Core, Remote: return -> pure

I wonder why type system needs to infer Monad constraint where Applicative would
suffice.

hnix-store-core/src/System/Nix/Internal/Nar/Effects.hs

@@ -89,7 +89,7 @@ streamStringOutIO f getChunk =
   go handle = do
     chunk <- getChunk
     case chunk of
-      Nothing -> return ()
+      Nothing -> pure ()
       Just c  -> do
         IO.liftIO $ Bytes.hPut handle c
         go handle

hnix-store-core/src/System/Nix/Internal/Nar/Parser.hs

@@ -89,7 +89,7 @@ runParser effs (NarParser action) h target = do
     Reader.runReaderT (Except.runExceptT $ State.evalStateT action state0) effs
       `Exception.Lifted.catch` exceptionHandler
   when (Either.isLeft unpackResult) cleanup
-  return unpackResult
+  pure unpackResult
 
  where
   state0 :: ParserState
@@ -103,7 +103,7 @@ runParser effs (NarParser action) h target = do
 
   exceptionHandler :: Exception.Lifted.SomeException -> m (Either String a)
   exceptionHandler e =
-    return $ Left $ "Exception while unpacking NAR file: " <> show e
+    pure $ Left $ "Exception while unpacking NAR file: " <> show e
 
   cleanup :: m ()
   cleanup =
@@ -141,7 +141,7 @@ data ParserState = ParserState
 
 -- | Parse a NAR byte string, producing @()@.
 --   Parsing a NAR is mostly used for its side-effect: producing
---   the file system objects packed in the NAR. That's why we return @()@
+--   the file system objects packed in the NAR. That's why we pure @()@
 parseNar :: (IO.MonadIO m, Fail.MonadFail m) => NarParser m ()
 parseNar = do
   expectStr "nix-archive-1"
@@ -179,7 +179,7 @@ parseSymlink = do
   currentDirectoryAndFile :: Monad m => NarParser m (FilePath, FilePath)
   currentDirectoryAndFile = do
     dirStack <- State.gets directoryStack
-    return (List.foldr1 (</>) (List.reverse $ drop 1 dirStack), head dirStack)
+    pure (List.foldr1 (</>) (List.reverse $ drop 1 dirStack), head dirStack)
 
 
 -- | Internal data type representing symlinks encountered in the NAR
@@ -224,7 +224,7 @@ parseFile = do
     getChunk = do
       bytesLeft <- IO.liftIO $ IORef.readIORef bytesLeftVar
       if bytesLeft == 0
-        then return Nothing
+        then pure Nothing
         else do
           chunk <- IO.liftIO $ Bytes.hGetSome narHandle $ fromIntegral $ min 10000 bytesLeft
           when (Bytes.null chunk) (Fail.fail "ZERO BYTES")
@@ -234,7 +234,7 @@ parseFile = do
           -- clean up chunks from previous runs. Without it, heap memory usage can
           -- quickly spike
           IO.liftIO $ Concurrent.threadDelay 10
-          return $ Just chunk
+          pure $ Just chunk
 
   target     <- currentFile
   streamFile <- Reader.asks Nar.narStreamFile
@@ -373,7 +373,7 @@ parens act = do
   expectStr "("
   r <- act
   expectStr ")"
-  return r
+  pure r
 
 
 -- | Sort links in the symlink stack according to their connectivity
@@ -406,7 +406,7 @@ createLinks = do
                         (linkPWD l </> linkTarget l)
       fileAbsPath   <- Directory.canonicalizePath
                         (linkFile l)
-      return (fileAbsPath, targetAbsPath)
+      pure (fileAbsPath, targetAbsPath)
     let linkGraph = Graph.edges canonicalLinks
     case Graph.topSort linkGraph of
       Left _            -> error "Symlinks form a loop"
@@ -414,7 +414,7 @@ createLinks = do
         let
           sortedLinks = flip Map.lookup linkLocations <$> sortedNodes
         in
-          return $ catMaybes sortedLinks
+          pure $ catMaybes sortedLinks
 
 
 ------------------------------------------------------------------------------
@@ -426,7 +426,7 @@ consume
   :: (IO.MonadIO m, Fail.MonadFail m)
   => Int
   -> NarParser m ByteString
-consume 0 = return ""
+consume 0 = pure ""
 consume n = do
   state0   <- State.get
   newBytes <- IO.liftIO $ Bytes.hGetSome (handle state0) (max 0 n)
@@ -434,7 +434,7 @@ consume n = do
     Fail.fail $
     "consume: Not enough bytes in handle. Wanted "
     <> show n <> " got " <> show (Bytes.length newBytes)
-  return newBytes
+  pure newBytes
 
