Haskell Scotty ‘Home.main’ is applied to too few arguments - haskell

Question

I need to start up my very simple webapp with Haskell's Scotty and I just can't seem to get the IO () ReaderT stuff workinng. I am basing this off of another example I found online, and am pretty new to Monads and Haskell overall.
My IDE is throwing this error:
Couldn't match expected type ‘IO t0’
with actual type ‘(m0 Network.Wai.Internal.Response
-> IO Network.Wai.Internal.Response)
-> IO ()’
• Probable cause: ‘Home.main’ is applied to too few arguments
In the expression: Home.main
When checking the type of the IO action ‘main’
It is also throwing this one but I think it should get fixed once I fixed the other one
Ambiguous occurrence ‘main’
It could refer to either ‘Home.main’,
imported from ‘Platform.Home’ at Main.hs:16:1-28
or ‘Main.main’, defined at Main.hs:28:1
I am leaving here the needed code, if there is anything else I should show please let me know.
In "Main.hs":
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
module Main
( main
) where
import Control.Monad (join)
import Control.Applicative ((<$>))
import Core.Item.Controller (routes)
import Core.Item.Controller as ItemController
import Core.Item.Service as ItemService
import Core.Item.DAO as ItemDAO
import Platform.Postgres as Postgres
import Platform.Home as Home
import Data.Maybe (fromMaybe)
import Network.Wai.Middleware.RequestLogger (logStdoutDev)
import Network.Wai.Middleware.Static (addBase, noDots, staticPolicy, (>->))
import System.Environment (lookupEnv)
import Text.Read (readMaybe)
import Web.Scotty (middleware, scotty)
import Language.Haskell.TH (Type(AppT))
import ClassyPrelude
main :: IO ()
main = do
pgEnv <- Postgres.init
let runner app = flip runReaderT pgEnv $ unAppT app
Home.main runner
type Environment = Postgres.Env
newtype AppT a = AppT
{ unAppT :: ReaderT Environment IO a
} deriving (Applicative, Functor, Monad, MonadIO, MonadReader Environment)
instance ItemController.Service AppT where
getItem = ItemService.getItem
getItems = ItemService.getItems
createItem = ItemService.createItem
instance ItemService.ItemRepo AppT where
findItems = ItemDAO.findItems
addItem = ItemDAO.addItem
instance ItemService.TimeRepo AppT where
currentTime = liftIO getCurrentTime
In "Postgres.hs"
type Env = Pool Connection
type Postgres r m = (MonadReader r m, Has Env r, MonadIO m)
init :: IO Env
init = do
pool <- acquirePool
migrateDb pool
return pool
And this is my "Home.hs":
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE ConstraintKinds #-}
module Platform.Home
( main
) where
import ClassyPrelude (MonadIO, LText, fromMaybe, readMay)
import Web.Scotty.Trans
import Network.HTTP.Types.Status
import Network.Wai.Handler.WarpTLS (runTLS, tlsSettings)
import Network.Wai.Handler.Warp (defaultSettings, setPort)
import Network.Wai (Response)
import Network.Wai.Middleware.Cors
import qualified Core.Item.Controller as ItemController
import System.Environment (lookupEnv)
type App r m = (ItemController.Service m, MonadIO m)
main :: (App r m) => (m Response -> IO Response) -> IO ()
main runner = do
port <- acquirePort
mayTLSSetting <- acquireTLSSetting
case mayTLSSetting of
Nothing ->
scottyT port runner routes
Just tlsSetting -> do
app <- scottyAppT runner routes
runTLS tlsSetting (setPort port defaultSettings) app
where
acquirePort = do
port <- fromMaybe "" <$> lookupEnv "PORT"
return . fromMaybe 3000 $ readMay port
acquireTLSSetting = do
env <- (>>= readMay) <$> lookupEnv "ENABLE_HTTPS"
let enableHttps = fromMaybe True env
return $ if enableHttps
then Just $ tlsSettings "secrets/tls/certificate.pem" "secrets/tls/key.pem"
else Nothing
routes :: (App r m) => ScottyT LText m ()
routes = do
-- middlewares
middleware $ cors $ const $ Just simpleCorsResourcePolicy
{ corsRequestHeaders = "Authorization":simpleHeaders
, corsMethods = "PUT":"DELETE":simpleMethods
}
options (regex ".*") $ return ()
-- errors
defaultHandler $ \str -> do
status status500
json str
-- feature routes
ItemController.routes
-- health
get "/api/health" $
json True

Answer 1

Actually, the errors are related. In Main.hs, change the import of Home to:
import qualified Platform.Home as Home
^^^^^^^^^-- add this
and it should fix both errors. The following minimal example gives the same pair of errors:
-- contents of Home.hs
module Home where
main :: (Int -> Int) -> IO ()
main = undefined
-- contents of Main.hs
import Home
main = Home.main id
but works if you change import Home to import qualified Home.
The issue appears to be that GHC tries to type-check Home.main as the program's main function (perhaps simply because it was the first one defined, having been imported before the definition of Main.main in the body of the module), and it generates this extra error message because Home.main's type doesn't match the required signature of IO t for a main function. This happens before it gets around to noticing that there are two definitions of main (i.e., the "ambiguous occurrence" error), and it's typechecked the wrong one.