Question #1: is XPath and/or XSLT designed such that I should never have
to write special case code?
Question #2: If I find myself writing special case code, should I stop
and say, "How can I modify this XPath and/or XSLT so that I do not have to
write special case code?
Reminds me of:
- Sentinel programming (See for example Programming Pearls, by J
Bentley)
- Null object pattern
Not sure support for sentinels is provided by any programming language.
This is more of an algorithmic feature.
Example: finding all (X)Html elements that have class $myClass:
//*[contains(concat(' ', @class, ' '), concat(' ', $myClass, ' ')]
This avoids having to handle special cases like $myClass at the start, end
or middle of a space+ -separated classnames.
Cheers,
Dimitre
On Tue, Jun 22, 2021 at 10:03 AM Roger L Costello costello(_at_)mitre(_dot_)org
<
xsl-list-service(_at_)lists(_dot_)mulberrytech(_dot_)com> wrote:
Hi Folks,
I think special case code is evil.
To explain what I mean by "special case code" let's take its opposite:
code selects what is desired under any condition.
So, by "special case code" I mean extra code that is written for dealing
with special conditions.
Last week I posted the following XPath expression to fetch the <Row>
element where Cell[1]/Data equals $element and Cell[2]/Data equals $parent.
$document/Row[Cell[1]/Data eq $element][Cell[2]/Data eq $parent]
However, when $parent is empty, the XPath expression fails.
I was all set to write special case code:
<xsl:if test="empty(Cell[2]/Data)"> do something </xsl:if>
Bad, bad, bad.
But then Mukul showed me an XPath expression that works correctly --
whether $parent is empty or not -- without any special case code:
$document/Row[Cell[1]/Data eq $element][Cell[2]/string(Data) eq $parent]
Awesome!
Today I was writing some XSLT to generate a bunch of rows showing, for
each element in an XML document, its name, the name of its parent, the name
of its grandparent, and the name of its great-grandparent:
<xsl:template match="*">
<row>
<element><xsl:value-of select="name(.)"/></element>
<parent-element><xsl:value-of select="name(..)"/></parent-element>
<grandparent-element><xsl:value-of
select="name(../../..)"/></grandparent-element>
<great-grandparent-element><xsl:value-of
select="name(../../../..)"/></great-grandparent-element>
</row>
<xsl:apply-templates select="*" />
</xsl:template>
Obviously as the XSLT traverses through an XML document some elements
don't have a great-grandparent or a grandparent or even a parent. But I
didn't need to write special case code to check those conditions. That is
terrific!
Question #1: is XPath and/or XSLT designed such that I should never have
to write special case code?
Question #2: If I find myself writing special case code, should I stop and
say, "How can I modify this XPath and/or XSLT so that I do not have to
write special case code?
/Roger
--
Cheers,
Dimitre Novatchev
---------------------------------------
Truly great madness cannot be achieved without significant intelligence.
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To invent, you need a good imagination and a pile of junk
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Never fight an inanimate object
-------------------------------------
To avoid situations in which you might make mistakes may be the
biggest mistake of all
------------------------------------
Quality means doing it right when no one is looking.
-------------------------------------
You've achieved success in your field when you don't know whether what
you're doing is work or play
-------------------------------------
To achieve the impossible dream, try going to sleep.
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Facts do not cease to exist because they are ignored.
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Typing monkeys will write all Shakespeare's works in 200yrs.Will they write
all patents, too? :)
-------------------------------------
Sanity is madness put to good use.
-------------------------------------
I finally figured out the only reason to be alive is to enjoy it.
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