问题描述
我需要使用 Haskell 的 Scotty 启动我的非常简单的 web 应用程序,但我似乎无法让 IO () ReaderT 东西正常工作。我基于我在网上找到的另一个示例,并且对 Monads 和 Haskell 整体而言还是很新的。
我的 IDE 抛出此错误:
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’
它也抛出了这个,但我认为一旦我修复了另一个就应该修复
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
我把需要的代码留在这里,如果还有什么我应该展示的,请告诉我。 在“Main.hs”中:
{-# 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
在“Postgres.hs”中
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
这是我的“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
解决方法
实际上,错误是相关的。在 Main.hs
中,将 import
的 Home
更改为:
import qualified Platform.Home as Home
^^^^^^^^^-- add this
它应该修复这两个错误。以下最小示例给出了相同的错误对:
-- contents of Home.hs
module Home where
main :: (Int -> Int) -> IO ()
main = undefined
-- contents of Main.hs
import Home
main = Home.main id
但如果您将 import Home
更改为 import qualified Home
,则有效。
问题似乎是 GHC 尝试将 Home.main
作为程序的 main
函数进行类型检查(可能只是因为它是第一个定义的函数,在 {{1} 的定义之前导入}} 在模块的主体中),并且它会生成此额外的错误消息,因为 Main.main
的类型与 Home.main
函数所需的 IO t
签名不匹配。这发生在它注意到 main
有两个定义(即“模棱两可”错误)之前,并且类型检查错误。