在 Haskeline 中,在 InputT monad 中执行简单的 IO,而不必求助于 unsafePerformIO

Perform simple IO in Haskeline, inside InputT monad, without having to resort to unsafePerformIO

鉴于下面的概念验证代码,我希望能够以某种方式执行我的 foo 函数,能够输出字符串 Paul! 并有可能获得它的 return InputT monad-transformer 中的值,而不使用 unsafePerformIOrunExceptT.

之后删除 IO 包装器
import Control.Monad.Except

import System.IO.Unsafe (unsafePerformIO)
import System.Console.Haskeline


type ErrorWithIO = ExceptT String IO


foo :: String -> ErrorWithIO String
foo "paul" = do liftIO $ putStrLn "Paul!"
                return "OK!"
foo _ = throwError "ERROR!"


runRepl :: IO ()
runRepl = runInputT defaultSettings $ loop


loop :: InputT IO ()
loop = do
    line <- getInputLine "> "
    case line of
        Nothing -> return ()
        Just input -> do return $ putStrLn "asd"
                         case unsafePerformIO $ runExceptT $ foo input of
                             Left err -> outputStrLn err >> loop
                             Right res -> do
                                 x <- outputStrLn . show $ res
                                 loop




main :: IO ()
main = runRepl >> putStrLn "Goodbye!"

我是不是漏掉了什么明显的东西?

由于 InputT IOMonadIO,您可以将 liftIO 与此类型一起使用:

liftIO :: IO a -> InputT IO a

所以,

do ...
   x <- liftIO $ runExceptT $ foo input
   case x of
     Left err  -> ...
     Right res -> ...

或者,使用 Control.Monad.Trans.lift