Tomasz Zielonka wrote:
On Thu, Apr 27, 2006 at 02:26:22AM +0300, Einar Karttunen wrote:
and handle options as functions from Config to Config:
Option ['i'] ["input"] (ReqArg (\x c -> c { infile = Just x }) "file") "input
file name"
I find this approach very convenient, but I push it a bit further. Some
time ago I wrote a small article about this:
http://www.haskell.org/pipermail/haskell/2004-January/013412.html
I was not the first one to use the approach but I felt that it should be
made more popular. Perhaps I should make a wiki page from it, but I seem
to never do such things and can't promise to do it this time :-/
You are dealing with more convenient option handling, validating and
defaulting on top of Sven Pannes famous GetOpt module. Nice stuff but
there is another important point: Your approach still needs a central
definition of an option list (or record) in the main (user) program. But
suppose you write some libraries that are used by a couple of user
programs. It becomes tedious and error prone to define the same lists of
options with descriptions and validating functions in all user programs
just to give it to the library. Moreover the user program in general
even don't know about the right validating function or option description.
So it would be much better to define the options in the library and to
provide this definitions to the user program somehow. I tought about
this topic several times and came up with a solution that works for me
but is far from being perfect. It uses existentials and a main
disadvantage is the need of explicit traversing. Moreover some new
boilerplate code is necessary.
You can find the interface in
http://liinwww.ira.uka.de/~rahn/src/Util/Option.hs
Sample library definitions of options are in
http://liinwww.ira.uka.de/~rahn/src/PCP/Fast/Env.hs
http://liinwww.ira.uka.de/~rahn/src/PCP/Fast/Description.hs
These definitions are combined in
http://liinwww.ira.uka.de/~rahn/src/PCP/Fast/Auto.hs
and finally used for example in the user programs
http://liinwww.ira.uka.de/~rahn/src/Prog/Eval.hs
http://liinwww.ira.uka.de/~rahn/src/Prog/Interesting.hs
Note, that the user programs just define options that are specific for
the program, e.g. both programs have options to define some search
bounds without definition.
As stated: Far from being perfect. Looking forward to get some new ideas!
Best regards, Mirko Rahn
--
-- Mirko Rahn -- Tel +49-721 608 7504 --
--- http://liinwww.ira.uka.de/~rahn/ ---
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