add a UUID type

Should have done this a long time ago.

Types/Remote.hs

@@ -15,6 +15,7 @@ import Data.Ord
 
 import qualified Git
 import Types.Key
+import Types.UUID
 
 type RemoteConfig = M.Map String String
 
@@ -25,15 +26,15 @@ data RemoteType a = RemoteType {
 	-- enumerates remotes of this type
 	enumerate :: a [Git.Repo],
 	-- generates a remote of this type
-	generate :: Git.Repo -> String -> Maybe RemoteConfig -> a (Remote a),
+	generate :: Git.Repo -> UUID -> Maybe RemoteConfig -> a (Remote a),
 	-- initializes or changes a remote
-	setup :: String -> RemoteConfig -> a RemoteConfig
+	setup :: UUID -> RemoteConfig -> a RemoteConfig
 }
 
 {- An individual remote. -}
 data Remote a = Remote {
 	-- each Remote has a unique uuid
-	uuid :: String,
+	uuid :: UUID,
 	-- each Remote has a human visible name
 	name :: String,
 	-- Remotes have a use cost; higher is more expensive

Types/UUID.hs

@@ -7,5 +7,15 @@
 
 module Types.UUID where
 
--- might be nice to have a newtype, but lots of stuff treats uuids as strings
-type UUID = String
+-- A UUID is either an arbitrary opaque string, or UUID info may be missing.
+data UUID = NoUUID | UUID String
+	deriving (Eq, Ord)
+
+instance Show UUID where
+	show (UUID u) = u
+	show NoUUID = ""
+
+instance Read UUID where
+	readsPrec _ s
+		| null s = [(NoUUID, "")]
+		| otherwise = [(UUID s, "")]