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Re: [xsl] Is it possible to store XSLT code fragments as data and then dynamically evaluate those code fragments?

2019-06-20 06:49:33
This is what higher-order functions are for in XSLT 3.0 / XPath 3.0. You write 
your mapping operations as functions (or write them in some other notation 
which you transform into functions), and then you parameterise your code so it 
executes a dynamically-supplied mapping operation using a dynamic function call.

Michael Kay

On 20 Jun 2019, at 12:38, Costello, Roger L. costello(_at_)mitre(_dot_)org 
<xsl-list-service(_at_)lists(_dot_)mulberrytech(_dot_)com> wrote:

Hello XSLT experts!

As you know, XPath expressions can be stored in an XML document and then an 
XSLT program can be written which inputs each expression and dynamically 
evaluates them. For example, I created several XPath expressions to query 
Book data; I stored the XPath expressions this way:

       <XPath>/Bookstore/Book[Binding eq 'paperback']</XPath>

Then I created an XSLT program which loops over each XPath expression and 
evaluates the expression against the Book document:

<xsl:variable name="queries" select="doc('XPath-Queries.xml')" />

<xsl:template match="/">
       <xsl:variable name="book-doc" select="."/>
       <xsl:for-each select="$queries//Query">
               <xsl:variable name="result" as="element()*">
                   <xsl:evaluate xpath="./XPath" context-item="$book-doc" />
               <xsl:sequence select="$result" />

The ability to store XPath expressions as data and then dynamically evaluate 
them is terrific!

I would like to take the next step and store XSLT code fragments as data and 
then dynamically evaluate them. For example, I have a code fragment for 
converting the cost of a Book in U.S. Dollars (USD) to the cost in Great 
Britain Pounds (GBP):

<Book currency="GBP">
   <xsl:value-of select="number($book-in-usd/Cost) * 0.79" />

I vaguely recall that long ago there was a proposal by the XSLT working group 
to provide an eval() function which would do exactly what I desire; but I 
checked the latest specification and it appears the eval() function wasn't 
accepted. Bummer.

The big picture is this: I want to perform various mapping operations on the 
Book data. I want the mapping operations expressed in a declarative manner so 
that Subject Matter Experts (SMEs) can review the mappings for correctness 
(e.g., upon reviewing the above code fragment the SME says: "Yes, that is the 
correct way to convert costs in USD to GBP"). Ideally, the mapping operations 
would be stored as data and then dynamically evaluated (after the SMEs have 
certified their correctness). What do you recommend? It appears that dynamic 
evaluation of XSLT code fragments is not possible, so what is the next best 


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