Excerpts from Magnus Therning's message of Fri Jan 08 16:25:31 +0100 2010:

On Fri, Jan 8, 2010 at 12:53 PM, Sebastian Fischer wrote:

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Dear Café,

Neil Mitchell's cmdargs package [1] is pretty neat. It can be used to parse command-line arguments into a user-defined data structure.

Is there something similar for parsing config files?

If you write one I most certainly will use it! ;)

Seriously, cmdargs is *brilliant*. It's also magic (to me).

Not only to you in fact it is black magic since it uses unsafePerformIO :( -- Nicolas Pouillard http://nicolaspouillard.fr

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Is there something similar for parsing config files?

If you write one I most certainly will use it! ;)

You (we) can already start using the cmdargs package to parse config files. Upon my feature request to add a function to the cmdargs package that allows to add default arguments, Neil pointed out that the function System.Environment.withArgs can be used to get the same effect without changes to the cmdargs package. Here is a complete example: {-# LANGUAGE DeriveDataTypeable #-} import System.Environment import System.Console.CmdArgs data Conf = Conf { option :: Bool } deriving (Show,Data,Typeable) myConf = mode $ Conf { option = enum False [True,False] } main = print =<< getConfig "my.conf" "My Program v0.0" myConf getConfig configFileName welcomeMsg modeDesc = do originalArgs <- getArgs argsFromFile <- words `fmap` readFile configFileName withArgs (argsFromFile ++ originalArgs) (cmdArgs welcomeMsg [modeDesc]) If you save the String '--true' in the file 'my.conf', this program reads the config from the file and prints it: # runhaskell typed-config.hs Conf {option = True} You can overwrite the default behaviour with command line arguments: # runhaskell typed-config.hs --false Conf {option = False} After parsing a config file into command-line arguments, the parsing of the typed `Config` comes for free. Sebastian P.S.: Instead of the `words` function one would use some smarter function that translates real config files into command-line arguments, but the fez-conf package (which provides such functionality) segfaults on my computer. Depending on how one specifies the mode value, one may not be able to overwrite default options. For example, the usual translation of the boolean field above is a single flag --option that can be present or absent. I did not find a way to unset a set flag other than declaring it as an enum flag. This could be improved if flags without arguments would support optional arguments like '--option=yes/no' or similar. (Btw. the documentation of enum seems wrong, the given example does not typecheck). -- Underestimating the novelty of the future is a time-honored tradition. (D.G.)

Ketil Malde writes:

- CmdArgs helpfully provides default --help, --version as well as --quite and --verbose. For the two former, there's also a nice default implementation, but presumably the latter two are for use in the program proper. Unfortunately, I don't know how to get at their values.

I couldn't find it in an example, but apparently it is done through global functions 'isLoud', 'isNormal', and 'isQuiet'. -k -- If I haven't seen further, it is by standing in the footprints of giants

Neil Mitchell writes:

The CmdArgs manual might help: http://community.haskell.org/~ndm/darcs/cmdargs/cmdargs.htm

Yes, this is what I used :-) Presenting examples is great, but gives me the hubris to rip off the example that seems to fit most closely, and modify it. This makes for a quick start, but also for the odd misunderstanding.

I'd describe cmdargs as referentially impure, but really concise.

Concise is nice - I've often been a bit annoyed that the otherwise nice GetOpt stuff forces me to write a lot of boilerplate, enough of it that my programs usually have a separate Options module. For a lazy slob like me, this means I often just hack something on top of the raw GetArgs or write half-baked but simple solutions like my simpleargs library.

'def' is the default value, empty has a particular semantic meaning and serves to change the options. I should document this more carefully. Perhaps empty should be renamed 'optional', since that's what it does.

'whenMissing'? 'unspecified'?

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- As I wanted a single file argument, I tried to use 'args' in  combination with a parameter of type FilePath.  Apparently 'args'  wants [FilePath] and appends command line arguments to the default  value.

Spoilt by Haskell, I'd have expected a type error here.

argPos 0 should do the trick.

Right. Will subsequent arguments be ignored?

CmdArgs is very much a 0.1 release. The documentation isn't polished, it does simple arguments nicely, but has flaws when you try and go more advanced. I want to spend some more time on it at some point.

That's great, I'm looking forward to it. -k -- If I haven't seen further, it is by standing in the footprints of giants