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.