    -- #hide
    --------------------------------------------------------------------------------
    -- |
    -- Module      :  Graphics.Rendering.OpenGL.GL.Extensions
    -- Copyright   :  (c) Sven Panne 2002-2009
    -- License     :  BSD-style (see the file libraries/OpenGL/LICENSE)
    --
    -- Maintainer  :  sven.panne@aedion.de
    -- Stability   :  stable
    -- Portability :  portable
    --
    -- This is a purely internal module for handling the OpenGL extension
    -- mechanism.
    --
    --------------------------------------------------------------------------------

    module Graphics.Rendering.OpenGL.GL.Extensions (
       FunPtr, unsafePerformIO,
       Invoker, getProcAddress   -- used only internally
    ) where

    import Foreign.C.String ( CString, withCString )
    import Foreign.Ptr ( FunPtr, nullFunPtr )
    import System.IO.Unsafe ( unsafePerformIO )

    #ifdef __HUGS__
    {-# CFILES cbits/HsOpenGL.c #-}
    #endif

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

    type Invoker a = FunPtr a -> a

    getProcAddress :: String -> String -> IO (FunPtr a)
    getProcAddress ext call =
       throwIfNull ("unknown OpenGL call " ++ call ++ ", check for " ++ ext) $
          withCString call hs_OpenGL_getProcAddress

    throwIfNull :: String -> IO (FunPtr a) -> IO (FunPtr a)
    throwIfNull msg act = do
       res <- act
       if res == nullFunPtr
          then ioError (userError msg)
          else return res

    foreign import ccall unsafe "hs_OpenGL_getProcAddress" hs_OpenGL_getProcAddress
       :: CString -> IO (FunPtr a)