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
Saxonica
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:
<Queries>
<Query>
<XPath>/Bookstore/Book</XPath>
</Query>
<Query>
<XPath>/Bookstore/Book[Binding eq 'paperback']</XPath>
</Query>
</Queries>
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="/">
<Results>
<xsl:variable name="book-doc" select="."/>
<xsl:for-each select="$queries//Query">
<Query-Result>
<xsl:variable name="result" as="element()*">
<xsl:evaluate xpath="./XPath" context-item="$book-doc" />
</xsl:variable>
<xsl:sequence select="$result" />
</Query-Result>
</xsl:for-each>
</Results>
</xsl:template>
The ability to store XPath expressions as data and then dynamically evaluate
them is terrific!
I would like to take the next step and store XSLT code fragments as data and
then dynamically evaluate them. For example, I have a code fragment for
converting the cost of a Book in U.S. Dollars (USD) to the cost in Great
Britain Pounds (GBP):
<Book currency="GBP">
<xsl:value-of select="number($book-in-usd/Cost) * 0.79" />
</Book>
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
thing?
/Roger
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