harbl-cli/src/Hosts.hs

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