How does one use pass by reference functions using the ffi?

[Manuel Chakravarty]

David Sankel wrote:

>   Say I have a function somewhere in c land with the following declaration:
> 
> void doStuff( int &a, int &b);
> 
>   What would the appropriate Haskell wrapper for this function look like? 

It'll be

  doStuffC :: Ptr Int -> Ptr Int -> IO ()

which really is a purely syntactic translation of the type
of the C function.  

> I'm looking to construct a haskell function of the type: 
> 
> doStuff :: Int -> Int -> IO( (Int,Int) )

You will need to do the work of allocating temporary storage
and passing pointers to that storage manually unless you use
one of the FFI tools (eg, c2hs
<http://www.cse.unsw.edu.au/~chak/haskell/c2hs/> or
GreenCard).  Doing it manually isn't quite as bad as it
might sound at first, as the FFI libraries contain
convenience functions for this type of situation:

  http://haskell.org/ghc/docs/latest/html/libraries/base/Foreign.Marshal.Utils.html#v%3Awith

So you get

  doStuff x y =
    with x $ \xPtr ->
    with y $ \yPtr -> do
      doStuffC xPtr yPtr	-- calling into C land
      x' <- peek xPtr
      y' <- peek yPtr
      return (x', y')

(The `with' brackets ensure proper deallocation of the
temporary storage in a stack-like manner.)

Good Luck!
Manuel