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HaXml: Haskell and XML
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<h2>DtdToHaskell tool</h2>
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<b>DtdToHaskell</b> is a tool (and <b>Text.XML.HaXml.Xml2Haskell</b>
provides the class framework) for translating any valid XML DTD
into equivalent Haskell types. This allows you to generate, edit,
and transform documents as normal typed values in programs, and to
read and write them as human-readable XML documents.
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Usage: <tt>DtdToHaskell [dtdfile [outfile]]</tt><br>
(Missing file arguments or dashes (<tt>-</tt>) indicate stdin
or stdout respectively.)
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The program reads and parses a DTD from <tt>dtdfile</tt> (which may be
either just a DTD, or a full XML document containing an internal DTD).
It generates into <tt>outfile</tt> a Haskell module containing a
collection of type definitions plus some class instance declarations
for I/O.
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In order to use the resulting module, you need to import it, and also
to <tt>import Text.XML.HaXml.Xml2Haskell</tt>. To read and write
XML files as values of the declared types, use some of the following
convenience functions:
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readXml :: XmlContent a => String -> Maybe a
showXml :: XmlContent a => a -> String
hGetXml :: XmlContent a => Handle -> IO a
hPutXml :: XmlContent a => Handle -> a -> IO ()
fReadXml :: XmlContent a => FilePath -> IO a
fWriteXml :: XmlContent a => FilePath -> a -> IO ()
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not forgetting to resolve the overloading in one of the usual
ways (e.g. by implicit context at point of use, by explicit type
signatures on values, use value as an argument to a function with
an explicit signature, use <tt>`asTypeOf`</tt>, etc.) (Also, note
the similarity between these signatures and those provided by the
<a href="Haskell2Xml.html">Haskell2Xml</a> library.)
<p>
You will need to study the automatically-generated type declarations to
write your own transformation scripts - most things are pretty obvious
parallels to the DTD structure.
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<b>Limitations</b><br>
The generated Haskell contains references to types like <tt>OneOf3</tt>
where there is a choice between <em>n</em> (in this case 3) different
tags. Currently, the module <b>Text.XML.HaXml.OneOfN</b> defines
these types up to <em>n</em>=20. If your DTD requires larger choices,
then use the tool <b>MkOneOf</b> to generate the extra size or range
of sizes you need.
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We mangle tag names and attribute names to ensure that they have the
correct lexical form in Haskell, but this means that (for instance) we
can't distinguish <tt>Myname</tt> and <tt>myname</tt>, which are
different names in XML but translate to overlapping types in Haskell
(and hence probably won't compile).
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Attribute names translate into named fields: but because Haskell doesn't
allow different types to have the same named field, this means your XML
document which uses the same name for similar attributes on different
tags would crash and burn. We have fixed this by incorporating the
tagname into the named field in addition to the attribute name, e.g.
<tt>tagAttr</tt> instead of just <tt>attr</tt>. Uglier, but more
portable.
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XML namespaces. Currently, we just mangle the namespace identifier into
any tag name which uses it. Probably the right way to do it is to
regard the namespace as a separate imported module, and hence translate
the namespace prefix into a module qualifier. Does this sound about
right? (It isn't implemented yet.)
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External subset. Since HaXml release 1.00, we support the
XML DTD external subset. This means we can read and parse a whole
bunch of files as part of the same DTD, and we respect INCLUDE and
IGNORE conditional sections.
<p>
There are some fringe parts of the DTD we are not entirely sure
about - Tokenised Types and Notation Types. In particular, there
is no validity checking of these external references. If you find
a problem, mail us: <tt>Malcolm.Wallace@cs.york.ac.uk</tt>
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