specialize Preparer a bit, so resourcePrepare can be added

The forall a. in Preparer made resourcePrepare not seem to be usable, so
I specialized a to Bool. Which works for both Preparer Storer and
Preparer Retriever, but wouldn't let the Preparer be used for hasKey
as it currently stands.

Remote/Directory.hs

@@ -6,7 +6,6 @@
  -}
 
 {-# LANGUAGE CPP #-}
-{-# LANGUAGE Rank2Types #-}
 
 module Remote.Directory (remote) where
 

Remote/Directory/LegacyChunked.hs

@@ -7,8 +7,6 @@
  - Licensed under the GNU GPL version 3 or higher.
  -}
 
-{-# LANGUAGE Rank2Types #-}
-
 module Remote.Directory.LegacyChunked where
 
 import qualified Data.ByteString.Lazy as L

Remote/Helper/ChunkedEncryptable.hs

@@ -14,6 +14,7 @@ module Remote.Helper.ChunkedEncryptable (
 	simplyPrepare,
 	ContentSource,
 	checkPrepare,
+	resourcePrepare,
 	fileStorer,
 	byteStorer,
 	fileRetriever,
@@ -49,6 +50,11 @@ checkPrepare checker helper k a = ifM (checker k)
 	, a Nothing
 	)
 
+-- Use to acquire a resource when preparing a helper.
+resourcePrepare :: (Key -> (r -> Annex Bool) -> Annex Bool) -> (r -> helper) -> Preparer helper
+resourcePrepare withr helper k a = withr k $ \r ->
+        a (Just (helper r))
+
 -- A Storer that expects to be provided with a file containing
 -- the content of the key to store.
 fileStorer :: (Key -> FilePath -> MeterUpdate -> Annex Bool) -> Storer

Types/StoreRetrieve.hs

@@ -5,8 +5,6 @@
  - Licensed under the GNU GPL version 3 or higher.
  -}
 
-{-# LANGUAGE Rank2Types #-}
-
 module Types.StoreRetrieve where
 
 import Common.Annex
@@ -16,7 +14,7 @@ import qualified Data.ByteString.Lazy as L
 
 -- Prepares for and then runs an action that will act on a Key's
 -- content, passing it a helper when the preparation is successful.
-type Preparer helper = forall a. Key -> (Maybe helper -> Annex a) -> Annex a
+type Preparer helper = Key -> (Maybe helper -> Annex Bool) -> Annex Bool
 
 -- A source of a Key's content.
 data ContentSource