Re: [xsl] Why are there no XSLT processors implemented in XSLT?

[Michael Kay <mike@xxxxxxxxxxxx>]

Back in 2007 I wrote a paper on writing one part of an XSLT processor, 
namely the optimizer, in XSLT: see

http://www.saxonica.com/papers/Extreme2007/EML2007Kay01.html

The main obstacle turned out to be that XSLT isn't good at doing "local 
rewrites", where most of the tree stays the same but a small part 
changes. It's possible that an implementation strategy based on creating 
deltas could solve that, and it might be generally useful. But I think 
one might need to modify some notions like node identity to make it 
really viable.

The other challenge is that while XSLT is good at doing XML, it's not 
all that good at a lot of the other jobs needed in a compiler, like 
parsing XPath. You can do it, but it's not the most natural tool for the 
job, and it's not the job that the tool was designed for.

Remember that sentence right at the start of the XSLT 1.0 spec:

"XSLT is not intended as a completely general-purpose XML transformation 
language. Rather it is designed primarily for the kinds of 
transformations that are needed when XSLT is used as part of XSL"

I've never been comfortable with that (which is why it's not there in 
2.0). I think it should be a general-purpose XML transformation 
language. But I don't think it should be a general-purpose programming 
language. What actually surprises me is how people have used it well 
outside its original intended application area, an example being 
creating programs for machine tools from the output of a CAD package.

Michael Kay
Saxonica


On 01/05/2012 12:49, Costello, Roger L. wrote:
> Hi Folks,
>
> I will take the following as an axiom:
>
>        Programs written to process XML should be implemented in XSLT.
>
> An XSLT processor is a program that processes XML.
>
> Therefore an XSLT processor should be implemented in XSLT.  (As an analogy, many C compilers are implemented in C.)
>
> However, there are no XSLT processors implemented in XSLT.
>
> Instead, XSLT processors are implemented in languages such as Java.
>
> The implication of this state of affairs is this: "For real, manly XML processing don't use XSLT; instead, use Java."
>
> Why are there no XSLT processors implemented in XSLT?
>
> /Roger