HXQ-0.12.0: index.html
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<head><title>HXQ: A Compiler from XQuery to Haskell</title></head>
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<h1>HXQ: A Compiler from XQuery to Haskell</h1>
<h3>Download <a href="/HXQ-0.12.0.tar.gz">HXQ-0.12.0.tar.gz</a></h3>
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<p>
<h2>Description</h2>
<p>
HXQ is a fast and space-efficient translator
from <a href="http://www.w3.org/XML/Query/">XQuery</a> (the standard
query language for XML) to embedded Haskell code. The translation is
based on Haskell templates. HXQ takes full advantage of Haskell's lazy
evaluation to keep in memory only those parts of XML data needed at
each point of evaluation, thus performing stream-based evaluation for
forward queries (queries that do not contain backward steps). This
results to an implementation that is as fast and space-efficient as
any stream-based implementation based on SAX filters or finite state
machines. Furthermore, the coding is far simpler and extensible since
its based on XML trees, rather than SAX events.
<p>
For example, the <a href="Test2.hs">XQuery given below</a>, which is
against the <a href="http://dblp.uni-trier.de/xml/">DBLP XML
file</a> (420MB), runs in 39 seconds on my laptop PC (using 18MB of max heap space).
To contrast this, <a href="http://www.gnu.org/software/qexo/">Qexo</a>, which
compiles XQueries to Java bytecode, took 1 minute 17 seconds (using no less than 1400MB of heap space).
Also <a href="http://xqilla.sourceforge.net/HomePage">XQilla</a>, which is written in C++, took 1 minute and 10 secs
(using 1150MB of heap space). (All results are taken on an Intel Core 2 Duo 2.2GHz 2GB running ghc-6.8.2 on Linux 2.6.23 kernel.)
<p>
Finally, HXQ can store XML documents in a relational database
(currently MySQL or SQLite), by shredding XML into relational tuples, and by
translating XQueries over the shredded documents into optimized SQL
queries.
<p>
<h2>Installation Instructions (HXQ without Database Connectivity)</h2>
<p>
HXQ can be installed on most platforms but I have only tested it on
Linux, MAC OS X, and Windows XP. The simplest installation is without database
connectivity (ie, it can only process XQueries against XML text
documents). If you want database connectivity
(over mysql or sqlite relational databases), look at the <a href="db.html">installation instructions with database connectivity</a>.
<p>
First, you need to install the Glasgow Haskell
Compiler, <a href="http://www.haskell.org/ghc/">ghc</a>. Optionally,
if you want to modify the XQuery parser, you need to install the
parser generator for Haskell,
<a href="http://www.haskell.org/happy/">happy</a>. Then,
download <a href="/HXQ-0.12.0.tar.gz">HXQ version 0.12.0</a> and untar
it (using <tt>tar xfz</tt> on Linux
or <a href="http://www.7-zip.org/">7z x</a> on Windows). Then,
you execute the following commands inside the HXQ directory:
<pre>
runhaskell Setup.lhs configure
runhaskell Setup.lhs build
runhaskell Setup.lhs install
</pre>
On linux, the last command must be run as root.
If you get an error during configuration that the readline package is missing, install
<a href="http://hackage.haskell.org/cgi-bin/hackage-scripts/package/readline">readline</a> before HXQ.
HXQ consists of the executable <tt>xquery</tt>, which is the XQuery interpreter, and the
HXQ library. To use the HXQ library in a Haskell program,
simply <tt>import Text.XML.HXQ.XQuery</tt>.
<p>
<h2><a href="db.html">Installation Instructions for Database Connectivity</a></h2>
<p>
<p>
<h2>Current Status</h2>
<p>
HXQ supports most essential XQuery features, although some system
functions are missing (but are easy to add). To see the list of
supported system functions, run <tt>xquery -help</tt> . HXQ does not
have static typechecking; it leaves all checking to Haskell. In
addition, the XQuery semantics requires duplicate elimination and
sorting by document order for every XPath step, which is very
expensive and unnecessary in most cases. This is not currently
supported by HXQ but will be addressed in the future (needs a static
analysis to determine when duplicate elimination is necessary). For
example, <tt>e//*//*</tt> may return duplicate elements in HXQ.
<p>
HXQ uses the <a href="http://www.flightlab.com/~joe/hxml/">HXML parser
for XML</a> (developed by Joe English), which is included in the
source. I have also tried hexpat, tagsoup, HXT, and HaXML Xtract, but
they all have space leaks.
