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diff --git a/harbl-cli/src/Hosts.hs b/harbl-cli/src/Hosts.hs
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+-- | 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