{-#LANGUAGE FlexibleContexts,MultiParamTypeClasses,FlexibleInstances,UndecidableInstances #-}
module PrivLib (
 getFoo, putFoo,
 runFoo, TyFoo,
 runFooT, TyFooT,
 MonadFoo(..)
) where

import Control.Monad.StateX

-- create the private index.  do not export.
data PRIV = PRIV  deriving (Show,Eq); instance Index PRIV where getVal = PRIV

-- create `wrapper' functions for all monadic behavior you want to
-- allow. This part is the most annoying.
getFoo :: (MonadStateX PRIV s m) => m s
getFoo = getx PRIV
putFoo :: (MonadStateX PRIV s m) => s -> m ()
putFoo = putx PRIV

-- create type synonyms to give controlled access. export.
type TyFoo  s   a = StateX  PRIV s   a
type TyFooT s m a = StateTX PRIV s m a

runFoo  start = flip (runStateX  PRIV) start
runFooT start = flip (runStateTX PRIV) start

-- create an empty type class only to `rename' the constraint without
-- having to reference the private index.
class    (MonadStateX PRIV s m) => MonadFoo s m where
instance (MonadStateX PRIV s m) => MonadFoo s m where

-- No Quadratic-sized set of instances to add, and yet you have
-- removed direct acces to the index! Safety is argued. Again, all the
-- `dangerous' pragmas are only needed in this file:
-- FlexibleInstances, UndecidableInstances.




{-
class (Monad m) => {-(MonadStateX PRIV s m) => -} MonadFoo s m where
  getFoo :: m s
  putFoo :: s -> m ()

instance (s1 ~ s2) => MonadFoo s1 (StateX PRIV s2) where
  getFoo = getx PRIV
  putFoo = putx PRIV

instance (Monad m, s1 ~ s2) => MonadFoo s1 (StateTX PRIV s2 m) where
  getFoo = getx PRIV
  putFoo = putx PRIV
-}

-- this gets an interesting type families nonsupported usage error.
--class (MonadStateX ix s1 m, s1 ~ s2 ) => Foo ix s1 s2 m where