[Haskell-cafe] Parsing arguments and reading configuration

[Antoine Latter]

2008/9/19 Magnus Therning <magnus at therning.org>:
>
> First I thought I'd treat the configuration in a similar way, but then I
> noticed a slight ordering problem.  The command line arguments should
> take priority over the contents of the configuration file, but the
> location of the configuration can be given as an argument.  I could read
> the arguments twice, first to get the correct location of the config
> file, then load the config, and then read the arguments again to make
> sure they take priority.  But that feels a little silly.  Are there any
> more elegant solutions people are using?

I'm not sure how well it would hold up under maintenance, but you coud
have a config sum-type which is itself a monoid, and then create two
of them:

> data UserConfig = UserConfig
>   { item1 :: Maybe Type1
>   , item2 :: Maybe Type2
>   , configFileLocation :: Maybe FilePath
>   }

> instance Monoid UserConfig where
>  {- not shown -}

> buildConfig :: IO UserConfig
> buildConfig = do
>   cmdLineCfg <- buildConfigFromCmdLine
>   fileCfg <- maybe (return mempty) buildConfigFromFile (configFileLocation cmdLineCfg)
>
>   return $ fileCfg `mappend` cmdLineCfg

> -- mappend is assumed to be left-biased

> buildConfigFromCmdLine :: IO UserConfig
> buildConfigFromFile :: FilePath -> IO UserConfig

Does that make sense?  or is it too complicated?

-Antoine