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 Data.Monoid ( Monoid(..) )
  15 import Data.Typeable ( Typeable )
  16 import System.Console.CmdArgs.Default ( Default(..) )
  17
  18
  19 -- | Wrapper around a list of strings (usernames).
  20 --
  21 newtype Usernames =
  22   Usernames { get_usernames :: [String] }
  23     deriving (Data, Show, Typeable)
  24
  25
  26 instance Default Usernames where
  27   -- | The default list of usernames is empty.
  28   --
  29   def = Usernames []
  30
  31
  32 -- | The 'Monoid' instance for 'Usernames' uses an
  33 --   'Monoid' instance for lists.
  34 --
  35 instance Monoid Usernames where
  36   -- | The \"empty\" 'Usernames' simply wraps an empty list.
  37   mempty = Usernames []
  38
  39   -- | This mappend is a little funny; it always chooses the second
  40   --   list if that list is nonempty. Otherwise, it chooses the
  41   --   first. This is actually associative!
  42   u1 `mappend` u2
  43     | null (get_usernames u2)  = u1
  44     | otherwise = u2
  45
  46
  47 instance DCT.Configured Usernames where
  48   -- | This allows us to read a 'Usernames' object out of a
  49   --   Configurator config file. By default Configurator wouldn't know
  50   --   what to do, so we have to tell it that we expect a list, and if
  51   --   that list has strings in it, we can apply the Usernames
  52   --   constructor to it.
  53   convert (DCT.List xs) =
  54     fmap Usernames (mapM convert_string xs)
  55     where
  56       convert_string :: DCT.Value -> Maybe String
  57       convert_string = DCT.convert
  58
  59   convert _ = Nothing