namespaces.xml 6.53 KB
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<?xml version="1.0" encoding="ISO-8859-1" standalone="yes"?>
<page name="namespaces">

<title>XML Namespaces</title>

<box title="Overview" link="ov">

<p>

CDuce fully implements the W3C <a
href="http://www.w3.org/TR/REC-xml-names/">XML Namespaces</a>
Recommendation. Atom names (hence XML element tags) and record labels
(hence XML attribute names) are logically composed of a namespace URI
and a local part. Syntactically, they are written as <em>qualified
names</em>, conforming to the QName production of the Recommendation:

</p>

<sample>
QName     ::= (Prefix ':')? LocalPart
Prefix    ::= NCName
LocalPart ::= NCName
</sample>

<p>

The prefix in a QName must be bound to a namespace URI. In XML, the
bindings from prefixes to namespace URIs are introduction through
special <code>xmlns:prefix</code> attributes. In CDuce, instead,
there are explicit namespace binders. For instance, the following
XML documents

</p>

<sample>
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&lt;p:a q:c="3" xmlns:p="http://a.com" xmlns:q="http://b.com"/>
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</sample>

<p>
can be written in CDuce:
</p>

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<sample>
namespace p = "http://a.com" in
namespace q = "http://b.com" in
&lt;p:a q:c="3">[]
</sample>

<p>
This element can be bound to a variable
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<code>x</code> by a <code>let</code> binding as follows: 
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</p>

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<sample>
let x = 
  namespace p = "http://a.com" in
  namespace q = "http://b.com" in
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  &lt;p:a q:c="3">[]
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</sample>

<p>
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In which case the namespace declarations are local to the scope
of the let.
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Alternatively, it is possible to use global prefix bindings:
</p>

<sample>
namespace p = "http://a.com"
namespace q = "http://b.com"
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let x = &lt;p:a q:c="3">[]
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</sample>

<p>
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Similarly, CDuce supports namespace <i>defaulting</i>. This is introduced
by a local or global <code>namespace "..."</code> construction.
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As in the XML, default namespace applies only to tags (atoms), not
attributes (record labels).
For instance, in the expression <code>namespace "A" in &lt;x
y="3">[]</code>, the namespace for the element tag is "A", and
the attribute has no namespace.
</p>

<p>
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The toplevel directive <code>#env</code> causes CDuce to print, among
others, the current set of global bindings.
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</p>

</box>

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<box title="Types for atoms" link="types">
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<p>
The type <code>Atom</code> represents all the atoms, in all the
namespaces. An underscore in tag position (as in
<code>&lt;_>[]</code>) stands for this type.
</p>

<p>
Each atom constitutes a subtype of <code>Atom</code>.
In addition to these singelton types, there are
the ``any in namespace'' subtypes, written:
<code>p:*</code> where <code>p</code> is a namespace prefix;
this type has all the atoms in the namespace denoted by <code>p</code>.

The token <code>.:*</code> represents all the atoms
in the current default namespace.
</p>

<p>
When used as atoms and not tags, the singleton types
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and ``any in namespace'' types must be prefixed by a backquote,
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as for atom values: <code>`p:x, `p:*, `.:*</code>.
</p>

</box>


<box title="Printing XML documents" link="print">

<p> The <code>print_xml</code> and <code>print_xml_utf8</code>
operators produce a string representation of an XML document.
They have to assign prefixes to namespace. In the current
implementation, CDuce produces XML documents with no
default namespace and only toplevel prefix bindings (that is,
<code>xmlns:p="..."</code> attributes are only produced for the root
element). Prefix names are chosen using several heuristics.
First, CDuce tries using the prefixes bound in the scope
of the <code>print_xml</code> operator. When this is not possible,
it uses global ``hints'': each time a prefix binding is encountered
(in the CDuce program or in loaded XML documents), it creates
a global hint for the namespace. Finally, it generates fresh
prefixes of the form <code>ns%%n%%</code> where <code>%%n%%</code>
is an integer.

For instance, consider the expression:
</p>

<sample>
print_xml (namespace "A" in &lt;a>[])
</sample>

<p>
As there is no available name the prefix URI "A", CDuce generates
a fresh prefix and produces the following XML documents:
</p>

<sample>
&lt;ns1:a xmlns:ns1="A"/>
</sample>

<p>
Now consider this expression:
</p>

<sample>
print_xml (namespace p = "A" in &lt;p:a>[])
</sample>

<p>CDuce produces:</p>

<sample>
&lt;p:a xmlns:p="A"/>
</sample>

<p>
In this case, the prefix binding for the namespace "A" is not
in the scope of <code>print_xml</code>, but the name <code>p</code>
is available as a global hint. Finally, consider:
</p>

<sample>
namespace q = "A" in print_xml (namespace p = "A" in &lt;p:a>[])
</sample>

<p>
Here, the prefix <code>q</code> is available in the scope of
the <code>print_xml</code>. So it is used in priority:
</p>

<sample>
&lt;q:a xmlns:q="A"/>
</sample>

<p>
As a final example, consider the following expression:
</p>

<sample>
print_xml (namespace p ="A" in &lt;p:a>[ (namespace p = "B" in &lt;p:a>[]) ])
</sample>

<p>
A single name <code>p</code> is available for both namespaces
"A" and "B". CDuce choses to assign it to "A", and it generates
a fresh name for "B", so as to produce:
</p>

<sample>
&lt;p:a xmlns:ns1="B" xmlns:p="A">&lt;ns1:a/>&lt;/p:a>
</sample>

<p>
Note that the fresh names are ``local'' to an application of
<code>print_xml</code>. Several application of <code>print_xml</code>
will re-use the same names <code>ns1, ns2</code>, ...
</p>

</box>

<box title="Pretty-printing of XML values and types" link="pretty">

<p>
The CDuce interpreter and toplevel uses an algorithm similar
to the one mentioned in the previous section to pretty-print
CDuce values and types that involve namespace.
</p>

<p>
The main difference is that it does <em>not</em> use by default the current
set of prefix bindings. The rationale is that this set can change
and this would make it difficult to understand the output of CDuce.
So only global hints are used to produce prefixes. Once a prefix has
been allocated, it is not re-used for another namespace. 
The toplevel directive <code>#env</code> causes CDuce to print, amongst
other, the table of prefixes used for pretty-printing.
It is possible to reinitialize this table with the directive
<code>#reinit_ns</code>. This directive also set
the current set if prefix bindings as a primary source of
hints for assigning prefixes for pretty-printing in the future.
</p>

</box>

<box title="Miscellaneous" link="misc">

<p>
Contrary to the W3C <a
href="http://www.w3.org/TR/xml-names11">Namespaces in XML 1.1</a>
Candidate Recommendation, a CDuce declaration <code>namespace p =
""</code> does <em>not</em> undeclare the prefix
<code>p</code>. Instead, it binds it to the null namespace
(that is, a QName using this prefix is interpreted as
having no namespace).
</p>

</box>

</page>