Hi Kutrt,
Dmitri,
We've talked on this issue before, and in the main I agree with you
about
the issue of side effects.
Yes, we agree on this issue and have essentially the same understanding
about it.
This holds true in certain contexts especially
(the use of an imperative variable as an incrementer, for instance,
something that isn't as big of an issue with XSLT2 and sequences),
The problem is not with XSLT or XSLT2 or with any functional language. The
problem is that once you allow external functions, you cannot have any
guarantees about their being side-effects free.
So, the side effect happens within the external environment of the
extension function and nothing can prevent it.
An example is storing and updating (state) data outside of XSLT.
The simplest and shortest example can be given with the use of the
date:time() EXSLT extension function.
In this case the programmer doesn't have to store and update the time --
this is done for them automatically :o) :((
Take this code:
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform";
xmlns:dt="http://exslt.org/dates-and-times";
exclude-result-prefixes="dt"
<xsl:variable name="vT1" select="dt:time()"/>
<xsl:variable name="vT2" select="dt:time()"/>
<xsl:template match="/">
<xsl:value-of select="$vT2"/>
<xsl:for-each select="document('')//node()
| document('')//@*
| document('')//namespace::*">
<xsl:for-each select="document('')//node()
| document('')//@*
| document'')//namespace::*">
<xsl:value-of select="concat('
', position())"/>
</xsl:for-each>
</xsl:for-each>
<xsl:value-of select="$vT1"/>
</xsl:template>
</xsl:stylesheet>
Any assumptions that $vT2 >= $vT1 are wrong.
In fact, when run with XalanJ 2.4.1 the result is:
09:11:50+01:00+01:00
[noise output]
09:11:51+01:00+01:00
and I
know that I work hard to educate my programmers about the dangers of
dealing
with side-effect systems. The problem that comes with XSLT2 is the fact
that
any access to a web service is implicitly a side-effect - the same call
with
the same parameters to the same web service will potentially return
different values and moreover potentially change the state of the called
service. Ditto the notion of <xsl:document-result> with a packet of
information being sent to a web service.
In other words, a side-effect free environment can only realistically be
maintained if the system itself is completely closed, yet most of the
interesting uses of XSLT only emerge when you open up the system a
bit ...
A side-effect free environment is completely useless, because it cannot
print (a side-effect operation!) any processing results.
We have to live with side-effects and we actually can use side-effects in
a controlled manner.
Unfortunately, some functional languages (XSLT included) lack a built-in
support for controlling side-effects.
Haskell has the notion of a Monad class built-into the language and any
programmer simply uses the IO (or any other suitable type) Monad, when
this is necessary.
I have my implementation of a Monad class - like support in XSLT and it
allows functions with side-effects to be used in a safe and controlled
manner.
however, I liken this to a novice trying to play jazz vs. a master jazz
musician -- the master will break the rules all the time, but only
because
he knows exactly why those rules exist in the first place and he weighs
the
risk of breaking them as the cost of achieving art.
Actually, there is no risk -- the master knows and follows *additional
rules* (Monads) that eliminate the risk completely. The master is a master
because he knows something in addition to the rules used by the novices.
The novice breaks
them
out of ignorance, and just creates bad music, but it is the only way
that
the novice will learn what those rules are in the first place.
Fortunately, false melody can be felt immediately.
However, in the case of programming everything may seem OK untill suddenly
(e.g. with the use of another XSLT processor or a new version of the same
XSLT processor) some completely surprizing results start to appear and it
is impossible for the novice to understand and explain them.
Dimitre Novatchev.
FXSL developer
http://fxsl.sourceforge.net/ -- the home of FXSL
Resume: http://fxsl.sf.net/DNovatchev/Resume/Res.html
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