For the time being such support will be provided by the FXSL
monadic implementation.
Before publishing the initial implementation I'm trying to
collect a number of good-use examples, like maintaining
state, update-in-place Arrays, IO from external files,
synchronous exceptions.
I've seen some examples recently where initializing a Java object
requires two calls: a constructor and an initialization.
<xsl:variable name="x" select="java:object.new()"/>
<xsl:do select="setProperty($x,17)"/>
<xsl:value-of select="$x"/>
Somehow the second instruction has to be forced to happen before the
third. The way I fudge it at present is to write the "xsl:do" as
"xsl:value-of", but I've had to tweak the optimizer so it doesn't
skip
the instruction because it recognizes statically that the result is
always empty.
Thank you Mike, for the good example.
BTW, what is xsl:do? I couldn't find it in the current WD spec.
Generally the Saxon optimizer has now got to the level where external
functions with side-effects are becoming very troublesome. I've been
thinking of introducing some kind of construct to force sequential,
unoptimized execution of a sequence of instructions. (And for the
time
being, I've overloaded the attribute saxon:assignable="yes" to
achieve
this effect).
I've heard horror stories of an XSLT processor completely discarding an
xsl:variable and then re-evaluating it. In such case even a monadic
implementation will not help -- there must be something to tell the
processor not to discard and "recreate" a certain variable, because
evaluating its value causes a side-effect.
In the absence of the ability to express that a variable is of type
IO a
this would be the way to say it.
=====
Cheers,
Dimitre Novatchev.
http://fxsl.sourceforge.net/ -- the home of FXSL
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