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