Hi - I'd like to be able to use Haskell for the project I'm working on but the problem is that I've already written a lot of code for a nice GUI using DirectX in Visual C++. I thought it might be possible to make up some kind of simple API for it which I could call from Haskell, so I started with the following simple Haskell module to try and see if I could understand the FFI: module Main where data Window = Window_Edit | Window_List | Window_Tree data Layout = Layout_Left | Layout_Right type Callback = () -> Bool foreign import ccall gui_init :: [(Window, Layout, Callback)] -> IO () main = gui_init [(Window_Edit, Layout_Left, \_->True)] Of course my "init" function is far too simple, but I was thinking this captures the essence of the kinds of things that would need to be passed to the C++ code ie a list of windows and call back functions etc. However I've immediately run into a major problem. When I try to compile with ghc -fglasgow-exts --make main.hs I get an error: "unacceptable argument type in foreign declaration..." My questions are: 1) Does this mean that the FFI can only pass Int, Char etc and not user defined data types or compound data types? 2) I'm also really confused by the different kinds of pointers available, and how to safely store a function closure in some C data structure, and how to use this to call the function from within C and access the result safely 3) When does GHC do garbage collection? Is the garbage collection done in the same thread as the executing program? Does GHC run a normal Haskell program using multiple threads? Would I need to link my C DLL with a multithreaded version of the C runtime to avoid problems? Alternatively, has anyone managed to use DirectX or COM from within a Haskell program? (because then I could perhaps rewrite all my gui code from scratch in Haskell...) (I'm loathe to switch to OpenGL because OpenGL is very poorly supported on windows - a default WinXP installation does not come with hardware accelerated OpenGL drivers, afaik, and also I can't find any documentation on the Haskell OpenGL bindings except the Haddock type signatures which are just not nearly enough to understand how to use it, and some out of date docs) Thanks, Brian.