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 .

The individual components of the static context are summarized below. A default initial value for each component may be specified by the host language. The scope of each component is specified in .

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 .

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

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

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 items are being processed by the expression.

Certain language constructs, notably the path expression E1/E2 and the predicate 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.

As described in , XPath defines a static analysis phase, which does not depend on input data, and a dynamic evaluation phase, which does depend on input data. Errors may be raised during each phase.

A static error is an error that must be detected during the static analysis phase. A syntax error is an example of a static error.

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

A type error may be raised during the static analysis phase or the dynamic evaluation phase. During the static 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 dynamic 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 outcome of the static analysis phase is either success or one or more type errors, static errors, or statically-detected dynamic errors. The result of the dynamic evaluation phase is either a result value, a type error, or a dynamic error.

If more than one error is present, or if an error condition comes within the scope of more than one error defined in this specification, then any non-empty subset of these errors may be reported.

During the static analysis phase, if the Static Typing Feature is in effect and the static type assigned to an expression other than () or data(()) is empty-sequence(), a static error is raised . This catches cases in which a query refers to an element or attribute that is not present in the in-scope schema definitions, possibly because of a spelling error.

Independently of whether the Static Typing Feature is in effect, if an implementation can determine during the static analysis phase that an expression, if evaluated, would necessarily raise a type error or a dynamic error, the implementation may (but is not required to) report that error during the static analysis phase. However, the fn:error() function must not be evaluated during the static analysis phase.

In addition to static errors, dynamic errors, and type errors, an XPath implementation may raise warnings, either during the static analysis phase or the dynamic 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 for a reason beyond the scope of this specification. For example, limitations may exist on the maximum numbers or sizes of various objects. Any such limitations, and the consequences of exceeding them, are implementation-dependent.

The errors defined in this specification are identified by QNames that have the form err:XPYYnnnn, where:

The method by which an XPath processor reports error information to the external environment is implementation-defined.

An error can be represented by a URI reference that is derived from the error QName as follows: an error with namespace URI NS and local part LP can be represented as the URI reference NS#LP. For example, an error whose QName is err:XPST0017 could be represented as http://www.w3.org/2005/xqt-errors#XPST0017.

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:

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.

In addition to its identifying QName, a dynamic error may also carry a descriptive string and one or more additional values called error values. An implementation may provide a mechanism whereby an application-defined error handler can process error values and produce diagnostic messages.

A dynamic error may be raised by a built-in function or operator. For example, the div operator raises an error if its operands are xs:decimal values and its second operand is equal to zero. Errors raised by built-in functions and operators are defined in .

A dynamic error can also be raised explicitly by calling the fn:error function, which only raises an error and never returns a value. This function is defined in . For example, the following function call raises a dynamic error, providing a QName that identifies the error, a descriptive string, and a diagnostic value (assuming that the prefix app is bound to a namespace containing application-defined error codes):

Because different implementations may choose to evaluate or optimize an expression in different ways, certain aspects of the detection and reporting of dynamic errors are implementation-dependent, as described in this section.

An implementation is always free to evaluate the operands of an operator in any order.

In some cases, a processor can determine the result of an expression without accessing all the data that would be implied by the formal expression semantics. For example, the formal description of filter expressions suggests that $s[1] should be evaluated by examining all the items in sequence $s, and selecting all those that satisfy the predicate position()=1. In practice, many implementations will recognize that they can evaluate this expression by taking the first item in the sequence and then exiting. If $s is defined by an expression such as //book[author eq 'Berners-Lee'], then this strategy may avoid a complete scan of a large document and may therefore greatly improve performance. However, a consequence of this strategy is that a dynamic error or type error that would be detected if the expression semantics were followed literally might not be detected at all if the evaluation exits early. In this example, such an error might occur if there is a book element in the input data with more than one author subelement.

The extent to which a processor may optimize its access to data, at the cost of not detecting errors, is defined by the following rules.

Consider an expression Q that has an operand (sub-expression) E. In general the value of E is a sequence. At an intermediate stage during evaluation of the sequence, some of its items will be known and others will be unknown. If, at such an intermediate stage of evaluation, a processor is able to establish that there are only two possible outcomes of evaluating Q, namely the value V or an error, then the processor may deliver the result V without evaluating further items in the operand E. For this purpose, two values are considered to represent the same outcome if their items are pairwise the same, where nodes are the same if they have the same identity, and values are the same if they are equal and have exactly the same type.

