XSLT: keys as maps

[michaelhkay]

I propose an XSLT function map-for-key('keyname', $root) which returns a map $M having the property that map:get($M, $key) returns the value of key('keyname', $key, $root).

This enables XSLT keys to be exploited in new ways: for example it becomes easy to merge the indexes for multiple documents, or to iterate over all the keys in a document.

These benefits can already be obtained by scrapping keys entirely and building maps instead; but keys do have some benefits (like remaining implicitly associated with particular documents, and being "more declarative") and if you've got a legacy application that makes extensive use of keys, this function gives you a bridging capability.

There are a few edge cases that will need ironing out, for example keys allow matching using a collation, which maps don't. (And the spec of xsl:key, now I come to think of it, says nothing about comparing date/time values in different timezones; I don't expect anyone has ever tried.)

[michaelhkay]

Separate but related: we should align keys with maps by using fn:atomic-equal() for comparisons rather than eq, at least in the case of numerics, to get rid of the non-transitivity problems. Timezones need more thought.