    {-# LANGUAGE ForeignFunctionInterface, CPP #-}
    -- #hide
    --------------------------------------------------------------------------------
    -- |
    -- Module      :  Graphics.UI.GLUT.Raw.APIEntry
    -- Copyright   :  (c) Sven Panne 2002-2009
    -- License     :  BSD-style (see the file LICENSE)
    --
    -- Maintainer  :  sven.panne@aedion.de
    -- Stability   :  provisional
    -- Portability :  portable
    --
    -- This is a purely internal module for handling an OpenGL-like extension
    -- mechanism for GLUT.
    --
    --------------------------------------------------------------------------------

    module Graphics.UI.GLUT.Raw.APIEntry (
       Invoker, getAPIEntry, getAPIEntryInternal,
       FunPtr, unsafePerformIO
    ) where

    import Foreign.C.String
    import Foreign.Marshal.Error
    import Foreign.Ptr
    import System.IO.Unsafe

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

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

    type Invoker a = FunPtr a -> a

    -- | Retrieve a GLUT API entry by name. Throws a userError when no entry with
    -- the given name was found.
    getAPIEntry :: String -> IO (FunPtr a)
    getAPIEntry extensionEntry =
       throwIfNullFunPtr ("unknown GLUT entry " ++ extensionEntry) $
          getAPIEntryInternal extensionEntry

    throwIfNullFunPtr :: String -> IO (FunPtr a) -> IO (FunPtr a)
    throwIfNullFunPtr = throwIf (== nullFunPtr) . const

    getAPIEntryInternal :: String -> IO (FunPtr a)
    getAPIEntryInternal extensionEntry =
       withCString extensionEntry hs_GLUT_getProcAddress

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