为什么一个Maybe monad被强制执行？

Question

我正在尝试使用Network.Linklater包实现一个基本的slackbot：

https://github.com/hlian/linklater

该包定义了以下功能：

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

我试图像这样消费：

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

这会产生（在编译时）：

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的签名是(Command -> IO Text) -> Application 。 kittensBot的签名不应该实现吗？ 为什么不呢？

Answer 1

尽管GitHub master上的slashSimple的定义与你报告的一样，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上使用这个包，你需要将kittensBot更新为：

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

或者，您可以从GitHub下载程序包并手动安装它。