[Haskell-cafe] Troubles with FFI

[Brian Hulley]

Matthew Bromberg wrote:
> foreign import ccall "matrix_c.h sumarr" sumarr :: Ptr CDouble ->
> CDouble [snip]
> If one changes the array in[] in the C code, e.g.
> double sumarr(double *in)
> {
>  in[0] = -10.0 ;
>  return(in[0] + in[1]) ;
> }
> The array is changed inside the haskell program as a side effect.
>
> main = do
>    arr <- newListArray (1 , 3) [3,2,1]:: IO (StorableArray Int
>    CDouble) -- extract the pointer to arr
>    withStorableArray arr $ print . sumarr
>    (readArray arr 1 ) >>= print

If you want to change the array in the C code, the foreign import should 
reflect the fact that something is being mutated by enclosing the result in 
the IO monad:

foreign import ccall "matrix_c.h sumarr" sumarr
          :: Ptr CDouble ->IO CDouble
                                     ^^

(You don't need to change the type of the C function itself though)
Also, the 'a' in withStorableArray:: StorableArray i e -> (Ptr e -> IO a) -> 
IO a is the result of your C function, which in this case is CDouble. It's 
not related to the type of elements of the array.

So you could use:

withStorableArray arr $ sumarr >>= print

Operator precedences are given in 
http://haskell.org/onlinereport/standard-prelude.html

I think FFI is one of these things that once you've understood it it seems 
really neat and well thought out but it's as you say very difficult to get 
into because you need to know everything all at once and the descriptions in 
the FFI addendum are extremely terse and also very general, since they deal 
with languages other than C as well.

Regards, Brian.