 
 -- | Pop a string off the token stack
@@ -442,10 +442,10 @@ popStr :: Monad m => NarParser m (Maybe Text)
 popStr = do
   s <- State.get
   case List.uncons (tokenStack s) of
-    Nothing      -> return Nothing
+    Nothing      -> pure Nothing
     Just (x, xs) -> do
       State.put $ s { tokenStack = xs }
-      return $ Just x
+      pure $ Just x
 
 
 -- | Push a string onto the token stack
@@ -472,7 +472,7 @@ popFileName =
 currentFile :: Monad m => NarParser m FilePath
 currentFile = do
   dirStack <- State.gets directoryStack
-  return $ List.foldr1 (</>) $ List.reverse dirStack
+  pure $ List.foldr1 (</>) $ List.reverse dirStack
 
 
 -- | Add a link to the collection of encountered symlinks

hnix-store-core/src/System/Nix/Internal/Nar/Streamer.hs

@@ -83,7 +83,7 @@ streamNarIO yield effs basePath = do
     yield $ str "("
     r <- act
     yield $ str ")"
-    return r
+    pure r
 
   -- Read, yield, and pad the file
   yieldFile :: FilePath -> Int64 -> m ()

hnix-store-core/tests/NarFormat.hs

@@ -159,7 +159,7 @@ unit_packSelfSrcDir = Temp.withSystemTempDirectory "nar-test" $ \tmpDir -> do
       let go dir = do
             srcHere <- doesDirectoryExist dir
             case srcHere of
-              False -> return ()
+              False -> pure ()
               True -> do
                 IO.withFile narFilePath IO.WriteMode $ \h ->
                   buildNarIO narEffectsIO "src" h
@@ -231,7 +231,7 @@ test_streamManyFilesToNar = HU.testCaseSteps "streamManyFilesToNar" $ \step ->
         IO.withFile "hnar" IO.WriteMode $ \h ->
           buildNarIO narEffectsIO narFilePath h
         filesPostcount <- countProcessFiles
-        return $ filesPostcount - filesPrecount
+        pure $ filesPostcount - filesPrecount
 
     step "create test files"
     Directory.createDirectory packagePath
@@ -316,7 +316,7 @@ assertBoundedMemory = do
       bytes <- max_live_bytes <$> getRTSStats
       bytes < 100 * 1000 * 1000 `shouldBe` True
 #else
-      return ()
+      pure ()
 #endif
 
 
@@ -358,7 +358,7 @@ packThenExtract testName setup =
         _narHandle <- IO.withFile nixNarFile IO.ReadMode $ \h ->
           unpackNarIO narEffectsIO h outputFile
 
-        return ()
+        pure ()
 
 -- | Count file descriptors owned by the current process
 countProcessFiles :: IO Int
@@ -366,7 +366,7 @@ countProcessFiles = do
   pid <- Unix.getProcessID
   let fdDir = "/proc/" ++ show pid ++ "/fd"
   fds  <- P.readProcess "ls" [fdDir] ""
-  return $ length $ words fds
+  pure $ length $ words fds
 
 
 -- | Read the binary output of `nix-store --dump` for a filepath
@@ -601,7 +601,7 @@ instance Arbitrary FileSystemObject where
         arbDirectory n = fmap (Directory . Map.fromList) $ replicateM n $ do
           nm <- arbName
           f <- oneof [arbFile, arbDirectory (n `div` 2)]
-          return (nm,f)
+          pure (nm,f)
 
 ------------------------------------------------------------------------------
 -- | Serialize Nar to lazy ByteString
@@ -615,7 +615,7 @@ putNar (Nar file) = header <> parens (putFile file)
                strs ["type", "regular"]
             >> (if isExec == Nar.Executable
                then strs ["executable", ""]
-               else return ())
+               else pure ())
             >> putContents fSize contents
 
         putFile (SymLink target) =
@@ -678,13 +678,13 @@ getNar = fmap Nar $ header >> parens getFile
                                                        >> assertStr "")
           assertStr_ "contents"
           (fSize, contents) <- sizedStr
-          return $ Regular (fromMaybe Nar.NonExecutable mExecutable) fSize contents
+          pure $ Regular (fromMaybe Nar.NonExecutable mExecutable) fSize contents
 
       getDirectory = do
           assertStr_ "type"
           assertStr_ "directory"
           fs <- many getEntry
-          return $ Directory (Map.fromList fs)
+          pure $ Directory (Map.fromList fs)
 
       getSymLink = do
           assertStr_ "type"
@@ -700,7 +700,7 @@ getNar = fmap Nar $ header >> parens getFile
               assertStr_ "node"
               file <- parens getFile
               maybe (fail $ "Bad FilePathPart: " ++ show name)
-                    (return . (,file))
+                    (pure . (,file))
                     (filePathPart $ E.encodeUtf8 name)
 