There is an exception to this rule: If a processor evaluates an operand E (wholly or in part), then it is required to establish that the actual value of the operand E does not violate any constraints on its cardinality. For example, the expression $e eq 0 results in a type error if the value of $e contains two or more items. A processor is not allowed to decide, after evaluating the first item in the value of $e and finding it equal to zero, that the only possible outcomes are the value true or a type error caused by the cardinality violation. It must establish that the value of $e contains no more than one item.

These rules apply to all the operands of an expression considered in combination: thus if an expression has two operands E1 and E2, it may be evaluated using any samples of the respective sequences that satisfy the above rules.

The rules cascade: if A is an operand of B and B is an operand of C, then the processor needs to evaluate only a sufficient sample of B to determine the value of C, and needs to evaluate only a sufficient sample of A to determine this sample of B.

The effect of these rules is that the processor is free to stop examining further items in a sequence as soon as it can establish that further items would not affect the result except possibly by causing an error. For example, the processor may return true as the result of the expression S1 = S2 as soon as it finds a pair of equal values from the two sequences.

Another consequence of these rules is that where none of the items in a sequence contributes to the result of an expression, the processor is not obliged to evaluate any part of the sequence. Again, however, the processor cannot dispense with a required cardinality check: if an empty sequence is not permitted in the relevant context, then the processor must ensure that the operand is not an empty sequence.

Examples:

An ordering called document order is defined among all the nodes accessible during processing of a given expression, which may consist of one or more trees (documents or fragments). Document order is defined in , and its definition is repeated here for convenience. 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. Informally, document order is the order in which nodes appear in the XML serialization of a document. Document order is stable, which means that the relative order of two nodes will not change during the processing of a given expression, even if this order is implementation-dependent.

Within a tree, document order satisfies the following constraints:

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

The semantics of some XPath operators depend on a process called atomization. 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 [err:FOTY0012]. Atomization of a sequence is defined as the result of invoking the fn:data function on the sequence, as defined in .

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:

Atomization is used in processing the following types of expressions:

Under certain circumstances (listed below), it is necessary to find the effective boolean value of a value. The effective boolean value of a value is defined as the result of applying the fn:boolean function to the value, as defined in .

The dynamic semantics of fn:boolean are repeated here for convenience:

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

XPath 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 .

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

The input functions supported by XPath are as follows:

The in-scope schema types in the static context are initialized with a set of predefined schema types that is determined by the host language. This set may include some or all of the schema types in the namespace http://www.w3.org/2001/XMLSchema, represented in this document by the namespace prefix xs. The schema types in this namespace are defined in and augmented by additional types defined in . The schema types defined in are summarized below.

The relationships among the schema types in the xs namespace are illustrated in Figure 2. A more complete description of the XPath type hierarchy can be found in .

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 string value of a node is a string and can be extracted by applying the fn:string function to the node. Definitions of fn:data and fn:string can be found in .

An implementation may store both the typed value and the string value of a node, or it may store only one of these and derive the other as needed. The string value of a node must be a valid lexical representation of the typed value of the node, but the node is not required to preserve the string representation from the original source document. For example, if the typed value of a node is the xs:integer value 30, its string value might be "30" or "0030".

The typed value, string value, and type annotation of a node are closely related. If the node was created by mapping from an Infoset or PSVI, the relationships among these properties are defined by rules in .

As a convenience to the reader, the relationship between typed value and string value for various kinds of nodes is summarized and illustrated by examples below.

Whenever it is necessary to refer to a type in an XPath expression, the SequenceType syntax is used.

With the exception of the special type empty-sequence(), a sequence type consists of an item type that constrains the type of each item in the sequence, and a cardinality that constrains the number of items in the sequence. Apart from the item type item(), which permits any kind of item, item types divide into node types (such as element()) and atomic types (such as xs:integer).

Item types representing element and attribute nodes may specify the required type annotations of those nodes, in the form of a schema type. Thus the item type element(*, us:address) denotes any element node whose type annotation is (or is derived from) the schema type named us:address.

