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{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeFamilies #-}
module ChangeVersion (main, test) where
import Data.Acid
import Control.Exception
import Control.Monad
import Data.SafeCopy
import System.Directory
import System.Environment
------------------------------------------------------
-- The Haskell structure that we want to encapsulate
data FirstState = FirstState String
deriving (Show)
data SecondState = SecondState Int
deriving (Show)
$(deriveSafeCopy 0 'base ''FirstState)
$(deriveSafeCopy 1 'base ''SecondState)
-- The version number difference is important here, as safecopy has no
-- way to reliably notice if we change a type and fail to update its
-- migration history. In some cases the serialised data will fail to
-- parse (see the "ChangeState" example), but it depends on the types
-- involved, and this will not always be the case.
------------------------------------------------------
-- The transaction we will execute over the state.
$(makeAcidic ''FirstState [])
$(makeAcidic ''SecondState [])
------------------------------------------------------
-- This is how AcidState is used:
main :: IO ()
main = do putStrLn "This example simulates what happens when you modify your safecopy"
putStrLn "version without specifying how to migrate from the old version to the new."
putStrLn "Hopefully this program will fail with a readable error message."
putStrLn ""
firstAcid <- openLocalStateFrom fp (FirstState "first state")
createCheckpoint firstAcid
closeAcidState firstAcid
secondAcid <- openLocalStateFrom fp (SecondState 42)
closeAcidState secondAcid
error "If you see this message then something has gone wrong!"
test :: IO ()
test = do
putStrLn "ChangeVersion test"
exists <- doesDirectoryExist fp
when exists $ removeDirectoryRecursive fp
handle hdl main
putStrLn "ChangeVersion done"
where
hdl (ErrorCall msg)
| msg == "Could not parse saved checkpoint due to the following error: Failed reading: safecopy: ChangeVersion.SecondState: Cannot find getter associated with this version number: Version {unVersion = 0}\nEmpty call stack\n"
= putStrLn $ "Caught error: " ++ msg
hdl e = throwIO e
fp = "state/ChangeVersion"
|
