Address hlint warnings/suggestions

.github/workflows/code-style.yaml

@@ -1,4 +1,4 @@
-name: Code style check
+name: Code style checks
 
 concurrency:
   group: formatting-${{ github.ref_name }}
@@ -20,4 +20,17 @@ jobs:
         uses: cachix/install-nix-action@v31
       - name: Check code formatting
         run: |
-          nix develop '#haskellFormatter' --command fourmolu --mode=check --check-idempotence servant servant-*
+          nix develop '#haskellFormatter' --command fourmolu --mode=check --check-idempotence servant servant-*
+
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout Code
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 1
+      - name: Install Nix
+        uses: cachix/install-nix-action@v31
+      - name: Run hlint check
+        run: |
+          nix develop '#haskellLinter' --command hlint servant servant-*

hlint.yaml → .hlint.yaml

@@ -45,15 +45,12 @@
 #
 # Generalise map to fmap, ++ to <>
 # - group: {name: generalise, enabled: true}
+#
+# Replace return with pure
+- group: {name: future, enabled: true}
 
 
 # Ignore some builtin hints
-- ignore: {name: Redundant do}
-- ignore: {name: Parse error}
-- ignore: {name: Use fmap}
-- ignore: {name: Use list comprehension}
-- ignore: {name: Use lambda-case}
-- ignore: {name: Eta reduce}
 # - ignore: {name: Use const, within: SpecialModule} # Only within certain modules
 
 

doc/cookbook/basic-auth/BasicAuth.lhs

@@ -71,7 +71,7 @@ api :: Proxy API
 api = Proxy
 
 server :: Server API
-server usr = return (site usr)
+server usr = pure (site usr)
 ```
 
 In order to protect our endpoint (`"mysite" :> Get '[JSON] Website`), we simply
@@ -105,10 +105,10 @@ checkBasicAuth db = BasicAuthCheck $ \basicAuthData ->
       password = decodeUtf8 (basicAuthPassword basicAuthData)
   in
   case Map.lookup username db of
-    Nothing -> return NoSuchUser
+    Nothing -> pure NoSuchUser
     Just u  -> if pass u == password
-               then return (Authorized u)
-               else return BadPassword
+               then pure (Authorized u)
+               else pure BadPassword
 ```
 
 This check simply looks up the user in the "database" and makes sure the

doc/cookbook/basic-streaming/Streaming.lhs

@@ -8,12 +8,12 @@ In other words, without streaming libraries.
 - Some basic usage doesn't require usage of streaming libraries,
   like `conduit`, `pipes`, `machines` or `streaming`.
   We have bindings for them though.
-- Similar example is bundled with each of our streaming library interop packages (see 
+- Similar example is bundled with each of our streaming library interop packages (see
 [servant-pipes](https://github.com/haskell-servant/servant/blob/master/servant-pipes/example/Main.hs),
 [servant-conduit](https://github.com/haskell-servant/servant/blob/master/servant-conduit/example/Main.hs) and
 [servant-machines](https://github.com/haskell-servant/servant/blob/master/servant-machines/example/Main.hs))
 - `SourceT` doesn't have *Prelude* with handy combinators, so we have to write
-  things ourselves. (Note to self: `mapM` and `foldM` would be handy to have). 
+  things ourselves. (Note to self: `mapM` and `foldM` would be handy to have).
 
 ## Code
 
@@ -64,19 +64,19 @@ server :: Server API
 server = fast :<|> slow :<|> readme :<|> proxy where
     fast n = liftIO $ do
         putStrLn $ "/get/" ++ show n
-        return $ fastSource n
+        pure $ fastSource n
 
     slow n = liftIO $ do
         putStrLn $ "/slow/" ++ show n
-        return $ slowSource n
+        pure $ slowSource n
 
     readme = liftIO $ do
         putStrLn "/proxy"
-        return (S.readFile "README.md")
+        pure (S.readFile "README.md")
 
     proxy c = liftIO $ do
         putStrLn "/proxy"
-        return c
+        pure c
 
