2008/9/19 Magnus Therning
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