       -- Fetch a length-prefixed, null-padded string
@@ -710,7 +710,7 @@ getNar = fmap Nar $ header >> parens getFile
           n <- getInt64le
           s <- getLazyByteString n
           _ <- getByteString . fromIntegral $ padLen n
-          return (n,s)
+          pure (n,s)
 
       parens m = assertStr "(" *> m <* assertStr ")"
 
@@ -718,5 +718,5 @@ getNar = fmap Nar $ header >> parens getFile
       assertStr s = do
           s' <- str
           if s == s'
-              then return s
+              then pure s
               else fail "No"

hnix-store-remote/src/System/Nix/Store/Remote.hs

@@ -170,7 +170,7 @@ buildDerivation p drv buildMode = do
     putInt 0
 
   res <- getSocketIncremental $ getBuildResult
-  return res
+  pure res
 
 ensurePath :: StorePath -> MonadStore ()
 ensurePath pn = do
@@ -189,13 +189,13 @@ findRoots = do
       <*> getPath sd
 
   r <- catRights res
-  return $ Data.Map.Strict.fromList r
+  pure $ Data.Map.Strict.fromList r
  where
   catRights :: [(a, Either String b)] -> MonadStore [(a, b)]
   catRights = mapM ex
 
   ex :: (a, Either [Char] b) -> MonadStore (a, b)
-  ex (x , Right y) = return (x, y)
+  ex (x , Right y) = pure (x, y)
   ex (_x, Left e ) = error $ "Unable to decode root: " ++ e
 
 isValidPathUncached :: StorePath -> MonadStore Bool
@@ -263,7 +263,7 @@ queryPathInfoUncached path = do
 
       trust = if ultimate then BuiltLocally else BuiltElsewhere
 
-  return $ StorePathMetadata{..}
+  pure $ StorePathMetadata{..}
 
 queryReferrers :: StorePath -> MonadStore StorePathSet
 queryReferrers p = do
@@ -311,7 +311,7 @@ queryMissing ps = do
   unknown        <- sockGetPaths
   downloadSize'  <- sockGetInt
   narSize'       <- sockGetInt
-  return (willBuild, willSubstitute, unknown, downloadSize', narSize')
+  pure (willBuild, willSubstitute, unknown, downloadSize', narSize')
 
 optimiseStore :: MonadStore ()
 optimiseStore = void $ simpleOp OptimiseStore

hnix-store-remote/src/System/Nix/Store/Remote/Binary.hs

@@ -47,7 +47,7 @@ getByteStringLen = do
   when (len `mod` 8 /= 0) $ do
     pads <- unpad $ fromIntegral $ 8 - (len `mod` 8)
     unless (all (== 0) pads) $ fail $ "No zeroes" ++ show (st, len, pads)
-  return $ BSL.toStrict st
+  pure $ BSL.toStrict st
   where unpad x = sequence $ replicate x getWord8
 
 getByteStrings :: Get [ByteString]

hnix-store-remote/src/System/Nix/Store/Remote/Logger.hs

@@ -49,8 +49,8 @@ processOutput = go decoder
   go :: Decoder Logger -> MonadStore [Logger]
   go (Done _leftover _consumed ctrl) = do
     case ctrl of
-      e@(Error _ _) -> return [e]
-      Last          -> return [Last]
+      e@(Error _ _) -> pure [e]
+      Last          -> pure [Last]
       Read _n       -> do
         (mdata, _) <- get
         case mdata of
@@ -61,12 +61,12 @@ processOutput = go decoder
             clearData
 
         next <- go decoder
-        return next
+        pure next
 
       -- we should probably handle Read here as well
       x -> do
         next <- go decoder
-        return $ x : next
+        pure $ x : next
   go (Partial k) = do
     soc   <- asks storeSocket
     chunk <- liftIO (Just <$> recv soc 8)

hnix-store-remote/src/System/Nix/Store/Remote/Parsers.hs

@@ -35,7 +35,7 @@ caText :: Parser ContentAddressableAddress
 caText = do
   _      <- "text:sha256:"
   digest <- decodeBase @'SHA256 Base32 <$> parseHash
-  either fail return $ Text <$> digest
+  either fail pure $ Text <$> digest
 
