AppState1.hs

{-#  LANGUAGE TemplateHaskell , FlexibleInstances, UndecidableInstances, OverlappingInstances,
  MultiParamTypeClasses, GeneralizedNewtypeDeriving, DeriveDataTypeable,
  TypeFamilies #-}
module StateVersions.AppState1 where

import Happstack.State
import Control.Monad
import Control.Monad.Reader
import Control.Monad.State
import Data.Typeable
import Data.Data

newtype AppState = AppState Int 
                  deriving (Read, Show, Typeable, Data)

instance Version AppState 
-- Now, I've said that Version is important for migrations
-- but the nice thing is that you don't have to plan in advance
-- that you may be migrating your state.  You can just use the
-- default instance of Version for the very first application
-- state you define.  It's only future versions that need 
-- anything other than the default instance for Version.
$(deriveSerialize ''AppState)

instance Component AppState where
    type Dependencies AppState = End --  no dependencies
    initialValue = AppState 0

askVal :: Query AppState Int
askVal = do
  AppState i <- ask
  return i

incVal :: Update AppState ()
incVal = do
  AppState i <- get
  put $ AppState (i+1)

$(mkMethods ''AppState ['askVal,'incVal])