     -- for some reason unfold leaks?
     fastSource = S.fromStepT . mk where
@@ -116,8 +116,8 @@ main = do
                     x <- S.unSourceT src (go (0 :: Int))
                     print x
                   where
-                    go !acc S.Stop        = return acc
-                    go !acc (S.Error err) = print err >> return acc
+                    go !acc S.Stop        = pure acc
+                    go !acc (S.Error err) = print err >> pure acc
                     go !acc (S.Skip s)    = go acc s
                     go !acc (S.Effect ms) = ms >>= go acc
                     go !acc (S.Yield _ s) = go (acc + 1) s

doc/cookbook/curl-mock/CurlMock.lhs

@@ -141,10 +141,10 @@ generateEndpoint :: Text -> Req Mocked -> IO Text
 generateEndpoint host req =
   case maybeBody of
     Just body ->
-      body >>= \b -> return $ T.intercalate " " [ "curl", "-X", method, "-d", "'" <> b <> "'"
+      body >>= \b -> pure $ T.intercalate " " [ "curl", "-X", method, "-d", "'" <> b <> "'"
                                                 , "-H 'Content-Type: application/json'", host <> "/" <> url ]
     Nothing ->
-      return $ T.intercalate " " [ "curl", "-X", method, host <> "/" <> url ]
+      pure $ T.intercalate " " [ "curl", "-X", method, host <> "/" <> url ]
   where
     method = decodeUtf8 $ req ^. reqMethod
 

doc/cookbook/custom-errors/CustomErrors.lhs

@@ -67,12 +67,12 @@ server :: Server TestApi
 server = helloH :<|> postGreetH :<|> deleteGreetH
 
   where helloH name Nothing = helloH name (Just False)
-        helloH name (Just False) = return . Greet $ "Hello, " <> name
-        helloH name (Just True) = return . Greet . Text.toUpper $ "Hello, " <> name
+        helloH name (Just False) = pure . Greet $ "Hello, " <> name
+        helloH name (Just True) = pure . Greet . Text.toUpper $ "Hello, " <> name
 
-        postGreetH greet = return greet
+        postGreetH greet = pure greet
 
-        deleteGreetH _ = return NoContent
+        deleteGreetH _ = pure NoContent
 ```
 
 ## Error formatters

doc/cookbook/db-mysql-basics/MysqlBasics.lhs

@@ -141,9 +141,9 @@ doMigration :: IO ()
 doMigration = runNoLoggingT $ runResourceT $ withMySQLConn connInfo $ runReaderT $ runMigration migrateAll
 
 server :: Server Api
-server = 
-  personGET :<|> 
-  personGETById :<|> 
+server =
+  personGET :<|>
+  personGETById :<|>
   personDELETE :<|>
   personPOST
     where
@@ -155,20 +155,20 @@ server =
 selectPersons :: Handler [Person]
 selectPersons = do
   personList <- runDB $ selectList [] []
-  return $ map (\(Entity _ u) -> u) personList
+  pure $ map (\(Entity _ u) -> u) personList
 
 selectPersonById :: Int -> Handler Person
 selectPersonById id = do
   sqlResult <- runDB $ get $ PersonKey id
   case sqlResult of
-    Just person -> return person
+    Just person -> pure person
     Nothing -> throwError err404 { errBody = JSON.encode "Person with ID not found." }
 
 createPerson :: Person -> Handler Person
 createPerson person = do
   attemptCreate <- runDB $ insert person
   case attemptCreate of
-    PersonKey k -> return person
+    PersonKey k -> pure person
     _           -> throwError err503 { errBody = JSON.encode "Could not create Person." }
 
 deletePerson :: Int -> Handler ()

doc/cookbook/db-postgres-pool/PostgresPool.lhs

@@ -51,7 +51,7 @@ initDB :: DBConnectionString -> IO ()
 initDB connstr = bracket (connectPostgreSQL connstr) close $ \conn -> do
   execute_ conn
     "CREATE TABLE IF NOT EXISTS messages (msg text not null)"
-  return ()
+  pure ()
 ```
 
