Config.hs

data ApiInfo = ApiInfo {
  name :: T.Text,
  version :: T.Text
} deriving (Eq, Generic)

instance FromJSON ApiInfo
instance ToJSON ApiInfo

data Config = Config {
   info :: ApiInfo
   , dbHost :: HostName
   , port :: Int
   }

data AppConfig = AppConfig {
    pool :: (Pool RethinkDBHandle)
    , config :: Config
    }

data AppOptions = AppOptions {
    env :: String
    } deriving (Data,Typeable)

type AppIO = ReaderT AppConfig (EitherT ServantErr IO)

type API =
    Get ApiInfo
    :<|> "users" :> UsersAPI

serverT :: ServerT API AppIO
serverT =
    getApiInfo
    :<|> usersServer
    where
        getApiInfo :: AppIO ApiInfo
        getApiInfo = apiInfo

appIOToEither' :: AppConfig -> AppIO a -> EitherT ServantErr IO a
appIOToEither' t r = runReaderT (hoist (EitherT . runEitherT) r) t

appIOToEither :: AppConfig -> AppIO :~> EitherT ServantErr IO
appIOToEither t = Nat $ appIOToEither' t

api :: Proxy API
api = Proxy

server :: AppConfig -> Server API
server t = enter (appIOToEither t) serverT

EitherT . runEitherT is equivalent to id isn't it? It doesn't do anything? I'm still confused about what hoist does. I looked at my code and appIOToEither' seems equivalent to my runAppT, and your code doesn't seem any shorter for using it. Can you take it out? Maybe it isn't doing anything.