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Re: Re: Re: RE: Higher-Order Functions in XPath 2.0

Subject: Re: Re: Re: RE: Higher-Order Functions in XPath 2.0
From: Dimitre Novatchev <dnovatchev@xxxxxxxxx>
Date: Thu, 17 Jan 2002 23:26:11 -0800 (PST)
cartesian product haskell
terje at in-progress dot com (Terje Norderhaug) wrote:

> At 7:56 PM 1/17/02, Dimitre Novatchev wrote:
> >> > Point 3.) This essentially says there is currently no facility
> >> > to iterate over two or more sequences in parallel rather than
> >> > on the cartesian product. This is especially bad because
> >> > there is no reasonable way to emulate this using the "for"
> >> > operator. [...]
> >
> >Another point: How can we produce ***more than one*** sequence in
> >parallel?
> >
> >For example, how to produce both:
> >
> >($a(i) + $b(i))-sequence
> >and
> >($a(i) * $b(i))-sequence
> >
> >In only one pass over $a and $b ?
> As there are no side effects, a smart XSL processor is already free 
> to optimize by evaluating the two using only one pass over the two 
> sequences, without requiring any special constructs in the language 
> to force parallel evaluation.

Two things here:

1. What is needed is not a "smart XSL processor", but a "smart" XPath
engine, as we're talking about XPath, and the above expressions may
beused outside/independently of XSLT.

2. Rather than to rely on the "smartness" of a particular XPath engine,
it's better to have the language guarantee that this is possible. 

What prevents such guarantee in the current WD of XPath 2.0 is the fact
that it prohibits an item of a sequence to be a sequence itself. Also,
there's no facility to define functions within XPath (so recursive
definitions are not supported).

In case these two factors are removed, we could have the same as

partition p xs    = foldr select ([],[]) xs
                      where select x (ts,fs) | p x       = (x:ts,fs)
		                             | otherwise = (ts,x:fs)

This function "partitions" a list into two lists -- one containing all
elements that satisfy the predicate p(), the other containing the rest.

The result is a pair (a sequence of exactly two elements) of the two
lists. And, of course, they are immediately accessible through the
functions fst() and snd() (first  and second).

The partition() function comes handy when defining a sort function
(e.g. quicksort) in a single line of code.

Dimitre Novatchev.

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