I don't know why the collection()/$a/title is working...I would have expected a
syntax error there--I'll look into that.
As for the context nodes, let me qualify what I said before. In 1.0-ml, the
whole database is the context. In 1.0, there is no default context node. The
spec does not allow for the whole database to be the context node-it requires a
single node. In MarkLogic, it does not make sense to choose a single node as
the context node, but it does make sense to choose the database as the context
("context nodes", if you will). That is why there is a difference between
1.0-ml and 1.0 here.
You are correct in thinking that explicitly supplying the context is a good
idea, as then that code will run in both 1.0 and 1.0-ml, and it will be clearer
what the intent is too.
Hope that helps,
-Danny
From: [email protected]
[mailto:[email protected]] On Behalf Of
[email protected]
Sent: Wednesday, February 09, 2011 2:25 PM
To: [email protected]
Subject: Re: [MarkLogic Dev General] using XPath with no context
Ok so it uses the default context is none is specified. but I still don't
understand why this works:
xquery version '1.0-ml';
let $test-doc :=
<article>
<title>Hello</title>
</article>
return xdmp:document-insert("/test-doc.xml", $test-doc)
;
let $a := /article
return fn:collection()/$a/title
=> <title>Hello</title>
But I think I will always specify a context anyway.
And I guess it is just for the 1.0-ml dialect
> From: [email protected]
> To: [email protected]
> Date: Wed, 9 Feb 2011 14:13:39 -0800
> Subject: Re: [MarkLogic Dev General] using XPath with no context
>
> In MarkLogic, the default context is all of the documents in the database to
> which your App Server is configured. So fn:collection() is equivalent to the
> default context.
>
> -Danny
>
> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]] On Behalf Of G. Ken Holman
> Sent: Wednesday, February 09, 2011 1:28 PM
> To: General MarkLogic Developer Discussion
> Subject: Re: [MarkLogic Dev General] using XPath with no context
>
> At 2011-02-09 14:21 -0700, [email protected] wrote:
> >What are the implications of doing:
> >
> >/article
>
> That is valid only if the processor gives you a source node tree at
> invocation time.
>
> >vs
> >
> >fn:collection()/article
>
> fn:collection() returns document nodes, so there is no "missing context".
>
> >or
> >
> >fn:doc()/article
>
> fn:doc() returns a document node, so there is no "missing context".
>
>
> >?
> >
> >Seems like the first one shouldn't be allowed since no context is
> >specified, yet it seems to work. Is that part of the XQuery spec or
> >is that part of the 1.0-ml dialect? (seems so because when I specify
> >1.0 I get an error). What is happening behind the scenes when no
> >context is specified?
>
> All depends on the processor. Saxon is an XQuery processor that
> accepts a source document specification on the command line and so
> "/article" is in the context of the document node of that source
> document. If you run Saxon without supplying a source document, you
> get an error.
>
> A processor defines what is in the dynamic context at the time
> document processing starts:
>
> http://www.w3.org/TR/2007/REC-xquery-20070123/#eval_context
>
> Looking at the list of components here:
>
> http://www.w3.org/TR/2007/REC-xquery-20070123/#id-xq-evaluation-context-components
>
> ... you can see that the current node, and therefore the tree of the
> current node, can be overwritten by the implementation.
>
> With Saxon, that current node and tree depends on the presence of the
> command line argument.
>
> I cannot quickly find the MarkLogic documentation of the value of the
> current node in the dynamic context at the start of
> processing. Perhaps someone else can post this.
>
> > From a stach trace it looks like fn:collection() is inserted behind
> > the scenes. If so, what's the thinking for that doing that?
>
> The static context defines what fn:collection() returns:
>
> http://www.w3.org/TR/2007/REC-xquery-20070123/#static_context
> http://www.w3.org/TR/2007/REC-xquery-20070123/#id-xq-static-context-components
> "Statically-known collections" - augmentable
>
> And so, again, it is up to the processor to define this. I believe
> MarkLogic returns the set of document nodes of every document.
>
> >Also, it looks like this is a way to be able to write somewhat
> >dynamic XQuery by doing something like this:
> >
> >let $query := /article/title
> >
> >return fn:collection()/$query
> >
> >Which apparently is the equivalent of fn:collection()//article/title
>
> That is not valid XPath 2.0 or XQuery 1.0 syntax.
>
> I hope this helps.
>
> . . . . . . . . . Ken
>
> --
> Contact us for world-wide XML consulting & instructor-led training
> Crane Softwrights Ltd. http://www.CraneSoftwrights.com/q/
> G. Ken Holman mailto:[email protected]
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