Re: [xsl] key() now has 3 arguments ... right

[Michael Kay <mike@xxxxxxxxxxxx>]

On 24/04/2012 20:41, ihe onwuka wrote:
> key() in an XSLT 2.0 pattern must have exactly two arguments
Correct. See http://www.w3.org/TR/xslt20/#pattern-syntax
> how come?
When people ask for reasons why the spec is as it is, there are two ways 
of answering:

(a) the historically accurate response. This involves searching the 
archives of working group discussions to see whether any relevant 
proposals were made, and whether they were accepted or rejected; if a 
proposal was rejected it then involves asking individual members of the 
WG why they voted one way or the other, since individual reasons for 
voting will not be recorded in the minutes; if no proposal was made it 
involves asking indivdual members of the WG why they did not see fit to 
make a proposal, and indeed it involves asking the user community why 
they did not request the feature in response to one of the many drafts 
issued for consultation.

(b) the post-hoc justification: "can you defend the fact that the spec 
is the way it is?". Here I think my answer would be that the syntax for 
patterns is a very restricted subset of the syntax of XPath expressions, 
and the WG has been traditionally conservative about extending it, 
probably in the belief that a smaller language is much more susceptible 
to the kind of analysis needed to build an efficient decision tree. 
There has also been historically little demand for a more powerful 
pattern language, with most users keeping patterns very simple most of 
the time.

key() in patterns is used very little, and serves very little purpose, 
since unlike key() in XPath expressions an index does not help 
evaluation and there is really no strong reason to use key() in 
preference to using a predicate. Adding the third argument to key() 
would be oddly asymmetric: since there is no way in XSLT 2.0 of rooting 
a pattern such as $x/a/b/c at a specific document node, it would be odd 
to allow a key() pattern to be rooted in that way.

Michael Kay
Saxonica