[xsl] New XSLT 3.0 Working Draft

[Michael Kay <mike@xxxxxxxxxxxx>]

There's a new Working draft of XSLT 3.0 - the first new public draft for 
2 years - at http://www.w3.org/TR/xslt-30/.

There's an enormous amount of new material here.

Big features:

- Streaming -

The analysis of streamability has been greatly simplified: it no longer 
requires any complex data flow analysis. This is achieved largely by not 
allowing variables to be bound to the nodes in a streamed document. 
Apart from that, most of the new features introduced for the benefit of 
streaming, such as xsl:iterate and xsl:stream, are largely intact.

A major innovation is the introduction of "accumulators", values 
associated with nodes that can be computed during a streaming pass of a 
document; they have the usability of mutable variables while being 
defined in a purely functional way, and are sufficiently constrained 
that they don't inhibit optimization.

- Packages -

Intended for independent compilation of stylesheet components: they 
allow stylesheets to distinguish which components are internal and which 
are visible to the outside world. Gives general software engineering 
benefits by separating interface from implementation; allows 
constraining of what can be overridden/customized and makes overriding 
type-safe.

- Maps -

A new data type, similar to the dictionaries or associative arrays in 
other languages. The keys in the map can be any atomic value; the 
associated value can be any value whatsoever. A particular motivation 
for maps was that with streaming, you only get to see each thing in a 
document once, so you need to remember what you've seen for use later 
(for example, in an accumulator); so you need a richer data structure 
for holding this data. Maps also provide a useful mechanism for 
importing/exporting data to/from JSON format (for which there are new 
functions).

- Higher-order functions -

More an XPath feature than an XSLT one, functions are now first-class 
values and can be passed as parameters to functions, returned by 
functions, held in maps, etc etc.

Other things include:

* xsl:try/catch

* xsl:evaluate

* xsl:assert