[Haskell-cafe] How can I get the mutable array out of an IOUArray for FFI use?

[Ryan Ingram]

I wrote some fast bit-twiddling functions in C because my Haskell
performance wasn't good enough.  Now I'm trying to recompile with
GHC6.8.3 and failing.   This code worked on GHC6.6.1.

I get the following error:

> ghc --make main.hs

Bitmap.hs:11:7:
    Could not find module `Data.Array.IO.Internals':
      it is hidden (in package array-0.1.0.0)

I suppose I can declare a copy of the internal type and use
unsafeCoerce#, but that seems like a terrible idea if there is a
better way.  What's the right way to make this work?  Can I force that
module to be unhidden?  Should I file a GHC bug?

  -- ryan

{-# OPTIONS_GHC -fffi -fglasgow-exts #-}
{-# INCLUDE "bitmap_operations.h" #-}

module Bitmap (
	clearBitmap,
) where
import Foreign.Ptr
import Data.Array.Base
import Data.Array.IO.Internals
import GHC.Exts
import Data.Word

foreign import ccall unsafe clear_bitmap :: MutableByteArray#
RealWorld -> Word32 -> Word32 -> IO ()

{-# INLINE unsafeGetMutableArray# #-}
unsafeGetMutableArray# :: IOUArray Int Word32 -> MutableByteArray# RealWorld
unsafeGetMutableArray# (IOUArray (STUArray _ _ array#)) = array#

clearBitmap :: IOUArray Int Word32 -> Word32 -> Word32 -> IO ()
clearBitmap a1 color size
	= clear_bitmap (unsafeGetMutableArray# a1) color size