Bob Smith wrote:
> hi all,
> I pretty new to xml, what does xmlns mean?
> like in:
> <rdf:RDF
>  xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
>  xmlns="http://purl.org/rss/1.0/"
>  xmlns:perljobs="http://jobs.perl.org/rss/"
>  xmlns:taxo="http://purl.org/rss/1.0/modules/taxonomy/"
>  xmlns:dc="http://purl.org/dc/elements/1.1/"
>  xmlns:syn="http://purl.org/rss/1.0/modules/syndication/"
>  xmlns:admin="http://webns.net/mvcb/"
> 
> tia

xmlns is used to bound prefixes to namespace URIs

it means that the *universal name* of the <rdf:RDF> element is
[ http://www.w3.org/1999/02/22-rdf-syntax-ns# , RDF ]
this notation [ URI , local-name ] denotes that the *universal name* is 
in 2 parts (in most XML papers, it is displayed with another notation : 
{http://www.w3.org/1999/02/22-rdf-syntax-ns#}RDF but I prefer the 
former) ; notice that these notations are only for explanation and NEVER 
appear in real XML documents

such names are *universal names* because URIs are unique ; notice that 
an URI is not necessary an URL :
xmlns:foo="urn:guid:88d969c9-f192-11d4-a65f-0040963251e6" 
xmlns:book="urn:isbn:2-84177-230-6" 
xmlns:office="urn:schemas-microsoft-com:office:office"
namespace URIs are just use to qualify XML names

the prefix is just an alias for the URI, but its mean is local to the 
xmlns declaration (that is to say, applies on its host element and its 
descendents)
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
<foo:RDF xmlns:foo="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
<RDF xmlns="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
are the same ; the last form doesn't use a prefix, it uses the default 
namespace

attributes are told "owned" by their host elements, so they don't need 
additional information ; that's why the default namespace doesn't apply 
to attributes ; sometimes, it is necessary to "add" to an element 
special purpose attributes, that are often called "foreign attributes" :
<image xmlns:xlink="http://www.w3.org/1999/xlink">
        xlink:type="simple"
        xlink:actuate="onLoad"
        xlink:show="embed"
        xlink:href="http://www.acme.com/foo/bar.jpg"
        width="123"
        height="234" />

some application may recognize a particular namespace URI :
-for example, an application might expose an hyperlink in conformance to 
the XLink specification if it encounters attributes like those above
-an XSLT processor will do something special when it encounters 
<xsl:template> not because the prefix is "xsl" but because this element 
is in the scope of the xmlns declaration : 
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
-a graphic application that recognize SVG could draw a rectangle :
<svg:svg xmlns:svg "http://www.w3.org/2000/svg"
     width="18cm" height="6cm" viewBox="0 0 200 120">
         <svg:rect x="1" y="1" width="198" height="118"
                   fill="#888888" stroke="blue" />
</svg:svg>

if you have to design XML datas, I encourage you to use your own 
namespace URI with your elements : thus, your application will be able 
to process these elements without ambiguity ; XML namespaces have been 
design for that purpose : distinguish elements that could have the same 
local name, and grouping a set of names "related to the same feature"

-- 
Cordialement,

            ///
           (. .)
  -----ooO--(_)--Ooo-----
|   Philippe Poulard    |
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