Having written my share of non-trivial recursive templates, I will attest that
xsl:iterate, once I worked how to use it, is much easier for me to both
understand and apply and I would expect most procedural programmers to have a
similar experience.
Which is not to discount Dimitre's experience in any way, just saying that my
experience was very different.
I do agree that trying to understand xsl:iterate from the spec alone is a
little challenging, but a few examples go a long way there.
Cheers,
E.
--
Eliot Kimber
http://contrext.com
On 8/13/20, 10:41 AM, "Dimitre Novatchev dnovatchev(_at_)gmail(_dot_)com"
<xsl-list-service(_at_)lists(_dot_)mulberrytech(_dot_)com> wrote:
>
>
What about XSLT 3's xsl:iterate? Isn't that, with its parameters that
>
> can change on each iteration step, close to a procedural, sequential loop?
>
>
I prefer to think of it as syntactic sugar for a functional fold operation,
or for a tail-recursive function.
>
It means that processing the Nth item has a functional dependency on
processing the (N-1)th,
>
but a functional dependency is not quite the same as a defined order of
execution.
If you read the spec for <xsl:iterate> it is quite likely you will find it
much more overwhelming and confusing than the clean and clear description of a
fold (in Haskell and also in the Functions and Operations spec).
In my case it cost me more than 2 hours to read the spec of <xsl:iterate>
and at the end I was not sure I properly understood what exactly this XSLT
instruction does and whether or not it was possible to use it in a few edge
cases. I spent another hour to construct one of these edge cases and made sure
this was handled properly in Saxon, however this confirmation came from Saxon,
not from the spec.
For me, the <xsl:iterate> instruction is a deviation from the spirit of
XSLT and I am not sure whether or not it will serve its stated goal to help
imperative programmers not learn and use recursion.
It would have been much better to (more or less) formally define the
concept of "tail recursion" and make it clear that a good implementation can
(or should) optimize tail recursion with iteration.
Dimitre
On Thu, Aug 13, 2020 at 3:55 AM Michael Kay mike(_at_)saxonica(_dot_)com
<xsl-list-service(_at_)lists(_dot_)mulberrytech(_dot_)com> wrote:
What about XSLT 3's xsl:iterate? Isn't that, with its parameters that
can change on each iteration step, close to a procedural, sequential loop?
I prefer to think of it as syntactic sugar for a functional fold operation,
or for a tail-recursive function. It means that processing the Nth item has a
functional dependency on processing the (N-1)th, but a functional dependency is
not quite the same as a defined order of execution.
For example, if the code did
<xsl:when test="self::H1">
<xsl:next-iteration>
<xsl:with-param name="pos" select="1"/>
</xsl:next-iteration>
</xsl:when>
then a smart optimizer could work out that processing the item following an
H1 doesn't depend on anything that went before, and could therefore be done in
parallel with processing of previous items. Unlike a loop, the functional
depenendency is explicit and it's therefore still possible in principle to
rearrange the order of execution.
Michael Kay
Saxonica
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