Re: [Haskelldb-users] Opinions/Guidance wanted for starting an undergraduate project

[<ch...@dt...>]

Hi. It's always nice to hear from someone is interested in HaskellDB.=20
I'll try to answer your questions as well as I can. As you may or may=20
not know, I am one of the HaskellDB developers. I am also an=20
undergraduate student starting my 4th year this autumn.

Adrian Hudnott wrote:

>Hi.  I'm new to Haskell but am exploring the
>possibilities for extending the functionality of
>functional programming languages in for my
>undergraduate 3rd year project.  Along with graphical
>user interfaces databases seems one area where
>functional languages are typically less developed than
>imperative languages.  While reading C J Date's
>classic book I was inspired by the example of Datalog
>applied to logic programming to think about similar
>correspondences between relational databases and
>functional programming, and realize that such
>correlations are strong.  I=92m looking to build a
>pre-compiler and database access extension for a
>mainstream functional language, based upon relational
>algebra and the essence of types, tuples, etc. as
>opposed to an SQL wrapper.  Then I came across
>HaskellDB and realized that this has already been
>done!  I also dug out some old papers which
>integrate the functional paradigm and databases even
>closer, such as FDM/DAPLEX (Shipman, 1981) and FQL
>(Buneman, 1979).  In a nutshell: Buneman reduces all
>database queries into terms of functional composition,
>mapping, filtering and tuple building without loosing
>much clarity!  There appear to be three possibilities
>for where I could go from here:
>
>1)Do you (the developers, including Dr Leijen) have
>any particular applications which you=92d like to see
>HaskellDB put to use for?  Perhaps one of these will
>inspire me to head off in a different direction.
> =20
>
I'm not sure Dr Leijen reads this mailing list, so you might want to=20
contact him directly if you wish for his opinion. There are of course=20
lots of applications that could be created using HaskellDB. Most of them=20
involve combining HaskellDB and some other Haskell library. I am=20
currently developing a combined HaskellDB/wxHaskell program (a rather=20
dull GUI program that needs to access a database). Some of my colleagues=20
made a basic web forum by combining HaskellDB and WASH/CGI.

>=20
>2)Investigate the possibilities for taking the
>HaskellDB project further in some way.
> =20
>
There is some interesting work to be done here, that we have been=20
thinking about, specifically we want to add a non-SQL backend to=20
HaskellDB, perhaps something like Hardbms (nice since it is defined=20
completely in Haskell) The problem with this is that it requires lots of=20
internal restructuring of HaskellDB, which might prove too difficult (or=20
rather, time-consuming) for someone not familiar with the code. This is=20
something that's on the top of my list for future developments I want to =
do.

>3)The most ambitious and my favourite option: To
>start from scratch at implementing something based
>upon
>similar principles to HaskellDB in NJ
>SML/MLj/Java/JDBC or SML.NET/J# and javacc or SML
>(which is much more the domain of my existing skills
>set).  My supervisor has some reservations, and asks
>how much HaskellDB relies on laziness, in determining
>whether this is practical?  The documentation on
>HaskellDB seems a little sparse (with me being new to
>Haskell syntax as well!). How are you modelling the
>=93dynamism=94 of a database?  Meaning: when [say] an
>insert operation is performed do you return =93a new
>database=94?  If so, what happens if you try to
>perform an operation using a variable which still
>points to =93the old database=94?
> =20
>
This sounds quite interesting, although it does seem like a rather large=20
project. I agree that the HaskellDB documentation is lacking, especially=20
for beginners. I recommend the following resources, in order:
- Daan's and Erik Meijer's original paper, see=20
http://www.cs.uu.nl/people/daan/download/papers/dsec.ps
- Daan's PhD thesis, chapter 5 talks about HaskellDB, see2
http://www.cs.uu.nl/people/daan/download/papers/phd-thesis.pdf
- Our as of yet unpublished paper, found at=20
http://haskelldb.sourceforge.net/haskelldb.ps

HaskellDB actually doesn't rely on laziness very much. It handles=20
dynamism through a Haskell concept called monads. I won't discuss them=20
further here. If you're not familiar with them, for now you may think of=20
them as a way to allow us to perform I/O in a purely functional way.

I'm going to try and explain with some examples from my own program.

A database is normally modeled with a function of type:
db :: (Database -> IO a) -> IO a
db =3D genericConnect "sqlite" ["./database","ReadWriteMode"]

So here "db" is my database function. It takes another function as an=20
argument. It then uses this function to perform some action in the real=20
world, returning that result. It is used in the following way:

addCustomer :: Customer -> IO ()
addCustomer c =3D db ( \db -> insert db C.customer
(C.iD << constant (cid c) #
C.name << constant (cname c) #
C.address << constant (caddr c) #
C.price << constant (cpric c)))

Here our database is applied to the insert function. What this actually=20
does is that db connects to the database and performs the actions=20
specified, then disconnects and returns the result (which here is=20
irrelevant).

getCustomers :: IO [Customer]
getCustomers =3D db ( \db ->
do t <- query db (table C.customer)
return (map toCustomer t))

Here is another example. This queries the database for all our=20
customers, applies a Haskell function to them via map, and returns the=20
result of that operation.

I hope this cleared things up a bit.

>(P.S: I haven=92t actually managed to get a working copy
>of HaskellDB installed yet.  GHC compiles under
>Mandrake but fails to work.  HSQL - both Windows
>binary and compiling from sources under Cygwin both
>fail to work. :( )
> =20
>
Too bad. If you provide me with some more information I might be able to=20
help you with this.

Regards,
Anders