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+{- git-annex internal data types
+ -
+ - Most things should not need this, using Types and/or Annex instead.
+ -}
+
+module TypeInternals where
+
+import Control.Monad.State (StateT)
+import Data.String.Utils
+
+import qualified GitRepo as Git
+
+-- command-line flags
+data Flag = Force deriving (Eq, Read, Show)
+
+-- git-annex's runtime state type doesn't really belong here,
+-- but it uses Backend, so has to be here to avoid a depends loop.
+data AnnexState = AnnexState {
+	repo :: Git.Repo,
+	backends :: [Backend],
+	supportedBackends :: [Backend],
+	flags :: [Flag]
+} deriving (Show)
+
+-- git-annex's monad
+type Annex = StateT AnnexState IO
+
+-- annexed filenames are mapped through a backend into keys
+type KeyFrag = String
+type BackendName = String
+data Key = Key (BackendName, KeyFrag) deriving (Eq)
+
+-- show a key to convert it to a string; the string includes the
+-- name of the backend to avoid collisions between key strings
+instance Show Key where
+	show (Key (b, k)) = b ++ ":" ++ k
+
+instance Read Key where
+	readsPrec _ s = [((Key (b,k)) ,"")]
+		where
+			l = split ":" s
+			b = l !! 0
+			k = join ":" $ drop 1 l
+
+-- pulls the backend name out
+backendName :: Key -> BackendName
+backendName (Key (b,k)) = b
+
+-- pulls the key fragment out
+keyFrag :: Key -> KeyFrag
+keyFrag (Key (b,k)) = k
+
+-- this structure represents a key-value backend
+data Backend = Backend {
+	-- name of this backend
+	name :: String,
+	-- converts a filename to a key
+	getKey :: FilePath -> Annex (Maybe Key),
+	-- stores a file's contents to a key
+	storeFileKey :: FilePath -> Key -> Annex Bool,
+	-- retrieves a key's contents to a file
+	retrieveKeyFile :: Key -> FilePath -> Annex Bool,
+	-- removes a key
+	removeKey :: Key -> Annex Bool,
+	-- checks if a backend is storing the content of a key
+	hasKey :: Key -> Annex Bool
+}
+
+instance Show Backend where
+	show backend = "Backend { name =\"" ++ (name backend) ++ "\" }"