 Next, our server implementation. It will be parametrised (take as
@@ -76,7 +76,7 @@ server conns = postMessage :<|> getMessages
             execute conn
                     "INSERT INTO messages VALUES (?)"
                     (Only msg)
-          return NoContent
+          pure NoContent
 
         getMessages :: Handler [Message]
         getMessages = fmap (map fromOnly) . liftIO $

doc/cookbook/db-sqlite-simple/DBConnection.lhs

@@ -59,7 +59,7 @@ server dbfile = postMessage :<|> getMessages
             execute conn
                     "INSERT INTO messages VALUES (?)"
                     (Only msg)
-          return NoContent
+          pure NoContent
 
         getMessages :: Handler [Message]
         getMessages = fmap (map fromOnly) . liftIO $

doc/cookbook/file-upload/FileUpload.lhs

@@ -82,7 +82,7 @@ upload multipartData = do
       let content = fdPayload file
       putStrLn $ "Content of " ++ show (fdFileName file)
       LBS.putStr content
-  return 0
+  pure 0
 
 startServer :: IO ()
 startServer = run 8080 (serve api upload)

doc/cookbook/https/Https.lhs

@@ -27,7 +27,7 @@ api :: Proxy API
 api = Proxy
 
 server :: Server API
-server = return 10
+server = pure 10
 
 app :: Application
 app = serve api server

doc/cookbook/managed-resource/ManagedResource.lhs

@@ -43,7 +43,7 @@ newHandle = do
   putStrLn "opening file"
   h <- openFile "test.txt" AppendMode
   putStrLn "opened file"
-  return h
+  pure h
 
 closeHandle :: Handle -> IO ()
 closeHandle h = do
@@ -65,7 +65,7 @@ server = writeToFile
             putStrLn "writing file"
             hPutStrLn h legalMsg
             putStrLn "wrote file"
-            return NoContent
+            pure NoContent
 ```
 
 Finally we run the server in the background while we post messages to it.

doc/cookbook/open-id-connect/OpenIdConnect.lhs

@@ -122,7 +122,7 @@ api = Proxy
 
 server :: OIDCEnv -> Server API
 server oidcEnv = serveOIDC oidcEnv handleOIDCLogin
-               :<|> return Homepage
+               :<|> pure Homepage
 
 -- | Then main app
 app :: OIDCEnv -> Application
@@ -161,7 +161,7 @@ initOIDC OIDCConf{..} = do
   mgr  <- newManager tlsManagerSettings
   prov <- O.discover "https://accounts.google.com" mgr
   let oidc     = O.setCredentials clientId clientPassword redirectUri (O.newOIDC prov)
-  return OIDCEnv { oidc = oidc
+  pure OIDCEnv { oidc = oidc
                  , mgr = mgr
                  , genState = genRandomBS
                  , prov = prov
@@ -208,13 +208,13 @@ genOIDCURL OIDCEnv{..} = do
   st <- genState -- generate a random string
   let oidcCreds = O.setCredentials clientId clientPassword redirectUri (O.newOIDC prov)
   loc <- O.getAuthenticationRequestUrl oidcCreds [O.openId, O.email, O.profile] (Just st) []
-  return (show loc)
+  pure (show loc)
 
 handleLogin :: OIDCEnv -> Handler NoContent
 handleLogin oidcenv = do
   loc <- liftIO (genOIDCURL oidcenv)
   redirects loc
-  return NoContent
+  pure NoContent
 ```
 