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

During evaluation of an expression, it is sometimes necessary to determine whether a value with a known dynamic type "matches" an expected sequence type. This process is known as SequenceType matching. For example, an instance of expression returns true if the dynamic type of a given value matches a given sequence type, or false if it does not.

QNames appearing in a sequence type have their prefixes expanded to namespace URIs by means of the statically known namespaces and (where applicable) the default element/type namespace. An unprefixed attribute QName is in no namespace. Equality of QNames is defined by the eq operator.

The rules for SequenceType matching compare the dynamic type of a value with an expected sequence type. These rules are a subset of the formal rules that match a value with an expected type defined in , because the Formal Semantics must be able to match values against types that are not expressible using the SequenceType syntax.

Some of the rules for SequenceType matching require determining whether a given schema type is the same as or derived from an expected schema type. The given schema type may be "known" (defined in the in-scope schema definitions), or "unknown" (not defined in the in-scope schema definitions). An unknown schema type 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 schema type is derived from the expected schema type. For example, an implementation might maintain a data dictionary containing information about type hierarchies.

The use of a value whose dynamic type is derived from an 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 definition of SequenceType matching relies on a pseudo-function named derives-from(AT, ET), which takes an actual simple or complex schema type AT and an expected simple or complex schema type ET, and either returns a boolean value or raises a type error . The pseudo-function derives-from is defined below and is defined formally in .

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

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

The value of a numeric literal containing no "." and no e or E character is an atomic value of type xs:integer. The value of a numeric literal containing "." but no e or E character is an atomic value of type xs:decimal. The value of a numeric literal containing an e or E character is an atomic value of type xs:double. The value of the numeric literal is determined by casting it to the appropriate type according to the rules for casting from xs:untypedAtomic to a numeric type as specified in .

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:

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

Values of other atomic types can be constructed by calling the constructor function for the given type. The constructor functions for XML Schema built-in types are defined in . 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:

Constructor functions can also be used to create special values that have no literal representation, as in the following examples:

xs:float("NaN") returns the special floating-point value, "Not a Number."

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

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 statically known 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:

The in-scope variables may be augmented by implementation-defined variables.

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

If a variable reference matches two or more variable 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.

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 .

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

If the context item is undefined, a context item expression raises a dynamic error .

The built-in functions supported by XPath are defined in . Additional functions may be provided in the static context. XPath per se does not provide a way to declare functions, but a host language may provide such a mechanism.

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 a function signature in the static context, a static error is raised .

A function call is evaluated as follows:

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

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:

A step is a part of a path expression that 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, working from left to right. A step may be either an axis step or a filter expression. Filter expressions are described in .

An axis step returns a sequence of 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 annotation. If the context item is a node, an axis step returns a sequence of zero or more nodes; otherwise, a type error is raised . The resulting node sequence is returned in document order. An axis step may be either a forward step or a reverse step, followed by zero or more predicates.

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

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 annotation 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 . The available node tests are described in . Examples of steps are provided in and .

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. In the case of multiple adjacent predicates, the predicates are applied from left to right, and the result of applying each predicate serves as the input sequence for the following predicate.

For each item in the input sequence, the predicate expression is evaluated using an inner focus, defined as follows: The context item is the item currently being tested against the predicate. The context size is the number of items in the input sequence. The context position is the position of the context item within the input sequence. For the purpose of evaluating the context position within a predicate, the input sequence is considered to be sorted as follows: into document order if the predicate is in a forward-axis step, into reverse document order if the predicate is in a reverse-axis step, or in its original order if the predicate is not in a step.

For each item in the input sequence, the result of the predicate expression is coerced to an xs: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:

Here are some examples of axis steps that contain predicates:

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 .

The abbreviated syntax permits the following abbreviations:

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

One way to construct a sequence is by using the comma operator, which evaluates each of its operands and concatenates the resulting sequences, in order, into a single result sequence. Empty parentheses can be used to denote an empty sequence.

A sequence may contain duplicate atomic 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:

A range expression 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?. If either operand is an empty sequence, or if the integer derived from the first operand is greater than the integer derived from the second operand, the result of the range expression is an empty sequence. If the two operands convert to the same integer, the result of the range expression is that integer. Otherwise, the result is a sequence containing the two integer operands and every integer between the two operands, in increasing order.

