haskell - 为什么要强制执行Monad？

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

https://github.com/hlian/linklater

该软件包定义了以下功能:

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

我正在尝试像这样消耗它:

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的签名不应该满足吗？为什么不呢？

最佳答案

尽管GitHub master上slashSimple的定义与您所报告的一样，但linklater-3.2.0.0中的Hackage版本为

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

如果要在Hackage上使用该软件包，则需要将kitesBot更新为以下内容:

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下载该软件包并手动安装。