Re: [xsl] Functional Programming: How do I convert an xsl:for-each loop into a functional style?

[David Carlisle <davidc@xxxxxxxxx>]

On 21/01/2010 12:38, Costello, Roger L. wrote:
> Hi Folks,
>
> My question is about writing XSLT in a functional programming style.
>
> My understanding is that a litmus test for whether a program is written in a functional style is whether it can be executed in any order.
>
> Consider this problem: convert a sequence of N numbers into a cumulative sequence, in which each number is the sum of the previous numbers.
>
> Here is a loop that solves the problem:
>
> <xsl:for-each select="Number">
>        <xsl:value-of select="sum((., preceding-sibling::Number))" />
> </xsl:for-each>
>
> Can this code be executed in any order?
>
> Suppose the loop is unraveled into a series of<xsl:value-of>  statements. If those statements are evaluated in arbitrary order then the output would not be in the desired sequence.  Thus, I conclude, this loop is not written in a functional style. Do you agree?
>
> How would the problem be solved in a functional style?

You are making the mistake of confusing execution order with the order 
of items in the result tree.

When making an xml result tree the order of the items almost always 
matters, you do not want the words in your paragraphs to be arbitrarily 
re-ordered for example.

However, the point about the side effect free model of programming is 
that the items in that for-each can be executed in any order, and in 
particular, most importantly these days, in parallel. of course the 
partial results that come in in arbitrary orders need to be marshalled 
into the correct output sequence which may or may not be trivial, but in 
general that's an issue with parallel computing: how to avoid spending 
longer working out who does what, and how to combine the results, than 
the time  you gain by by having multiple threads.

You say "here is a loop" but xsl:for-each is not a loop in the sense of 
a C for or fortran do loop. It is a specification how each item in a 
sequence is to be processed, and an implied specification how all the 
results are merged. It is not an instruction to process the items in a 
particular order. And if you add a non side-effect free instruction such 
as xsl;message into the loop, it is possible that you see that in fact 
things are not processed in the order you expect.

David


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