src/Usernames.hs

   1 {-# LANGUAGE DeriveDataTypeable #-}
   2
   3 -- | A newtype around a list of Strings which represent the usernames
   4 --   to watch. This is all to avoid an orphan instance of Configured
   5 --   for [String] if we had defined one in e.g. OptionalConfiguration.
   6 --
   7 module Usernames ( Usernames(..) )
   8 where
   9
  10 -- DC is needed only for the DCT.Configured instance of String.
  11 import qualified Data.Configurator as DC()
  12 import qualified Data.Configurator.Types as DCT
  13 import Data.Data ( Data )
  14 import System.Console.CmdArgs.Default ( Default(..) )
  15 import Data.Typeable ( Typeable )
  16
  17
  18 -- | Wrapper around a list of strings (usernames).
  19 --
  20 newtype Usernames =
  21   Usernames { get_usernames :: [String] }
  22     deriving (Data, Show, Typeable)
  23
  24
  25 instance Default Usernames where
  26   -- | The default list of usernames is empty.
  27   --
  28   def = Usernames []
  29
  30
  31
  32 instance DCT.Configured Usernames where
  33   -- | This allows us to read a 'Usernames' object out of a
  34   --   Configurator config file. By default Configurator wouldn't know
  35   --   what to do, so we have to tell it that we expect a list, and if
  36   --   that list has strings in it, we can apply the Usernames
  37   --   constructor to it.
  38   convert (DCT.List xs) =
  39     fmap Usernames (mapM convert_string xs)
  40     where
  41       convert_string :: DCT.Value -> Maybe String
  42       convert_string = DCT.convert
  43
  44   convert _ = Nothing