{-#LANGUAGE FlexibleContexts,FlexibleInstances,UndecidableInstances #-}
-- MultiParamTypeClasses,

{-
Module      :  examples/PrivLib
Copyright   :  (c) Mark Snyder 2012.
License     :  BSD-style
Maintainer  :  Mark Snyder, msnyde14@gmu.edu
Stability   :  experimental
Portability :  non-portable (multi-param classes, functional dependencies)
-}


module PrivLib( getV, putV, TyPos, TyPosT, runPos, runPosT, MonadPos(..)
              ) where

import Control.Monad.StateX

data PRIV = PRIV deriving (Show, Eq)
instance Index PRIV where getVal = PRIV

getV :: (MonadStateX PRIV Int m) => m Int
getV = getx PRIV

putV :: (MonadStateX PRIV Int m) => Int -> m ()
putV v = putx PRIV (if v<=0 then abs v else v) 

-- 'unprivileged' types for the regular and transformer versions.
type TyPos    a = StateX  PRIV Int   a
type TyPosT m a = StateTX PRIV Int m a

-- 'unprivileged' run functions for the regular and transformer versions.
runPos  start = flip (runStateX  PRIV) (abs start)
runPosT start = flip (runStateTX PRIV) (abs start)

-- 'unprivileged' class that hides our use of the StateX monad.
class (MonadStateX PRIV Int m) => MonadPos m where
instance (MonadStateX PRIV Int m) => MonadPos m where