Haskell websockets 标头

Question

我正在尝试在 haskell 中读取 websockets 标头。

当前代码如下

import Network
import Network.Socket.Internal
import Text.Printf
import Control.Monad
import Control.Monad.IO.Class
import Data.Maybe
import Data.List
import Data.Digest.Pure.SHA (bytestringDigest, sha1)
import Text.Regex
import System.IO

port :: Int
port = 8080

bufferLength :: Int
bufferLength = 2048

keyRegex :: String
keyRegex = "Sec-WebSocket-Key: (.+)(\r)"

response :: String
response = "HTTP/1.1 101 Web Socket Protocol Handshake\r\nUpgrade:     WebSocket\r\nConnection: Upgrade\r\nsec-websocket-accept:     changethis\r\n\r\n"

guidString :: String
guidString = "258EAFA5-E914-47DA-95CA-C5AB0DC85B11"

talk :: MonadIO m => Handle -> m String
talk handle = do liftIO $ hGetContents handle

main :: IO()
main = withSocketsDo $ do
  sock <- listenOn (PortNumber (fromIntegral port))
  printf "Listening on port %d\n" port
  forever $ do
    (handle, host, port) <- accept sock
    printf "Accepted connection from %s: %s\n" host (show port)
    hSetBuffering handle NoBuffering
    putStrLn $ talk handle

代码报错

Main.hs:38:16:
    Couldn't match type `[Char]' with `Char'
    Expected type: String
      Actual type: [String]
    In the return type of a call of `talk'
    In the second argument of `($)', namely `talk handle'
    In a stmt of a 'do' block: putStrLn $ talk handle

如何打印标题字符串？谈话函数应该获取标题字符串并将其返回以进行打印。

Answer 1

talk handle 的返回类型为(MonadIO m => m String)。

putStrLn 的类型为 String -> IO () 或 [Char] -> IO ()。

[] 是一个单子。

putStrLn 需要一个字符列表，但它的值是 (MonadIO m => m String)。

m 变成一个列表，putStrLn 得到一个 [String] 类型的值，这是错误。

---

正如@Zeta 建议的那样，改用talk handle >>= putStrLn：

>>= 是允许使用另一个计算的结果继续计算的运算符。在您的情况下，您可以阅读talk handle >>= putStrLn，如下所示：

talk handle -- first compute it
  >>= putStrLn -- then take string from result of `talk handle` and print it

putStrLn $ talk handle 不会出现任何错误：

(>>=)                        :: Monad m => m a -> (a -> m b) -> m b
talk handle                  :: MonadIO m => m String
(>>=) (talk handle)          :: MonadIO m => (String -> m b) -> m b  
putStrLn                     :: String -> IO ()
(>>=) (talk handle) putStrLn :: IO () -- `m` becomes `IO`    

---

编辑：

在这里你可以找到IO monad 的介绍：LYHFGG, Input and Output

在这里您可以找到对 monad 的一般介绍：LYHFGG, A Fistful of Monads。

您应该了解Functor 和Applicative 才能阅读A Fistful of Monads。无论如何，您都可以在书的内容中找到它。