Question

我已经写了一个文件的地图，现在我正在尝试对它进行读取。那可能吗？其中一个问题就是使用ByteString和ByteString.Char8编写和读取代码。我一直收到以下错误

fromList *** Exception: Prelude.read: no parse

我的代码如下：

import qualified Data.ByteString.Char8 as BSC
import qualified Data.ByteString as BS
import qualified Data.Map as Map

type Code = Map.Map Char [Bit]

writeCode :: FilePath -> Code -> IO ()
writeCode fp tr = BS.writeFile ("code_" ++ fp)
                  (BSC.pack (show (tr :: M.Map Char [Bit])))

readCode :: FilePath -> IO Code
readCode f = do s <- BS.readFile ("code_" ++ f)
                let s' = BSC.unpack s
                return (read s' :: Code)

Answer 1

问题是你覆盖了Show的默认实现，但是依赖于Read的默认实现，它需要＆＃34; One＆＃34;或＆＃34;零＆＃34;，不是＆＃34; 1＆＃34;或＆＃34; 0＆＃34;。

解决方案是编写自己的Read实现（你可以保留Eq和Ord）：

import qualified Data.ByteString.Char8 as BSC
import qualified Data.ByteString as BS
import qualified Data.Map as Map

data Bit = One | Zero deriving (Eq,Ord)

instance Show Bit where
  show One = "1"
  show Zero = "0"

instance Read Bit where
  readsPrec _ ('1':xs) = [(One, xs)]
  readsPrec _ ('0':xs) = [(Zero, xs)]
  readsPrec _ _        = []

type Code = Map.Map Char [Bit]

writeCode :: FilePath -> Code -> IO ()
writeCode fp tr = BS.writeFile ("code_" ++ fp) (BSC.pack (show tr))

readCode :: FilePath -> IO Code
readCode f = do s <- BS.readFile ("code_" ++ f)
                let s' = BSC.unpack s
                return (read s' :: Code)

Haskell从文件中读取一个Map

1 个答案: