Maps in XPath

An important new feature in XPath 3.1 is the definition of maps. (This feature is in fact included in the XPath 3.0 implementation in Saxon, and thus was available as an advanced feature since Saxon 9.5, before the full implementation of XPath 3.1 in Saxon 9.7).

A map is a new kind of XDM item (alongside nodes and atomic values). In fact, a map is a kind of function: you can think of it as a function defined extensionally (by tabulating the value of the function for all possible arguments) rather than intensionally (by means of an algorithm).

A map is a set of entries. Each entry is a key-value pair. The key is always an atomic value. The value is any XDM value: a sequence of nodes, atomic values, functions, or maps.

Maps, like sequences, are immutable. When you add an entry to a map, you get a new map; the original is unchanged. Saxon provides an efficient implementation of maps that achieves this without copying the whole map every time an entry is added.

Also like sequences, maps do not have an intrinsic type of their own, but rather have a type that can be inferred from what they contain. A map conforms to the type map(K, V) if all the keys are of type K and all the values are of type V. For example if the keys are all strings, and the values are all employee elements, then the map conforms to the type map(xs:string, element(employee)).

There are several ways to create a map:

It is also possible to create maps using the XSLT instructions xsl:map and xsl:map-entry.

Given a map $M, the value corresponding to a given key $K can be found either by invoking the map as a function: $M($K), or by calling map:get($M, $K).

The summary of the full list of functions that operate on maps is as follows; for full details see the Functions Library. The prefix map represents the namespace URI