 -- | Parser for @fixed:<r?>:<ht>:<h>@
 caFixed :: Parser ContentAddressableAddress
@@ -43,7 +43,7 @@ caFixed = do
   _           <- "fixed:"
   narHashMode <- (pure Recursive <$> "r:") <|> (pure RegularFile <$> "")
   digest      <- parseTypedDigest
-  either fail return $ Fixed narHashMode <$> digest
+  either fail pure $ Fixed narHashMode <$> digest
 
 parseTypedDigest :: Parser (Either String SomeNamedDigest)
 parseTypedDigest = mkNamedDigest <$> parseHashType <*> parseHash

hnix-store-remote/src/System/Nix/Store/Remote/Protocol.hs

@@ -122,7 +122,7 @@ opNum QueryMissing                = 40
 
 
 simpleOp :: WorkerOp -> MonadStore Bool
-simpleOp op = simpleOpArgs op $ return ()
+simpleOp op = simpleOpArgs op $ pure ()
 
 simpleOpArgs :: WorkerOp -> Put -> MonadStore Bool
 simpleOpArgs op args = do
@@ -137,7 +137,7 @@ simpleOpArgs op args = do
     err
 
 runOp :: WorkerOp -> MonadStore ()
-runOp op = runOpArgs op $ return ()
+runOp op = runOpArgs op $ pure ()
 
 runOpArgs :: WorkerOp -> Put -> MonadStore ()
 runOpArgs op args =
@@ -179,7 +179,7 @@ runStoreOpts sockPath storeRootDir code = do
         0
 
     Network.Socket.connect soc (SockAddrUnix path)
-    return StoreConfig
+    pure StoreConfig
         { storeSocket = soc
         , storeDir = storeRootDir
         }

hnix-store-remote/src/System/Nix/Store/Remote/Util.hs

@@ -36,7 +36,7 @@ genericIncremental :: (MonadIO m) => m (Maybe ByteString) -> Get a -> m a
 genericIncremental getsome parser = go decoder
  where
   decoder = runGetIncremental parser
-  go (Done _leftover _consumed x  ) = return x
+  go (Done _leftover _consumed x  ) = pure x
   go (Partial k                   ) = do
     chunk <- getsome
     go (k chunk)
@@ -76,14 +76,14 @@ sockGetPath = do
   pth <- getSocketIncremental (getPath sd)
   either
     throwError
-    return
+    pure
     pth
 
 sockGetPathMay :: MonadStore (Maybe StorePath)
 sockGetPathMay = do
   sd  <- getStoreDir
   pth <- getSocketIncremental (getPath sd)
-  return $
+  pure $
     either
       (const Nothing)
       Just

hnix-store-remote/tests/NixDaemon.hs

@@ -48,7 +48,7 @@ createProcessEnv fp proc args = do
       { P.cwd = Just $ fp
       , P.env = Just $ mockedEnv mPath fp
       }
-  return ph
+  pure ph
 
 mockedEnv :: Maybe String -> FilePath -> [(String, FilePath)]
 mockedEnv mEnvPath fp =
@@ -66,7 +66,7 @@ waitSocket fp x = do
   ex <- doesFileExist fp
   bool
     (threadDelay 100000 >> waitSocket fp (x - 1))
-    (return ())
+    (pure ())
     ex
 
 writeConf :: FilePath -> IO ()
@@ -98,7 +98,7 @@ startDaemon fp = do
   writeConf (fp </> "etc" </> "nix.conf")
   p <- createProcessEnv fp "nix-daemon" []
   waitSocket sockFp 30
-  return (p, runStoreOpts sockFp (fp </> "store"))
+  pure (p, runStoreOpts sockFp (fp </> "store"))
  where
   sockFp = fp </> "var/nix/daemon-socket/socket"
 
@@ -142,7 +142,7 @@ it
   -> (a -> Bool)
   -> Hspec.SpecWith (m () -> IO (a, b))
 it name action check =
-  Hspec.it name $ \run -> (run (action >> return ())) `checks` check
+  Hspec.it name $ \run -> (run (action >> pure ())) `checks` check
 
 itRights
   :: (Show a, Show b, Show c, Monad m)
@@ -168,7 +168,7 @@ dummy :: MonadStore StorePath
 dummy = do
   let Right n = makeStorePathName "dummy"
   res <- addToStore @'SHA256 n "dummy" False (pure True) False
-  return res
+  pure res
 
 invalidPath :: StorePath
 invalidPath =
@@ -203,7 +203,7 @@ spec_protocol = Hspec.around withNixDaemon $
         verifyStore True True `shouldReturn` False
 
     context "addTextToStore" $
-      itRights "adds text to store" $ withPath $ const return ()
+      itRights "adds text to store" $ withPath $ const pure ()
 
     context "isValidPathUncached" $ do
       itRights "validates path" $ withPath $ \path -> do