XQuery 3.1

This article is part of the XQuery Portal. It summarizes the new features of the XQuery 3.1 Working Draft that are already supported by BaseX.

Maps

A map is an additional kind of item. It comprises a collation and a set of entries. Each entry comprises a key which is an arbitrary atomic value, and an arbitrary sequence called the associated value. Within a map, no two entries have the same key, when compared using the eq operator under the map's collation. It is not necessary that all the keys should be mutually comparable (for example, they can include a mixture of integers and strings). Key values will never be of type xs:untypedAtomic, and they will never be the xs:float or xs:double value NaN.

The function call map:get($map, $key) can be used to retrieve the value associated with a given key.

A map can also be viewed as a function from keys to associated values. To achieve this, a map is also a function item. The function corresponding to the map has the signature function($key as xs:anyAtomicType) as item()*. Calling the function has the same effect as calling the get function: the expression $map($key) returns the same result as map:get($map, $key). For example, if $books-by-isbn is a map whose keys are ISBNs and whose associated values are book elements, then the expression $books-by-isbn("0470192747") returns the book element with the given ISBN. The fact that a map is a function item allows it to be passed as an argument to higher-order functions that expect a function item as one of their arguments. As an example, the following query uses the higher-order function fn:map($f, $seq) to extract all bound values from a map:

let $map := map { 'foo': 42, 'bar': 'baz', 123: 456 }
return fn:map($map, map:keys($map))

This returns some permutation of (42, 'baz', 456).

Like all other values, maps are immutable. For example, the map:remove function creates a new map by removing an entry from an existing map, but the existing map is not changed by the operation.

Like sequences, maps have no identity. It is meaningful to compare the contents of two maps, but there is no way of asking whether they are "the same map": two maps with the same content are indistinguishable.

Because a map is a function item, functions that apply to functions also apply to maps. A map is an anonymous function, so fn:function-name returns the empty sequence; fn:function-arity always returns 1.

Maps may be compared using the fn:deep-equal function. The semantics for this function are extended so that when two items are compared, at any level of recursion, the items compare equal if they are both maps, if both use the same collation, if both contain the same set of keys (compared using the eq operator), without regard to ordering, and if for each key that is present in both maps, the associated values are deep-equal. When comparing maps, the maps' collation is used rather than the collation supplied as an argument to the fn:deep-equal function.

Some examples use the map $week defined as:

declare variable $week as map(*) := map {
  0: "Sonntag",
  1: "Montag",
  2: "Dienstag",
  3: "Mittwoch",
  4: "Donnerstag",
  5: "Freitag",
  6: "Samstag"
};

The Map Module describes all map functions.

Changelog

Introduced with Version 8.0.