fix storing files in .git/annex by key

Annex.hs

@@ -9,14 +9,13 @@ import System.Directory
 import GitRepo
 import Utility
 
-{- An annexed file's content is stored in .git/annex/. -}
-annexedFileLocation repo file = do
+{- An annexed file's content is stored somewhere under .git/annex/ -}
+annexLoc repo key = do
 	dir <- gitDir repo
-	return $ dir ++ "/annex/" ++ (gitRelative repo file)
+	return $ dir ++ "/annex/" ++ key
 
 {- Annexes a file, storing it in a backend, and then moving it into
- - the annex directory and setting up the symlink pointing to its
- - content. -}
+ - the annex directory and setting up the symlink pointing to its content. -}
 annexFile :: [Backend] -> GitRepo -> FilePath -> IO ()
 annexFile backends repo file = do
 	alreadyannexed <- lookupBackend backends repo file
@@ -24,12 +23,12 @@ annexFile backends repo file = do
 		Just _ -> error $ "already annexed " ++ file
 		Nothing -> do
 			stored <- storeFile backends repo file
-			if (not stored)
-				then error $ "no backend could store " ++ file
-				else symlink
+			case (stored) of
+				Nothing -> error $ "no backend could store " ++ file
+				Just key -> symlink key
 	where
-		symlink = do
-			dest <- annexedFileLocation repo file
+		symlink key = do
+			dest <- annexLoc repo key
 			createDirectoryIfMissing True (parentDir dest)
 			renameFile file dest
 			createSymbolicLink dest file

Backend.hs

@@ -44,21 +44,29 @@ backendFile :: Backend -> GitRepo -> FilePath -> String
 backendFile backend repo file = gitStateDir repo ++
 	(gitRelative repo file) ++ "." ++ (name backend)
 
-{- Attempts to Stores a file in one of the backends. -}
-storeFile :: [Backend] -> GitRepo -> FilePath -> IO (Bool)
-storeFile [] _ _ = return False
+{- Attempts to store a file in one of the backends, and returns
+ - its key. -}
+storeFile :: [Backend] -> GitRepo -> FilePath -> IO (Maybe Key)
+storeFile [] _ _ = return Nothing
 storeFile (b:bs) repo file = do
 	try <- (getKey b) (gitRelative repo file)
 	case (try) of
-		Nothing -> storeFile bs repo file
+		Nothing -> nextbackend
 		Just key -> do
-			(storeFileKey b) file key
+			stored <- (storeFileKey b) file key
+			if (not stored)
+				then nextbackend
+				else do
+					bookkeeping key
+					return $ Just key
+	where
+		nextbackend = storeFile bs repo file
+		backendfile = backendFile b repo file
+		bookkeeping key = do
 			createDirectoryIfMissing True (parentDir backendfile)
 			writeFile backendfile key
-			return True
-				where backendfile = backendFile b repo file
 
-{- Attempts to retrieve an file from one of the backends, saving it to 
+{- Attempts to retrieve an file from one of the backends, saving it to
  - a specified location. -}
 retrieveFile :: [Backend] -> GitRepo -> FilePath -> FilePath -> IO (Bool)
 retrieveFile backends repo file dest = do