This section outlines a number of general concepts that apply throughout this specification.
In this document, examples and material labeled as “Note” are provided for explanatory purposes and are not normative.
Every value manipulated by XPath, XQuery, or XSLT is a sequence comprising zero or more items.
[Definition: A sequence type constrains the set of permitted sequences, by defining the permitted item types and the permitted number of items in the sequence (exactly zero, exactly one, zero-or-more, one-or-more, zero-or-one).]
Every item is an instance of one or more item types:
All items are instances of the type item().
Every node is an instance of the type node(), and more specifically it is an instance of one of seven node kinds: document(), element(*), attribute(*), text(), comment(), processing-instruction(), or namespace(). Nodes may also be instances of more specific types characterized by the node name and type annotation.
Every atomic item is an instance of a specific atomic type determined by its type annotation; it is also an instance of every type from which that type is derived by restriction (directly or indirectly), and of every union type that includes that type as a member type.
Every function item is an instance of the generic type function(*), and also of a specific function type defining the types of the function's parameters and the type of the result.
A map item, as well as being a function, is also an instance of the generic map type map(*), of more specific map types map(K, V) defining the types of the keys and values, and perhaps of one or more record types that associate a type with specific key values.
An array item, as well as being a function, is also an instance of the generic array type array(*), and also of more specific array types array(M) defining the type of the array's members.
This section describes how item types relate to each other.
The diagrams below show how nodes, functions, primitive simple types, and user defined types fit together into a type system. In the diagrams, connecting lines represent relationships between derived types and the types from which they are derived; the latter are always higher and to the left of the latter.
The xs:IDREFS, xs:NMTOKENS, xs:ENTITIES types, and xs:numeric, and both the user-defined list types and user-defined union types are special types in that these types are lists or unions rather than types derived by extension or restriction.
The first diagram illustrates the relationship of various item types. Item types in the data model form a directed graph, rather than a hierarchy or lattice: in the relationship defined by the derived-from(A, B) function, some types are derived from more than one other type. Examples include functions (function(xs:string) as xs:int is substitutable for function(xs:NCName) as xs:int and also for function(xs:string) as xs:decimal), and union types (A is substitutable for the union type (A | B) and also for the union type (A | C)). In XDM, item types include node types, function types, and built-in atomic types. The list, which shows only hierarchic relationships, is therefore a simplification of the full model.
The XPath Data Model is the abstraction over which XPath expressions are evaluated. Historically, all of the items in the data model could be derived directly (nodes) or indirectly (typed values, sequences) from an XML document. However, as the XPath expression language has matured, new features have been added which require additional types of items to appear in the data model. These items have no direct XML serialization, but they are never the less part of the data model.
The next diagram shows all of the atomic types, including the primitive simple types and the built-in types derived from the primitive simple types. This includes all the built-in datatypes defined in [Schema Part 2]. Atomic types act both as item types (meaning they can be used to declare the types of variables and function arguments), and as schema types (meaning they can be used as type annotations on nodes).
Changes in 4.0 ⬇ ⬆
Constructors are added, and the single accessor function is now an iterator over the key/value pairs in the map. [Issue 1335 20 July 2024]
Ordered maps are introduced. [Issue 1651 PR 1703 14 January 2025]
[Definition: A map item (also called simply a map) is an item that represents an ordered sequence of key/value pairs, in which the keys are unique.] In other languages this is sometimes called a hash, dictionary, or associative array. The keys are atomic items, and each key in the map is unique (there is no other key to which it is equal). Each key is associated with a value that may be any sequence of zero or more items. There is no uniqueness constraint on values, only on keys. The semantics of equality when comparing keys are described in Section 13.2.1 fn:atomic-equalFO.
[Definition: The key/value pairs in a map are referred to as entries.]
[Definition: A map containing exactly one entry is referred to as a single-entry map.][Definition: A map containing exactly one entry is referred to as a single-entry map.][Definition: A map containing exactly one entry is referred to as a single-entry map.]
[Definition: A map containing no entries is referred to as an empty map.][Definition: A map containing no entries is referred to as an empty map.][Definition: A map containing no entries is referred to as an empty map.]
Note:
Maps have no intrinsic identity separate from their content. A map can be given a transient identity, represented by an id property in its label, by applying the fn:pin function. This property is expected to be used in defining operations for deep update of maps.
[Definition: The order of entries in a map is referred to as entry order.] The entry order affects the result of functions such as map:keys and map:for-each, and also determines the order of entries when a map is serialized using the JSON output method.
Constructor and accessor functions for maps are defined in the following sections.
The dm:empty-map constructor returns an entry, that is, a map containing no key/value pairs.The dm:empty-map constructor returns an empty map, that is, a map containing no key/value pairs.The dm:empty-map constructor returns an entryempty map, that is, a map containing no key/value pairs.
Note:
In XPath an entry may be constructed using the expression {} or map {}.In XPath an empty map may be constructed using the expression {} or map {}.In XPath an entryempty map may be constructed using the expression {} or map {}.
Changes in 4.0 ⬇ ⬆
Constructors are added, and the single accessor function is now an iterator over the members of the array. [Issue 1335 20 July 2024]
[Definition: An array item (also called simply an array) is a value that represents an array.] [Definition: An array is an ordered list of values; these values are called the members of the array.] Unlike sequences, a member of an array can be any value (including a sequence or an array). The number of members in an array is called its size, and they are referenced by their position, in the range 1 to the size of the array.
[Definition: An array containing exactly one member is referred to as a single-member array.][Definition: An array containing exactly one member is referred to as a single-member array.][Definition: An array containing exactly one member is referred to as a single-member array.]
[Definition: An array containing no members is referred to as an empty array.][Definition: An array containing no members is referred to as an empty array.][Definition: An array containing no members is referred to as an empty array.]
Note:
Arrays have no intrinsic identity separate from their content. An array can be given a transient identity, represented by an id property in its label, by applying the fn:pin function. This property is expected to be used in defining operations for deep update of arrays.
Constructor and accessor functions for arrays are defined in the following sections.
dm:empty-array() as array(*)
The dm:empty-array constructor returns an entry, that is, an array item containing no members.The dm:empty-array constructor returns an empty array, that is, an array item containing no members.The dm:empty-array constructor returns an entryempty array, that is, an array item containing no members.
The function is exposed in XPath as an empty array constructor, written [] or array {}.