RemoveEvent.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)

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-- The Haskell structure that we want to encapsulate

data FirstState = FirstState
    deriving (Show)

data SecondState = SecondState
    deriving (Show)

$(deriveSafeCopy 0 'base ''FirstState)
$(deriveSafeCopy 0 'base ''SecondState)

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-- The transaction we will execute over the state.

firstEvent :: Update FirstState ()
firstEvent = return ()

$(makeAcidic ''FirstState ['firstEvent])
$(makeAcidic ''SecondState [])

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-- This is how AcidState is used:

main :: IO ()
main = do putStrLn "This example simulates what happens when you remove an event"
          putStrLn "that is required to replay the journal."
          putStrLn "Hopefully this program will fail with a readable error message."
          putStrLn ""
          firstAcid <- openAcidStateFrom "state/RemoveEvent" FirstState
          update firstAcid FirstEvent
          closeAcidState firstAcid

          secondAcid <- openAcidStateFrom "state/RemoveEvent" SecondState
          closeAcidState secondAcid
          putStrLn "If you see this message then something has gone wrong!"