Re: [xsl] How do I pass the mode as a string?

Subject: Re: [xsl] How do I pass the mode as a string?
From: "Michael Kay mike@xxxxxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>
Date: Tue, 3 Mar 2020 16:32:01 -0000
The two things that come closest to polymorphism in XSLT (3.0) are (a) the
template rule despatch mechanism, and (b) higher order functions. Both of
these can be used to achieve dynamic selection of different functionality
based on the input data encountered. Exactly how you would apply either
mechanism in your scenario depends on having a deeper understanding of your
specific transformation requirement. Modes weren't designed for this
requirement and so it's hard to give you a forwards directions that doesn't
start "If I were you, I wouldn't start from here". If you do have to start
from here (i.e. you've got a collection of interchangeable modes and want to
select one of them dynamically) then I'm afraid you're stuck with some kind of
xsl:choose equivalent.

Michael Kay

> On 3 Mar 2020, at 15:38, Kerry, Richard richard.kerry@xxxxxxxx
<xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx> wrote:
> Ok,
> Thanks all for the explanations.  I'll look at the alternatives.
> If this was C++, which is what I use most of the time (XSL is infrequent,
just when I want to manipulate some XML) I would use some kind of polymorphism
to achieve this.  I'd define a couple of one-line functions with the different
variables, or calls, and pass the appropriate one to the outer function so
they could be run on its results (somewhat garbled, abbreviated explanation,
sorry).  I'm not aware of anything in XSL that looks like polymorphism, but I
would be interested to know if there was a construct that might be similar, or
usable for cases like this one.
> Granted I can't construct variable names dynamically, but I can have a
number of variables existing simultaneously and select one and pass it around
by address if I need to.  I don't see an analogous construct in XSL, except to
use choose/when to select a line of code based on an actual variable.
> I could send my examples if that would be best, but I think that would
provide a lot of unnecessary code just to show a small bit of context.
> What I am actually trying to do is to get a number of XML files processed.
The files are in similar folders, called 'input' and 'output'.  The files'
names follow a couple of name patterns, acq_(\d{4}) and out_(\d{4}). There is
identical code to scan through the folders and identify the files I want to
process (and extract the numbers from the filenames).  They are then passed to
apply-templates with the different modes as mentioned.
> So the file scanning, while not especially complicated, is not entirely
trivial and I'd like to have it written just once.  And then, because the
contents of the files differs in structure, they are handled by different
templates, with different modes.
> Richard Kerry
> BNCS Engineer, SI SOL Telco & Media Vertical Practice
> M: +44 (0)7812 325518
> 2nd Floor, MidCity Place, 71 High Holborn, London, WC1V 6EA
> richard.kerry@xxxxxxxx
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> From: Michael Kay mike@xxxxxxxxxxxx
> Sent: 03 March 2020 11:48
> To: xsl-list@xxxxxxxxxxxxxxxxxxxxxx <xsl-list@xxxxxxxxxxxxxxxxxxxxxx>
> Subject: Re: [xsl] How do I pass the mode as a string?
> Section B'6.6.2 in the XSLT 3.0 spec states:
> The xsl:apply-templates
3o%3D&reserved=0> element also has an optional mode attribute. The value of
this attribute must be one of the following:
> an EQName
rved=0>, which is expanded as described in 5.1.1 Qualified Names
to define the name of a mode
> the token #default, to indicate that the default mode for the stylesheet
3D&reserved=0> is to be used
> the token #unnamed, to indicate that the unnamed mode
D&reserved=0> is to be used
> the token #current, to indicate that the current mode
%3D&reserved=0> is to be used
> It does not allow an expression, and it does not allow an attribute value
> It's similar to trying to refer to a variable by name: you can't construct
variable names dynamically either, however much you feel it would make your
life easier.
> You haven't described your problem so it's difficult to suggest a different
approach to solving it. Higher-order functions might be the right way.
> Michael Kay
> Saxonica
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