[Haskell-cafe] c2hs seems to ignore the types I give it when generating foreign import declarations

[Cale Gibbard]

On 11/12/05, Manuel M T Chakravarty <chak at cse.unsw.edu.au> wrote:
> Cale Gibbard:
> > Hello, I've just started writing bindings to the Enlightenment
> > Foundation Libraries, using c2hs. I've run into the following problem
> > with the types in the foreign import declarations generated.
> >
> > In Imlib2.h, there is a function prototype:
> > int imlib_get_visual_depth(Display * display, Visual * visual);
> >
> > In Imlib.chs, I have (among other things)
> > import qualified Graphics.X11.Xlib.Types as Xlib
> > -- ...
> > {# fun imlib_get_visual_depth as getVisualDepth { id `Xlib.Display',
> > id `Xlib.Visual' } -> `Int' #}
> >
> > I run:
> > c2hs -k /usr/local/include/Imlib2.h Imlib.chs
> >
> > and in the resulting Imlib.hs, I get:
> >
> > --- this part is correct:
> > getVisualDepth :: Xlib.Display -> Xlib.Visual -> IO (Int)
> > getVisualDepth a1 a2 =
> >   let {a1' = id a1} in
> >   let {a2' = id a2} in
> >   getVisualDepth'_ a1' a2' >>= \res ->
> >   let {res' = cIntConv res} in
> >   return (res')
> >
> > --- this part is not:
> > foreign import ccall safe "Imlib.h imlib_get_visual_depth"
> >   getVisualDepth'_ :: ((Ptr ()) -> ((Ptr ()) -> (IO CInt)))
> >
> > Why did it choose the type Ptr (), when I gave the type Xlib.Display?
> > I could do an additional cast, but it will get tedious, as I have a
> > lot of functions to import, and there shouldn't be any need for it
> > anyway. As this is, it doesn't typecheck at all. If by hand, I change
> > it to read:
> >
> > foreign import ccall safe "Imlib.h imlib_get_visual_depth"
> >   getVisualDepth'_ :: Xlib.Display -> Xlib.Visual -> IO CInt
> >
> > (which is what I expected it to produce in the first place) then it
> > works as expected.
> >
> > Am I making a grievous error, or is this a bug/lack of a feature?
>
> You need to tell c2hs to what Haskell type a C pointer type maps before
> it can generate the right signature for the import declaration.  This is
> done using a pointer hook:
>
>   http://www.cse.unsw.edu.au/~chak/haskell/c2hs/docu/c2hs-3.html#ss3.10
>
> In your case, you would usually do
>
>   {#pointer *Display as Display newtype#}
>
> However, when you use this form, c2hs will generate the newtype
> declaration for you that you import from `Graphics.X11.Xlib.Types' (and
> you would have to cast between your version of `Display' and that of
> `Xlib').
>
> Are you actually going to use functions from `Graphics.X11.Xlib' on
> these types or did you just want to use the existing types because it is
> more elegant?  It is a awkward to use existing types that have *not*
> been generated by c2hs in that way at the moment.  However, it should be
> quite easy to extend c2hs to enable this quite easily if it is
> important.
>
> Manuel
>
> PS: I started a binding of Ecore and Evas a while ago.  Still
>     incomplete, but if you meant to do these, too, then we should maybe
>     exchange code instead of duplicating work.
>

Hi,

I wanted to use the existing types so as to have compatibility with
any other libraries which might use them. It would be a shame if every
Haskell binding to an X library had different, incompatible versions
of the X types, and thus they couldn't be used together.

I managed to get something which worked, but I've been distracted by
other random things lately. Perhaps I'll come back to it now :)

The binding to Ecore and Evas sounds good, I was hoping to eventually
bind the whole EFL.

 - Cale

I would like to do bindings on Ecore an