Thanks to everyone, especially Bulat Ziganshin. In http://haskell.org/haskellwiki/Modern_array_libraries there is enough information to do what I want. It specifically mentions that it's OK to pass ByteArray# and MutableByteArray# to an "unsafe foreign" procedure as long as that procedure doesn't save the pointer, and that worked for me. Here is what I ended up using, which worked great and the FFI usage for a couple of key functions sped up my code by a large factor: import Data.Array.Base import Data.Array.IO.Internals import GHC.Exts {-# INLINE unsafeByteArrayToPtr #-} unsafeByteArrayToPtr :: IOUArray Int Word32 -> Ptr Word32 unsafeByteArrayToPtr (IOUArray (STUArray _ _ array#)) = Ptr (unsafeCoerce# array#) Possibly a better thing to do would be to declare that the call takes a MutableByteArray# directly in the foreign import statement, which I believe would let me avoid using unsafeCoerce# at all, but this was good enough for my purposes. Afterwards I used -ddump-simpl to check on the generated Core for the foreign call and it looked good. -- ryan