為什么一個Maybe monad被強制執行？

Question

我正在嘗試使用Network.Linklater包實現一個基本的slackbot：

https://github.com/hlian/linklater

該包定義了以下功能：

slashSimple :: (Command -> IO Text) -> Application
slashSimple f =
  slash (\command _ respond -> f command >>= (respond . responseOf status200))

我試圖像這樣消費：

kittensBot :: Command -> IO Text
kittensBot cmd = do
           putStrLn("+ Incoming command: " ++ show cmd)
           return "ok"

main :: IO ()
main = do
     putStrLn ("Listening on port: " ++ show port)
     run port (slashSimple kittensBot)
     where
       port = 3001

這會產生（在編譯時）：

Main.hs:20:28:
    Couldn't match type ‘Maybe Command’ with ‘Command’
    Expected type: Maybe Command -> IO Text
      Actual type: Command -> IO Text
    In the first argument of ‘slashSimple’, namely ‘kittensBot’
    In the second argument of ‘run’, namely ‘(slashSimple kittensBot)’

但是slashSimple的簽名是(Command -> IO Text) -> Application 。 kittensBot的簽名不應該實現嗎？ 為什么不呢？

Answer 1

盡管GitHub master上的slashSimple的定義與你報告的一樣，但是linklater-3.2.0.0中的Hackage版本是

slashSimple :: (Maybe Command -> IO Text) -> Application

如果你想在Hackage上使用這個包，你需要將kittensBot更新為：

kittensBot :: Maybe Command -> IO Text
kittensBot Nothing = ...
kittensBot (Just cmd) = do
       putStrLn("+ Incoming command: " ++ show cmd)
       return "ok"

或者，您可以從GitHub下載程序包並手動安裝它。