Subject: RE: [xsl] Re: First public working draft of XSLT 2.1 From: "Michael Kay" <mike@xxxxxxxxxxxx> Date: Sat, 15 May 2010 20:51:29 +0100 |
> I think, that the new xsl:evaluate instruction is the most > harmful addition, as it either rules out the idea of > compilation of xslt in native/IL/other language code, or > demands for interpreter or compiler to be available at > stylesheet execution time. > > This makes execution environment much more heavier than necessary. You may have noticed we have an open issue (issue 13) saying that we have not yet decided whether this will be a mandatory or optional feature of the language. The main argument for making it optional is to allow stylesheet execution in run-time environments where having an XPath compiler would be excessive overhead. > > But probably the worst effect will be from developers who > will widely practice the "easiest" way to achieve desired > effect with dynamic xpath. There are many use cases for dynamic evaluation; it is one of the most widely-used and widely-implemented extensions in existing XSLT processors, and it seems reasonable to offer users a portable specification for the facility. Of course there is a risk that some users will misuse the feature, but I don't think that's a very good argument for not making it available. > > On the other hand indirect function calls introduced in xpath > 2.1 give enough power to model dynamic flexibility, if required. > -- Indeed, and hopefully that will reduce the temptation to use dynamic evaluation for solving problems in cases where higher-order functions are a more appropriate solution. But there are many applications where reading XPath expressions from data files or constructing then dynamically from user-supplied input is a requirement, and currently such applications cannot be written in a portable way. For an example of such an application, consider the errata publication system used for the XSLT and related specifications. The XML master file for an erratum identifies the text to be changed or deleted by means of an XPath expression embedded in the XML document, and the stylesheet uses this to find the text and display it. This is a perfectly reasonable design, and it's frankly embarrassing that the build system for the XSLT specification should have to rely on vendor extensions to the language. Regards, Michael Kay http://www.saxonica.com/ http://twitter.com/michaelhkay
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