<p>
HXQ has two parsers: one that generates simple rose trees from XML
documents, which can be processed by forward queries without space
leaks, and another parser where each tree node has a reference to its
parent. Some, but not all, backward axis steps (such as the parent
axis /..) are removed from a query using optimization rules. If there
are backward axis steps left in the query, then HXQ uses the latter
parser, which may result to a performance penalty due to space leaks.
<p>
<h2>XQuery Documentation</h2>
<p>
The complete XQuery syntax in HXQ is described in <a href="hxq-manual.pdf">hxq-manual.pdf</a>.
Here some tutorials on <a href="http://lambda.uta.edu/cse6339/queries2.pdf">XPath</a>
and <a href="http://lambda.uta.edu/cse6339/xquery2.pdf">XQuery</a>.
Here is a nice <a href="http://www.dbis.ethz.ch/education/ws0708/xml_db_ws2007">course on XML and XQuery</a>.
<p>
<h2>Using the Compiler</h2>
<p>
The main functions for embedding XQueries in Haskell are:
<ul>
<li> <tt>$(xe query) :: XSeq</tt>
<li> <tt>$(xq query) :: IO XSeq</tt>
</ul>
where <tt>query</tt> is a string value (a Haskell expression that
evaluates to a string <b>at compile-time</b>). They both translate the
query into Haskell code, which is compiled and optimized into machine
code directly. The code that xe generates has type <tt>XSeq</tt> (a
sequence of XML trees of type <tt>[XTree]</tt>) while the code that xq
generates has type <tt>(IO XSeq)</tt>. If the query reads a
document (using doc(...)) or calls an external function, then you should use xq since it requires
IO. You can use the value of a Haskell variable <tt>v</tt> inside
a query using <tt>$v</tt> as long as <tt>v</tt> has
type <tt>XSeq</tt>. To call a function in a query, it should be
defined in Haskell with type <tt>(XSeq,...,XSeq) -> IO XSeq</tt>.
<p>
Here is an example of a main program:
<pre>
f(x,y) = $(xq "<article><first>{$x}</first><second>{$y}</second></article>")
main = do a <- $(xq ("<result>{ "
++" for $x at $i in doc('data/dblp.xml')//inproceedings "
++" where $x/author = 'Leonidas Fegaras' "
++" order by $x/year descending "
++" return <paper>{ $i, ') ', $x/booktitle/text(), "
++" ': ', $x/title/text() "
++" }</paper> "
++" }</result> "))
putXSeq a
b <- $(xq " f( $a/paper[10], $a/paper[8] ) ")
putXSeq b
</pre>
Another example, can be found in <a href="Test1.hs">Test1.hs</a>. You compile it using
<tt>ghc --make Test1.hs -o a.out</tt>.
<p>
You can compile an XQuery file into a Haskell program
(<tt>Temp.hs</tt>) using <tt>xquery -c file</tt>. Or better, you can
use the script <tt>compile</tt> (on either Unix or Windows) to compile the XQuery file
to an executable. For example:
<pre>
compile data/q1.xq
</pre>
will compile the XQuery file <a href="data/q1.xq">data/q1.xq</a> into the executable <tt>a.out</tt>.
<p>
<h2>Using the Interpreter</h2>
<p>
The HXQ interpreter is far more slower than the compiler; use it only
if you need to evaluate ad-hoc XQueries read from input or from files.
The main functions are:
<ul>
<li> <tt>xquery :: String -> IO XSeq</tt> -- Evaluates an XQuery in a string
<li> <tt>xfile :: String -> IO XSeq</tt> -- Evaluates an XQuery in a file
</ul>
The HXQ interpreter doesn't recognize Haskell variables and functions
(but you may declare XQuery variables and functions using the XQuery
'declare' syntax). The main HXQ program, called <tt>xquery</tt>,
evaluates an XQuery in a file using the interpreter. For example:
<pre>
xquery data/q1.xq
</pre> Without an argument, it reads and evaluates XQueries and
variable/function declarations from input. With <tt>xquery -p
xpath-query xml-file</tt> you evaluate an XPath query against an XML
file, eg. <tt>xquery -p "//inproceedings[100]" data/dblp.xml</tt>.
With <tt>xquery -help</tt> you get the list of system functions and
usage information.
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<address>Last modified: 12/06/08 by <a href="http://lambda.uta.edu/">Leonidas Fegaras</a></address>