[Haskell-cafe] Haskell DB and XML libs: one user's experience

Robert Dockins robdockins at fastmail.fm
Tue May 16 16:57:18 EDT 2006

Hello all,

I recently found myself needing to do some data manipulation; I  
needed to take some data from a database and generate a series of XML  
files from it.  In the past I've done most of this sort of work in  
Java, but this time I decided I'd take the opportunity to explore the  
state of the art of Haskell DB and XML libraries.

As to DB, I tried using HDBC first.  I was actually a little  
surprised how straightforward it was.  My database (PostgreSQL) is  
directly supported, and the compile/install went smoothly.  My first  
test connection program that typechecked worked as expected (!) and I  
was soon executing queries doing useful work with the results.  I'd  
just like to take a moment to congratulate John Goerzen for creating  
a product with a low barrier of entry for using databases in  
Haskell.  As I didn't really do anything beyond simple queries, can't  
comment on more advanced features.

For XML, I wavered between HaXml and HXT (the Haskell XML Tookbox).   
I initially decided to use HXT because it has support for xml  
namespaces, which I was going to need, and because it just seems to  
be the most complete and advanced package available.  The HXT install  
suffers a little bit from transitive-closureitis, but, overall wasn't  
too bad.  However, I had a really hard time using it!  The API is  
_really_ intimidating, and I couldn't find any basic tutorial-style  
documentation.  The API docs are a little hard to use because related  
definitions are spread out over a bunch of modules, and the links  
don't always work.  Also, the theses are nice, but they read like  
theses ;-)  That's not what I want when I have a job to complete.   
Long story made short; I couldn't figure out how to create and XML  
document and serialize it to disk.  I was reasonably motivated and  
I'm a pretty experienced Haskell programmer, but I had to call it  
quits after about 3 hours of struggling with it.  Most of my programs  
would mysteriously fail to produce output OR errors!  It was really  

I ended up using HaXml instead and shoehorning in the namespaces by  
using attributes named "xmlns:xyz" etc. on the document root element  
(which is OK, but not ideal).  The HaXml API was also tough to work  
with but was less mystifying than HXT's, and I eventually got it to  
work.  I was a little disappointed by the results, because the pretty  
printer does some fairly bizarre things to ensure that it doesn't  
introduce extra whitespace into the DOM.  I also had to do some  
futzing to make HaXml correctly escape literal text.  Finally, the  
using the HaXml API to generate XML results in verbose code that's  
hard to read.  I was hoping that I'd get results comparable to using  
xmlenc (http://xmlenc.sourceforge.net/) in Java, but I was  
disappointed by fairly low signal-to-noise ratio (although in all  
fairness, its probably comparable to using the DOM or SAX Java  
APIs).  Overall, HaXml works, but feels a bit awkward, at least for  
this use case.

Now taking a slightly closer look at HXML, I see that it may be the  
best choice for what I was attempting to do (although it also doesn't  
support namespaces).  The simplified representation looks  
particularly nice for building XML from scratch.  I may try rewriting  
with HXML and see how that goes.

So that's it.  I don't have any deep conclusions, but I thought I'd  
share my experiences in the hopes that they will be helpful to somebody.

Rob Dockins

Speak softly and drive a Sherman tank.
Laugh hard; it's a long way to the bank.
           -- TMBG

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