I was already writing about tuples and maps in the xslt (see Tuples and maps - Status: CLOSED, WONTFIX, and Tuples and maps in Saxon).

Now, I want to argue on a use case, and on how xslt processor can detect such a use case and implement it as map. This way, for a certain conditions, a sequences could be treated as maps (or as sets).

Use case.

There are two stages:

• a logic collecting nodes/values satisfying some criteria.
• process data, and take a special action whenever a node/value is collected on the previous stage.

Whenever we're talking of nodes than result of the first stage is a sequence $set as node()*. The role of this sequence is a set of nodes (order is not important).

The second stage is usually an xsl:for-each, an xsl:apply-templates, or something of this kind, which repeatedly verifies whether a some $node as node()? belongs to the $set, like a following: $node intersect $set, or $node except $set.

In spite of that we're still using regular xpath 2.0, we have managed to express a set based operation. It's a matter of xslt processor's optimizer to detect such a use case and consider a sequence as a set. In fact the detection rule is rather simple.

Thus what to do? Well, I do not think I'm the smartest child, quite opposite... however it worth to hint this idea to xslt implementers (see Suggest optimization). I still do not know if it was fruitful...

P.S. A very similar use case exists for a function index-of($collection, $item).