    {-# LANGUAGE ForeignFunctionInterface, CPP #-}
    -- #hide
    --------------------------------------------------------------------------------
    -- |
    -- Module      :  Graphics.Rendering.GLU.Raw.APIEntry
    -- Copyright   :  (c) Sven Panne 2009
    -- License     :  BSD-style (see the file LICENSE)
    --
    -- Maintainer  :  sven.panne@aedion.de
    -- Stability   :  stable
    -- Portability :  portable
    --
    -- This module offers a portable way to retrieve GLUT entries, providing a
    -- portability layer upon platform-specific mechanisms like @dlsym@,
    -- @GetProcAddress@ or @NSAddressOfSymbol@.
    --
    --------------------------------------------------------------------------------

    module Graphics.Rendering.GLU.Raw.APIEntry (
       Invoker, getAPIEntry,
       FunPtr, unsafePerformIO
    ) where

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

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

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

    type Invoker a = FunPtr a -> a

    -- | Retrieve a GLU 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 GLU entry " ++ extensionEntry) $
          withCString extensionEntry hs_GLU_getProcAddress

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

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