Now, for "for each" and "map" to be 100% tautologous, would, imho, require
that
(in the XPath environment) sequences allow "null" or the empty sequence to be
a valid item within a sequence, or, in other words, sequences may contain
nested
sequences, including the empty sequence itself. As XPath has no notion of
nested
sequences, and, from my memory of the spec, null-ness is represented by
empty-ness
(i.e. sequences of exactly zero items).
Accordingly, foreach and map are not quite the same thing when the XPath
data model
does not allow for nested sequences. Whether the omission of nested
sequences from
the XPath model was a good thing, a pragmatic thing, or a bad thing, I am
not quite sure.
In XSLT context <xsl:for-each> and implementations of map() *are* the
same thing, because their results are indistinguishable (identical).
For example, these two sequences are identical in the XPath 2.0 data model:
1, 2, (), 3
1, 2, 3
Also, there is no way to claim that <xsl:for-each> has really produced
the second sequence...
--
Cheers,
Dimitre Novatchev
---------------------------------------
Truly great madness cannot be achieved without significant intelligence.
---------------------------------------
To invent, you need a good imagination and a pile of junk
-------------------------------------
Never fight an inanimate object
-------------------------------------
You've achieved success in your field when you don't know whether what
you're doing is work or play
On Thu, May 29, 2008 at 7:43 AM, Justin Johansson
<procode(_at_)adam(_dot_)com(_dot_)au> wrote:
Thanks folks for explanations.
Dimitre
There is ordering of the *results*, not of the application of the body
of <xsl:for-each> to the elements of the sequence.
Yes, I understand. It was the result ordering that got me thinking about
I/O monads controlling the sequential output (and boiling down to joins
on multi-processor threads). Sounds like Colin has a take on managing
this (parallelism) as well.
Michael Kay
The use of the English words "for each" is a mixed blessing. On the one
hand, it makes your typical user less uncomfortable than if it were named
xsl:map. On the other hand, it gives people the wrong impression that it is
procedural.
Okay, I went away thinking that "for each" and "map" were synonyms
but upon reflection me thinks that these terms/names are subtlely different.
Refer your example
<xsl:for-each select="S">
<instructions/>
</xsl:for-each>
The concept of "for each" means, at least to me, to do something and to
produce
a result (sequence) of zero or more items against corresponding items in
the input
sequence to "for each".
"instructions" has a general result signature of item()*. Conceivably, the
overall result
could be the output of zero items for a non-empty argument sequence given
to "for each".
The concept of "map" however, generally suggests, say, a 1-N mapping
of input items to result items as produced by the higher-order application of
"instructions", N generally being a constant >= 1, certainly not variable and
generally not zero either.
In a "mapping" context, "instructions" has a signature like
instructions ( arg as item()) as item() and not
instructions ( arg as item()) as item()? as may be a use case for "for each".
Trustingly this argument is clear enough to demonstrate the different between
the notions of "for each" and "map".
Now, for "for each" and "map" to be 100% tautologous, would, imho, require
that
(in the XPath environment) sequences allow "null" or the empty sequence to be
a valid item within a sequence, or, in other words, sequences may contain
nested
sequences, including the empty sequence itself. As XPath has no notion of
nested
sequences, and, from my memory of the spec, null-ness is represented by
empty-ness
(i.e. sequences of exactly zero items).
Accordingly, foreach and map are not quite the same thing when the XPath
data model
does not allow for nested sequences. Whether the omission of nested
sequences from
the XPath model was a good thing, a pragmatic thing, or a bad thing, I am
not quite sure.
Cheers
Justin Johansson
---------------------------------------------------
A horse with no name is called Lambda.
What was the name of the town in the song?
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