ChangeState.hs

{-# LANGUAGE DeriveDataTypeable, TypeFamilies, TemplateHaskell #-}
module Main (main) where

import Data.Acid

import Control.Monad.State
import System.Environment
import Data.SafeCopy

import Data.Typeable

import Control.Exception
import Prelude hiding (catch)

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 <- openAcidStateFrom "state/ChangeState" (FirstState "first state")
          createCheckpoint firstAcid
          closeAcidState firstAcid

          secondAcid <- openAcidStateFrom "state/ChangeState" (SecondState (Text.pack "This initial value shouldn't be used"))
          closeAcidState secondAcid
          putStrLn "If you see this message then something has gone wrong!"