    --------------------------------------------------------------------------------
    -- |
    -- Module      :  Graphics.Rendering.OpenGL.GL.StateVar
    -- Copyright   :  (c) Sven Panne 2002-2006
    -- License     :  BSD-style (see the file libraries/OpenGL/LICENSE)
    --
    -- Maintainer  :  sven.panne@aedion.de
    -- Stability   :  stable
    -- Portability :  portable
    --
    --------------------------------------------------------------------------------

    module Graphics.Rendering.OpenGL.GL.StateVar (
       HasGetter(..),
       GettableStateVar, makeGettableStateVar,
       HasSetter(..), set,
       SettableStateVar, makeSettableStateVar,
       StateVar, makeStateVar,
       ($~), ($=!), ($~!)
    ) where

    import Data.IORef ( IORef, readIORef, writeIORef )

    --------------------------------------------------------------------------------

    infixr 2 $=

    --------------------------------------------------------------------------------

    class HasGetter g where
       get :: g a -> IO a

    --------------------------------------------------------------------------------

    newtype GettableStateVar a = GettableStateVar (IO a)

    instance HasGetter GettableStateVar where
       get (GettableStateVar g) = g

    makeGettableStateVar :: IO a -> GettableStateVar a
    makeGettableStateVar = GettableStateVar

    --------------------------------------------------------------------------------

    class HasSetter s where
       ($=) :: s a -> a -> IO ()

    {-# DEPRECATED set "use `sequence_' instead" #-}
    set :: [IO ()] -> IO ()
    set = sequence_

    --------------------------------------------------------------------------------

    newtype SettableStateVar a = SettableStateVar (a -> IO ())

    instance HasSetter SettableStateVar where
       ($=) (SettableStateVar s) a = s a

    -- | A strict variant of '$='.
    ($=!) :: HasSetter s => s a -> a -> IO ()
    v $=! x = x `seq` v $= x

    makeSettableStateVar :: (a -> IO ()) -> SettableStateVar a
    makeSettableStateVar = SettableStateVar

    --------------------------------------------------------------------------------

    data StateVar a =
       StateVar (GettableStateVar a) (SettableStateVar a)

    instance HasGetter StateVar where
       get (StateVar g _) = get g

    instance HasSetter StateVar where
       ($=) (StateVar _ s) a = s $= a

    makeStateVar :: IO a -> (a -> IO ()) -> StateVar a
    makeStateVar g s = StateVar (makeGettableStateVar g) (makeSettableStateVar s)

    --------------------------------------------------------------------------------

    -- | A modificator convenience function.

    ($~) :: (HasGetter v, HasSetter v) => v a -> (a -> a) -> IO ()
    v $~ f = do
       x <- get v
       v $= f x

    -- | A strict variant of '$~'.
    ($~!) :: (HasGetter v, HasSetter v) => v a -> (a -> a) -> IO ()
    v $~! f = do
       x <- get v
       v $=! f x

    --------------------------------------------------------------------------------

    instance HasGetter IORef where
       get = readIORef

    instance HasSetter IORef where
       ($=) = writeIORef