Avoid unneeded writes

src/Control/Addressbook/Streaming.hs

@@ -29,6 +29,7 @@ import qualified Data.ByteString.Lazy.Char8 as LBC
 import System.IO.Unsafe (unsafeInterleaveIO)
 import Control.Parallel.Strategies (rseq, parMap)
 import qualified Data.List as L
+import Control.Monad (unless)
 
 combine :: (MonadUnliftIO m, MonadResource m, MonadThrow m, MonadIO m) => ConduitM FilePath Header m ()
 combine = await >>= \case
@@ -47,11 +48,12 @@ run = do
   original <- Set.fromList . map LBS.toStrict . lbsLines <$> LBS.readFile datFile
   xs <- LBS.getContents >>= stream
   let set = original `Set.union` F.fold (parMap rseq F.fold (chunks 20 xs))
-  runResourceT $
-    runConduit $
-         CL.sourceList (Set.elems set)
-      .| C.map (<> "\n")
-      .| CB.sinkFileCautious datFile
+  unless (original == set) $
+    runResourceT $
+      runConduit $
+           CL.sourceList (Set.elems set)
+        .| C.map (<> "\n")
+        .| CB.sinkFileCautious datFile
   where
     separate = \case
       From x -> [x]