Maps in XPath 3.0

The XSL Working Group has proposed extensions to XPath to handle maps. These extensions have not yet been accepted into the XPath 3.0 working drafts, but they are implemented in Saxon 9.4.

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:

• If the number of entries is known, you can use the constructor syntax map { key := value; key := value; ... }. Here the keys and values can be any "simple expression" (an expression not containing a top-level comma). If the keys and values are all known statically, you might write: map { "a" := 1; "e: := 2; "i" := 3; "o" := 4; "u" := 5 }. You can use this construct anywhere an XPath expression can be used, for example in the select attribute of an xsl:variable element.

• The function map:new() takes a number of maps as input, and combines them into a single map. This can be used to construct a map where the number of entries is not known statically: for example for $i in 1 to 10 return map{$i := codepoints-to-string($i)}.

• A single-entry map can also be constructed using the function map:entry($key, $value)

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 full list of functions that operate on maps is as follows. The prefix map represents the namespace URI http://www.w3.org/2005/xpath-functions/map

• map:new($maps as map(*)) as map(*): takes a sequence of maps as input and combines them into a single map.

• map:new($maps as map(*), $collation as xs:string ) as map(*): takes a sequence of maps as input and combines them into a single map, using the specified collation to compare key values.

• map:collation($map as map(*)) as xs:string: returns the collation of a map.

• map:keys($map as map(*)) as xs:anyAtomicType*: returns the keys that are present in a map, in unpredictable order.

• map:contains($map as map(*), $key as xs:anyAtomicType) as xs:boolean: returns true if the given key is present in the map.

• map:get($map as map(*), $key as xs:anyAtomicType) as xs:item()*: returns the value associated with the given key if present, or the empty sequence otherwise. Equivalent to calling $map($key).

• map:entry($key as xs:anyAtomicType, $value as item()*): creates a singleton map. Useful as an argument to map:new()

• map:remove($map as map(*), $key as xs:anyAtomicType) as map(*): removes an entry from a map (if it was present), returning a new map; if not present, returns the existing map unchanged.