爲什麼強制實施Maybe monad？

Question

我試圖實現使用Network.Linklater包一個基本slackbot：

https://github.com/hlian/linklater

此包定義了以下功能：

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

我試圖消耗這個像這樣：

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的簽名不應該履行嗎？爲什麼不呢？

Answer 1

雖然slashSimple在GitHub上主的定義是你的報道，Hackage版本linklater-3.2.0.0是

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

如果你想使用Hackage包，你將需要更新kittensBot喜歡的東西：

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下載軟件包並手動安裝它。