passing Strings with FFI

[Corneliu Popeea]

>foreign import "fooble" fooble
>    fooble :: PackedString -> Int -> IO Int
>
>type PackedString = ByteArray Int

With the code above I'm getting the following error:

-- Unacceptable argument type in foreign declaration: PackedString
-- When checking declaration:
--      foreign import _ccall "fooble" fooble :: PackedString -> Int -> IO Int

Is there any option that I must include when I launch ghc (I'm very new to 
this compiler, as to Haskell)?
-- I'm trying with "ghc -c -fglasgow-exts test.hs"

Best regards,
Corneliu