 The `AuthInfo` is about the infos we can grab from OIDC provider.
@@ -236,7 +236,7 @@ instance FromJSON AuthInfo where
     email   :: Text <- v .: "email"
     email_verified   :: Bool <- v .: "email_verified"
     name :: Text <- v .: "name"
-    return $ AuthInfo (toS email) email_verified (toS name)
+    pure $ AuthInfo (toS email) email_verified (toS name)
   parseJSON invalid    = AeT.typeMismatch "Coord" invalid
 instance JSON.ToJSON AuthInfo where
   toJSON (AuthInfo e ev n) =
@@ -289,7 +289,7 @@ handleLoggedIn oidcenv handleSuccessfulId err mcode =
             if emailVerified authInfo
               then do
                 user <- liftIO $ handleSuccessfulId authInfo
-                either forbidden return user
+                either forbidden pure user
               else forbidden "Please verify your email"
       Nothing -> do
         liftIO $ putText "No code param"
@@ -371,7 +371,7 @@ We need some helpers to generate random string for generating state and API Keys
 genRandomBS :: IO ByteString
 genRandomBS = do
   g <- Random.newStdGen
-  Random.randomRs (0, n) g & take 42 & fmap toChar & readable 0 & toS & return
+  Random.randomRs (0, n) g & take 42 & fmap toChar & readable 0 & toS & pure
   where
     n = length letters - 1
     toChar i = letters List.!! i
@@ -394,7 +394,7 @@ genRandomBS = do
 customerFromAuthInfo :: AuthInfo -> IO Customer
 customerFromAuthInfo authinfo = do
   apikey <- genRandomBS
-  return Customer { account = toS (email authinfo)
+  pure Customer { account = toS (email authinfo)
                   , apiKey = apikey
                   , mail = Just (toS (email authinfo))
                   , fullname = Just (toS (name authinfo))
@@ -404,8 +404,8 @@ handleOIDCLogin :: LoginHandler
 handleOIDCLogin authInfo = do
   custInfo <- customerFromAuthInfo authInfo
   if emailVerified authInfo
-    then return . Right . customerToUser $ custInfo
-    else return (Left "You emails is not verified by your provider. Please verify your email.")
+    then pure . Right . customerToUser $ custInfo
+    else pure (Left "You emails is not verified by your provider. Please verify your email.")
   where
     customerToUser :: Customer -> User
     customerToUser c =

doc/cookbook/sentry/Sentry.lhs

@@ -39,7 +39,7 @@ server = breakHandler
   where breakHandler :: Handler ()
         breakHandler = do
           throw MyException
-          return ()
+          pure ()
 ```
 
 First thing we need to do if we want to intercept and log this exception, we need to look in the section of our code where we run the `warp` application, and instead of using the simple `run` function from `warp`, we use the `runSettings` functions which allows to customise the handling of requests

doc/cookbook/structuring-apis/StructuringApis.lhs

@@ -81,10 +81,10 @@ type that `Server FactoringAPI` "resolves to" by typing
 factoringServer :: Server FactoringAPI
 factoringServer x = getXY :<|> postX
 
-  where getXY Nothing  = return x
-        getXY (Just y) = return (x + y)
+  where getXY Nothing  = pure x
+        getXY (Just y) = pure (x + y)
 
-        postX = return (x - 1)
+        postX = pure (x - 1)
 ```
 
 If you want to avoid the "nested types" and the need to manually
@@ -164,19 +164,19 @@ related sections of the API.
 ``` haskell
 userServer :: Server (SimpleAPI "users" User UserId)
 userServer = simpleServer
-  (return [])
-  (\userid -> return $
+  (pure [])
+  (\userid -> pure $
       if userid == 0
       then User "john" 64
       else User "everybody else" 10
   )
-  (\_user -> return NoContent)
+  (\_user -> pure NoContent)
 
 productServer :: Server (SimpleAPI "products" Product ProductId)
 productServer = simpleServer
-  (return [])
-  (\_productid -> return $ Product "Great stuff")
-  (\_product -> return NoContent)
+  (pure [])
+  (\_productid -> pure $ Product "Great stuff")
+  (\_product -> pure NoContent)
 ```
 
 Finally, some dummy types and the serving part.