src/MxList.hs

   1 {-# LANGUAGE DeriveDataTypeable #-}
   2
   3 -- | A newtype around a list of Strings which represent the MXes whose
   4 --   remote forwards we don't report. This is all to avoid an orphan
   5 --   instance of Configured for [String] if we had defined one in
   6 --   e.g. 'OptionalConfiguration'.
   7 --
   8 module MxList (
   9   MxList(..) )
  10 where
  11
  12 -- DC is needed only for the DCT.Configured instance of String.
  13 import qualified Data.Configurator as DC()
  14 import qualified Data.Configurator.Types as DCT (
  15   Configured,
  16   Value( List ),
  17   convert )
  18 import Data.Data (Data)
  19 import System.Console.CmdArgs.Default (Default(..))
  20 import Data.Typeable (Typeable)
  21
  22
  23 -- | A (wrapper around a) list of MX hostnames.
  24 --
  25 newtype MxList =
  26   MxList { get_mxs :: [String] }
  27     deriving (Data, Show, Typeable)
  28
  29
  30 -- | The default (empty) list of MXes.
  31 --
  32 instance Default MxList where
  33   def = MxList []
  34
  35
  36 instance DCT.Configured MxList where
  37   -- | This allows us to read a MxList object out of a Configurator
  38   --   config file. By default Configurator wouldn't know what to do,
  39   --   so we have to tell it that we expect a list, and if that list
  40   --   has strings in it, we can apply the MxList constructor to
  41   --   it.
  42   convert (DCT.List xs) =
  43     -- mapM gives us a Maybe [String] here.
  44     fmap MxList (mapM convert_string xs)
  45     where
  46       convert_string :: DCT.Value -> Maybe String
  47       convert_string = DCT.convert
  48
  49   -- If we read anything other than a list of values out of the file,
  50   -- fail.
  51   convert _ = Nothing