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+{- git-annex backend data types
+ -
+ - Mostly only backend implementations should need to import this.
+ -}
+
+module BackendTypes where
+
+import Control.Monad.State
+import Data.String.Utils
+import qualified GitRepo as Git
+
+-- 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]
+} deriving (Show)
+
+-- git-annex's monad
+type Annex = StateT AnnexState IO
+
+-- annexed filenames are mapped into keys
+data Key = Key String deriving (Eq)
+
+-- show a key to convert it to a string
+instance Show Key where
+	show (Key v) = v
+
+-- 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
+}
+
+instance Show Backend where
+	show backend = "Backend { name =\"" ++ (name backend) ++ "\" }"