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{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeFamilies #-}
module ChangeState (main, test) where
import Data.Acid
import Control.Exception
import Control.Monad
import Data.SafeCopy
import System.Directory
import System.Environment
import Data.List (isSuffixOf)
import qualified Data.Text as Text
------------------------------------------------------
-- The Haskell structure that we want to encapsulate
data FirstState = FirstState String
deriving (Show)
data SecondState = SecondState Text.Text
deriving (Show)
$(deriveSafeCopy 0 'base ''FirstState)
$(deriveSafeCopy 0 'base ''SecondState)
------------------------------------------------------
-- 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 state type"
putStrLn "without telling AcidState 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 (Text.pack "This initial value shouldn't be used"))
closeAcidState secondAcid
error "If you see this message then something has gone wrong!"
test :: IO ()
test = do
putStrLn "ChangeState test"
exists <- doesDirectoryExist fp
when exists $ removeDirectoryRecursive fp
handle hdl main
putStrLn "ChangeState done"
where
hdl (ErrorCall msg)
| "Could not parse saved checkpoint due to the following error: too few bytes\nFrom:\tChangeState.SecondState:\n\tdemandInput\n\n" `isSuffixOf` msg
= putStrLn $ "Caught error: " ++ msg
hdl e = throwIO e
fp = "state/ChangeState"
|
