), its typed value is treated as a sequence of the atomic type from which it is derived (such as xs:IDREF).

4. 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 xdt:untyped or denotes a complex type with mixed content (including xs:anyType), 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 xdt:untyped 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 from the atomic 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 and its string value is the zero-length string.

   4. If the type annotation denotes a complex type with element-only 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 element-only. 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.

2.4.3 SequenceType Syntax

[Definition: When it is necessary to refer to a type in an XQuery expression, the SequenceType syntax is used. The name SequenceType suggests that this syntax is used to describe the type of an XQuery value, which is always a sequence.]

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

• xs:date refers to the built-in atomic Schema type named xs: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 and has the type annotation po:address (or a type derived from po:address)

• element(*, po:address) refers to an element of any name that has the type annotation po:address (or a type derived from po:address)

• element(customer) refers to an element named customer of any type

• schema-element(customer) refers to an element named customer whose type annotation matches the type declared for a customer element in the in-scope element declarations

• 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 SequenceType Matching

[Definition: During evaluation of an expression, it is sometimes necessary to determine whether a value with a known type "matches" an expected type, expressed in the SequenceType syntax. This process is known as SequenceType matching.] For example, an instance of expression returns true if the actual type of a given value matches a given type, or false if it does not.

QNames appearing in a SequenceType have their prefixes expanded to namespace URIs by means of the statically known namespaces and the default element/type namespace. As usual, two QNames are considered to be equal if their local parts are the same and their namespace URI's are the same.

[Definition: The use of a value whose actual type is derived from the expected type is known as subtype substitution.] Subtype substitution does not change the actual type of a value. For example, if an xs:integer value is used where an xs:decimal value is expected, the value retains its type as xs:integer.

The rules for SequenceType matching compare the actual type of a value with an expected type. These rules are a subset of the formal rules that match a value with an expected type defined in [XQuery 1.0 and XPath 2.0 Formal Semantics], because the Formal Semantics must be able to match a value with any XML Schema type, whereas the rules below only match values against those types expressible by the SequenceType syntax.

Some of the rules for SequenceType matching require determining whether a given type name is the same as or derived from an expected type name. The given type name may be "known" (defined in the in-scope schema definitions), or "unknown" (not defined in the in-scope schema definitions). An unknown type name might be encountered, for example, if a source document has been validated using a schema that was not imported into the static context. In this case, an implementation is allowed (but is not required) to provide an implementation-dependent mechanism for determining whether the unknown type name is derived from the expected type name. For example, an implementation might maintain a data dictionary containing information about type hierarchies.

The definition of SequenceType matching relies on a pseudo-function named derives-from(AT, ET), which takes an an actual simple or complex type name AT and an expected simple or complex type name ET, and either returns a boolean value or raises a type error. [err:XP0004][err:XP0006] The pseudo-function derives-from is defined below and is defined formally in [XQuery 1.0 and XPath 2.0 Formal Semantics].

• derives-from(AT, ET) returns true if any of the following three conditions is true:

  1. AT is a type name found in the in-scope schema definitions, and is the same as ET or is derived by restriction or extension from ET

  2. AT is a type name not found in the in-scope schema definitions, and an implementation-dependent mechanism is able to determine that AT is derived by restriction from ET

  3. There exists some type name IT such that derives-from(IT, ET) and derives-from(AT, IT) are true.

• derives-from(AT, ET) returns false if either the first and third or the second and third of the following conditions are true:

  1. AT is a type name found in the in-scope schema definitions, and is not the same as ET, and is not is derived by restriction or extension from ET

  2. AT is a type name not found in the in-scope schema definitions, and an implementation-dependent mechanism is able to determine that AT is not derived by restriction from ET

  3. No type name IT exists such that derives-from(IT, ET) and derives-from(AT, IT) are true.

• derives-from(AT, ET) raises a type error [err:XP0004][err:XP0006] if:

  1. ET is an unknown type, or

  2. AT is an unknown type, and the implementation is not able to determine whether AT is derived by restriction from ET.

The rules for SequenceType matching are given below, with examples (the examples are for purposes of illustration, and do not cover all possible cases).