Add a local binary store

Same as using file://$DIRECTORY

hnix-store-binary/hnix-store-binary.cabal

@@ -15,10 +15,13 @@ cabal-version:       >=1.10
 
 library
   exposed-modules:     System.Nix.Store.Binary
+                     , System.Nix.Store.Binary.Local
 
   build-depends:       base >=4.10 && <4.12
                      , binary
                      , bytestring
+                     , directory
+                     , filepath
                      , lzma
                      , mtl
                      , text

hnix-store-binary/src/System/Nix/Store/Binary/Local.hs

@@ -0,0 +1,26 @@
+{-|
+Description : Interact with a local binary Nix store.
+Maintainer  : Brian McKenna <[email protected]>
+-}
+module System.Nix.Store.Binary.Local (
+  localBinaryStoreEffects
+) where
+
+import Control.Monad.Trans (MonadIO(..))
+import System.Directory
+import System.FilePath
+import System.Nix.Store.Binary
+import qualified Data.ByteString as BS
+import qualified Data.Text as T
+
+localBinaryStoreEffects
+  :: (MonadIO m)
+  => FilePath
+  -> BinaryStoreEffects m
+localBinaryStoreEffects cacheDir =
+  BinaryStoreEffects upsertFile'
+  where
+    upsertFile' pth d _ = liftIO $ do
+      let pth' = cacheDir </> T.unpack pth
+      createDirectoryIfMissing True $ takeDirectory pth'
+      BS.writeFile pth' d