A filter expression consists simply of a primary expression followed by zero or more predicates. The result of the filter expression consists of the items returned by the primary expression, filtered by applying each predicate in turn, working from left to right. If no predicates are specified, the result is simply the result of the primary expression. The ordering of the items returned by a filter expression is the same as their order in the result of the primary expression. Context positions are assigned to items based on their ordinal position in the result sequence. The first context position is 1.

Here are some examples of filter expressions:

XPath provides the following operators for combining sequences of nodes:

All these operators eliminate duplicate nodes from their result sequences based on node identity. The resulting sequence is returned in document order.

If an operand of union, intersect, or except contains an item that is not a node, a type error is raised .

Here are some examples of expressions that combine sequences. Assume the existence of three element nodes that we will refer to by symbolic names A, B, and C. Assume that the variables $seq1, $seq2 and $seq3 are bound to the following sequences of these nodes:

Then:

In addition to the sequence operators described here, 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 items from a sequence.

The value comparison operators are eq, ne, lt, le, gt, and ge. Value comparisons are used for comparing single values.

The first step in evaluating a value comparison is to evaluate its operands. The order in which the operands are evaluated is implementation-dependent. Each operand is evaluated by applying the following steps, in order:

Next, if possible, the two operands are converted to their least common type by a combination of type promotion and subtype substitution. For example, if the operands are of type hatsize (derived from xs:integer) and shoesize (derived from xs:float), their least common type is xs:float.

Finally, if the types of the operands are a valid combination for the given operator, the operator is applied to the operands. The combinations of atomic types that are accepted by the various value comparison operators, and their respective result types, are listed in together with the operator functions that define the semantics of the operator for each type combination. The definitions of the operator functions are found in .

Informally, if both atomized operands consist of exactly one atomic value, then 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.

If the types of the operands, after evaluation, are not a valid combination for the given operator, according to the rules in , a type error is raised .

Here are some examples of value comparisons:

The general comparison operators are =, !=, <, <=, >, and >=. 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.

If XPath 1.0 compatibility mode is true, a general comparison is evaluated by applying the following rules, in order:

If XPath 1.0 compatibility mode is false, a general comparison is evaluated by applying the following rules, in order:

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 that have the required magnitude relationship. 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:

Node comparisons are used to compare two nodes, by their identity or by their document order. The result of a node comparison is defined by the following rules:

Here are some examples of node comparisons:

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

Occasionally it is necessary to convert a value to a specific datatype. For this purpose, XPath provides a cast expression that creates a new value of a specific type based on an existing value. A cast expression takes two operands: an input expression and a target type. The type of the input expression is called the input type. The target type must be an atomic type that is in the in-scope schema types . In addition, the target type cannot be xs:NOTATION or xs:anyAtomicType . The optional occurrence indicator "?" denotes that 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:

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:FORG0001] 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.

XPath provides an expression that tests whether a given value is castable into a given target type. The target type must be an atomic type that is in the in-scope schema types . In addition, the target type cannot be xs:NOTATION or xs:anyAtomicType . The optional occurrence indicator "?" denotes that an empty sequence is permitted.

The expression E castable as T returns true if the result of evaluating E can be successfully cast into the target type T by using a cast expression; otherwise it returns false. If evaluation of E fails with a dynamic error, the castable expression as a whole fails. The castable expression can be used as a predicate 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:

For every atomic type in the in-scope schema types (except xs:NOTATION and xs:anyAtomicType, which are not instantiable), a constructor function is implicitly defined. In each case, the name of the constructor function is the same as the name of its target type (including namespace). The signature of the constructor function for type T is as follows:

The constructor function for a given type is used to convert instances of other atomic types into the given type. The semantics of the constructor function call T($arg) are defined to be equivalent to the expression (($arg) cast as T?).

The constructor functions for xs:QName and for types derived from xs:QName and xs:NOTATION require their arguments to be string literals or to have a base type that is the same as the base type of the target type; otherwise a type error is raised. This rule is consistent with the semantics of cast expressions for these types, as defined in .

The following examples illustrate the use of constructor functions:

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

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

The semantics of expr1 treat as type1 are as follows: