diff --git a/main/Main.hs b/main/Main.hs
index f67fd501..99c592c8 100644
--- a/main/Main.hs
+++ b/main/Main.hs
@@ -5,7 +5,9 @@
 
 module Main where
 
-import Control.Concurrent (Chan, forkIO, newChan, readChan, writeChan)
+import Control.Monad (unless)
+import Control.Monad.Trans.Class (lift)
+import Control.Monad.Trans.State.Strict (StateT, evalStateT, get, put)
 import Data.ByteString.Char8 (unpack)
 import Data.Either (lefts)
 import Data.FileEmbed
@@ -87,19 +89,31 @@ data Target = Target
   }
 
 -- | Recursively collect nix files in a directory
-collectNixFiles :: FilePath -> IO [FilePath]
+collectNixFiles :: FilePath -> StateT Bool IO [FilePath]
 collectNixFiles path = do
-  dir <- doesDirectoryExist path
+  dir <- lift $ doesDirectoryExist path
   if
     | dir -> do
-        files <- listDirectory path
+        warnDeprecated
+        files <- lift $ listDirectory path
         concat <$> mapM collectNixFiles ((path </>) <$> files)
     | ".nix" `isSuffixOf` path -> pure [path]
-    | otherwise -> pure []
+    | otherwise -> do
+        warnDeprecated
+        pure []
+  where
+    warnDeprecated = do
+      warningPrinted <- get
+      -- We don't want to print the warning more than once
+      unless warningPrinted $ do
+        lift $ hPutStrLn stderr $ "\ESC[33mPassing directories or non-Nix files (such as \"" <> path <> "\") is deprecated and will be unsupported soon, please use https://treefmt.com/ instead, e.g. via https://github.com/numtide/treefmt-nix\ESC[0m"
+        put True
 
 -- | Recursively collect nix files in a list of directories
 collectAllNixFiles :: [FilePath] -> IO [FilePath]
-collectAllNixFiles paths = concat <$> mapM collectNixFiles paths
+collectAllNixFiles paths =
+  -- The state represents whether a deprecation warning was already printed for a directory being passed
+  flip evalStateT False $ concat <$> mapM collectNixFiles paths
 
 formatTarget :: Formatter -> Target -> IO Result
 formatTarget format Target{tDoRead, tPath, tDoWrite} = do
@@ -159,37 +173,16 @@ toWriteError Nixfmt{quiet = True} = const $ return ()
 toJobs :: Nixfmt -> IO [IO Result]
 toJobs opts = map (toOperation opts $ toFormatter opts) <$> toTargets opts
 
--- TODO: Efficient parallel implementation. This is just a sequential stub.
--- This was originally implemented using parallel-io, but it gave a factor two
--- overhead.
-doParallel :: [IO a] -> IO [a]
-doParallel = sequence
-
-errorWriter :: (String -> IO ()) -> Chan (Maybe String) -> Chan () -> IO ()
-errorWriter doWrite chan done = do
-  item <- readChan chan
-  case item of
-    Nothing -> return ()
-    Just msg -> doWrite msg >> errorWriter doWrite chan done
-  writeChan done ()
-
-writeErrorBundle :: Chan (Maybe String) -> Result -> IO Result
-writeErrorBundle chan result = do
+writeErrorBundle :: (String -> IO ()) -> Result -> IO Result
+writeErrorBundle doWrite result = do
   case result of
     Right () -> return ()
-    Left err -> writeChan chan $ Just err
+    Left err -> doWrite err
   return result
 
 -- | Run a list of jobs and write errors to stderr without interleaving them.
 runJobs :: (String -> IO ()) -> [IO Result] -> IO [Result]
-runJobs writeError jobs = do
-  errChan <- newChan
-  doneChan <- newChan
-  _ <- forkIO $ errorWriter writeError errChan doneChan
-  results <- doParallel $ map (>>= writeErrorBundle errChan) jobs
-  writeChan errChan Nothing
-  _ <- readChan doneChan
-  return results
+runJobs writeError = mapM (>>= writeErrorBundle writeError)
 
 main :: IO ()
 main = withUtf8StdHandles $ do
diff --git a/nixfmt.cabal b/nixfmt.cabal
index 57073e6f..97e312a4 100644
--- a/nixfmt.cabal
+++ b/nixfmt.cabal
@@ -37,6 +37,7 @@ executable nixfmt
     , nixfmt
     , unix             >= 2.7.2 && < 2.9
     , text             >= 1.2.3 && < 2.2
+    , transformers
 
     -- for System.IO.Atomic
     , directory        >= 1.3.3 && < 1.4