add

BackendType.hs

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+{- git-annex backend data types
+ - -}
+
+module BackendType (
+	-- the entire types are exported, for use in backend implementations
+	Key(..),
+	Backend(..)
+) where
+
+import GitRepo
+
+-- annexed filenames are mapped into keys
+type Key = FilePath
+
+-- this structure represents a key/value backend
+data Backend = Backend {
+	-- name of this backend
+	name :: String,
+	-- converts a filename to a key
+	getKey :: GitRepo -> FilePath -> IO (Maybe Key),
+	-- stores a file's contents to a key
+	storeFileKey :: GitRepo -> FilePath -> Key -> IO Bool,
+	-- retrieves a key's contents to a file
+	retrieveKeyFile :: Key -> FilePath -> IO Bool,
+	-- removes a key
+	removeKey :: Key -> IO Bool
+}
+
+instance Show Backend where
+	show backend = "Backend { name =\"" ++ (name backend) ++ "\" }"
+