haskell - 使用 Pipes 的简单程序挂起

Question

我有以下程序，使用 runhaskell Toy.hs 运行时不产生任何输出，而是无限期挂起。据我了解，程序应该打印“hi”然后退出。我将不胜感激有关如何调试此类问题的答案和/或建议。我正在使用来自 github ( github.com/Gabriel439/Haskell-Pipes-Library ) 的 Pipes 4.0.0。

module Toy where

import Pipes
import Control.Monad.State

type Request = String
type Response = String

serveChoice :: Request -> Server Request Response IO ()
serveChoice = forever go
  where go req = do
        lift $ putStrLn req
        respond req

run :: Monad m => () -> Client Request Response (StateT Int m) ()
run () = do
    request "hi"
    return ()

main :: IO ()
main = evalStateT (runEffect $ hoist lift . serveChoice >-> run $ ()) 0

Answer 1

您需要使用 foreverK而不是 forever ， 像这样:

module Toy where

import Pipes
import Pipes.Prelude (foreverK)
import Control.Monad.State

type Request = String
type Response = String

serveChoice :: Request -> Server Request Response IO ()
serveChoice = foreverK go
  where go req = do
        lift $ putStrLn req
        respond req

run :: Monad m => () -> Client Request Response (StateT Int m) ()
run () = do
    request "hi"
    return ()

main :: IO ()
main = evalStateT (runEffect $ hoist lift . serveChoice >-> run $ ()) 0

您的原始版本挂起的原因是您使用了 forever在 Reader monad(即 ((->) a) monad)而不是管道 monad。在这个单子(monad)中， forever相当于 ⊥ :

-- i.e.        m b  ->     m c
forever :: (a -> b) -> (a -> c)
forever m = m >> forever m
          = m >>= \_ -> forever m
          = \a -> (\_ -> forever m) (m a) a
          = \a -> forever m a
          = forever m

foreverK可能是您想要的，因为它与 Server 的习语相同s 在 pipes-3.3.0 中介绍教程。

此更改修复了现在正常完成的程序:

>>> main
hi
>>>