Question

我正在尝试使用safecopy haskell库，但是当我尝试将字符串迁移到bytestring时，最后4个字符丢失，4'\ NULL'字符前置于字符串：

{-# LANGUAGE TemplateHaskell, DeriveDataTypeable, TypeFamilies #-}
import Data.SafeCopy
import Data.Acid
import Data.Typeable
import Control.Monad.State.Class
import Control.Monad.Reader.Class

data T = T { str :: String }
         deriving (Show, Typeable)

getT :: Query T String
getT = fmap str ask

setT :: String -> Update T ()
setT str = put $ T str

deriveSafeCopy 0 'base  ''T

makeAcidic ''T ['setT, 'getT]

main :: IO ()
main = do
  state <- openLocalState (T "string set with default")
  update state (SetT "string set with SetT")
  str <- query state GetT
  putStrLn str

此输出：string set with SetT，但在此之后运行以下修改后的版本：

{-# LANGUAGE TemplateHaskell, DeriveDataTypeable, TypeFamilies #-}
import Data.SafeCopy
import Data.Acid
import Data.Typeable
import Control.Monad.State.Class
import Control.Monad.Reader.Class
import Data.ByteString.Char8 as B

data T_v0 = T_v0 String
         deriving (Show, Typeable)
deriveSafeCopy 0 'base  ''T_v0

data T = T { str :: B.ByteString }
         deriving (Show, Typeable)
deriveSafeCopy 1 'extension ''T

instance Migrate T where
  type MigrateFrom T = T_v0
  migrate (T_v0 str) = T $ B.pack str

getT :: Query T B.ByteString
getT = fmap str ask

setT :: B.ByteString -> Update T ()
setT str = put $ T str
makeAcidic ''T ['setT, 'getT]

main :: IO ()
main = do
  state <- openLocalState (T $ B.pack "bytestring set with default")
  str <- query state GetT
  print str

输出："\NUL\NUL\NUL\NULstring set with "。我不知道为什么会这样。我在迁移步骤中做错了吗？我试图尽可能地靠近example code。有谁知道为什么会这样？

P.S：对于大量的代码感到抱歉，但我想不出更好的方法来传达这个问题。

Answer 1

这是因为acid-state。您已经在混淆事务日志的实现之间更改了getT和setT的类型。在没有先使用createCheckpoint清除事务日志（使用较旧版本的代码库）的情况下，您基本上无法更改使用makeAcidic标记的任何函数。

Safecopy从字符串迁移到bytestring

1 个答案: