-- | A newtype around a list of Strings which represent hosts to look up.
--   This is all to avoid an orphan instance of 'Configured' for
--   [String] if we had defined one in e.g. 'OptionalConfiguration'.
--
{-# LANGUAGE DeriveDataTypeable #-}

module Hosts ( Hosts(..) )
where

-- DC is needed only for the DCT.Configured instance of String.
import qualified Data.Configurator as DC()
import qualified Data.Configurator.Types as DCT (
  Configured,
  Value( List ),
  convert )
import Data.Data ( Data )
import System.Console.CmdArgs.Default ( Default(..) )
import Data.Typeable ( Typeable )


-- | A (wrapper around a) list of hosts.
--
newtype Hosts =
  Hosts { get_hosts :: [String] }
    deriving (Data, Show, Typeable)


-- | The default list of hosts. It's empty.
--
instance Default Hosts where def = Hosts []

instance DCT.Configured Hosts where
  -- | This allows us to read a Hosts object out of a Configurator
  --   config file. By default Configurator wouldn't know what to do,
  --   so we have to tell it that we expect a list, and if that list
  --   has strings in it, we can apply the Hosts constructor to
  --   it.
  convert (DCT.List xs) =
    -- mapM gives us a Maybe [String] here.
    fmap Hosts (mapM convert_string xs)
    where
      convert_string :: DCT.Value -> Maybe String
      convert_string = DCT.convert

  -- If we read anything other than a list of values out of the file,
  -- fail.
  convert _ = Nothing