Hi Simon,
1) How to convert the Haskell string to a C string? - There's a function
packCString# :: [Char] -> ByteArray# packCString# str = case (packString str) of { ByteArray _ _ bytes -> bytes }
in CStrings.lhs / PrelPack.lhs, which I probably have to use.
Yes (but use packString rather than packString#).
Okay, so I have use: packString :: [Char] -> ByteArray Int btw: what's the difference between packString and packCString then?
2) How to pass a pointer (Addr) of the converted string back to the outside world?
You can pass the ByteArray directly to a foreign imported function (which *must* be declared unsafe - you can't pass ByteArrays to safe ccalls).
I think, here the context got lost: I want to expose a Haskell function with a string result rather than calling a C function from Haskell.
3) Who is responsible for freeing the memory of the string result? I suppose this should be done outside. But what to do that GHC's garbage collection doesn't free it automatically?
The ByteArray is freed automatically by GHC's garbage collector. If you need something more permanent, use malloc/free and the functions from Storable to do the marshalling.
For instance, how would you define a function like revstr :: String -> String revstr s = reverse s for foreign export? (The conversion of the string argument I handled like this:) strlength :: Addr -> IO Int strlength cstr = do s <- unpackCStringIO cstr return (length s) foreign export stdcall strlength :: Addr -> IO Int Cheers, Christian P.S. I think, some information on the handling of typical string conversion problems would be an interesting topic for GHC's user's manual, too.