[Gtk2hs-devel] Re: [C2hs] Re: patch to c2hs to support asm constructs - .h files in foreign declarations

[Manuel M T C. <ch...@cs...>]

[This reply only address the second issue, to keep the discussion better
structured.]

Duncan Coutts:
> > Not including any .h in foreign imports is not a portable way of using
> > the FFI.
> 
> Sorry, I don't think I was clear enough.
> 
> We use the -#include on the command line (and in the package.conf file)
> to specify the header files to include. So we are compiling with the
> correct .h files specified. So the point is that we want to use the .h
> files specified in the package.conf file (or on the commandline during
> the build itself) rather than lots of little .h files that just #include
> the main .h file.
> 
> So for example we build the modules like this:
> 
> ghc -c gtk/Graphics/UI/Gtk/Buttons/Button.hs -o
> gtk/Graphics/UI/Gtk/Buttons/Button.o -fffi -iglib:gtk -package-name gtk
> '-#include<gtk/gtk.h>' -I/usr/include/glib-2.0
> -I/usr/lib/glib-2.0/include -DXTHREADS -D_REENTRANT -DXUSE_MTSAFE_API
> -I/usr/include/gtk-2.0 -I/usr/lib/gtk-2.0/include
> -I/usr/include/pango-1.0 -I/usr/include/freetype2 -I/usr/include/atk-1.0
> 
> So we specify the '-#include<gtk/gtk.h>' and all the -I dirs on the
> command line. And the gtk.package.conf file contains:

Well, the problem is that -#include is GHC specific.

> extra-libraries: "gtk-x11-2.0","gdk-x11-2.0","atk-1.0","pangoxft-1.0","pangox-1.0","pango-1.0","gdk_pixbuf-2.0","m","gmodule-2.0","dl"
> include-dirs: "/usr/include/gtk-2.0", "/usr/lib/gtk-2.0/include", "/usr/include/pango-1.0", "/usr/include/freetype2", "/usr/include/atk-1.0"
> includes: gtk/gtk.h
> 
> So that importing modules will get the correct include (and search
> path).
> 
> So we want to generate import decls like this:
> 
> foreign import ccall safe "gtk_button_pressed"
>   gtk_button_pressed :: ((Ptr Button) -> (IO ()))
> 
> instead of like this:
> 
> foreign import ccall safe "gtk/Graphics/UI/Gtk/Buttons/Button.h gtk_button_pressed"
>   gtk_button_pressed :: ((Ptr Button) -> (IO ()))
> 
> where gtk/Graphics/UI/Gtk/Buttons/Button.h contains nothing but:
> #include <gtk/gtk.h>
> 
> Since that way we do not need to distribute lots of little .h files. We
> directly include the apprpriate system header.

I understand that you want to get rid of the many .h files.  My proposal
is to do that by generating

  foreign import ccall safe "gtk/gtk.h gtk_button_pressed"
  gtk_button_pressed :: ((Ptr Button) -> (IO ()))

This gets rid of the .h file, but is still portable.

Obviously, this can only be done in cases, where the "little .h file"
contains only a single #include.  And it should only be done when a
special command line option is given, but that should not be a problem
for you.

> Another reason for this is that ghc corrently does not inline FFI calls
> which are declared in the latter form above (since it would also have to
> propogate the .h files across module boundaries, and it currently
> doesn't do that).
> 
> See:
> http://haskell.org/ghc/docs/latest/html/users_guide/sec-ffi-ghc.html#glasgow-foreign-headers
> 
> > The only feasible "optimisation" for c2hs seems to be that if
> > it generates a "foo.h" with only
> > 
> >   #include "lib.h"
> 
> We currently do not put the #include "lib.h" into the .chs module since
> we already have to pre-process the .chs module with cpp. That's why we
> specify it on the c2hs and ghc command line.

Why do you do that?

(The solution that I propose above will still work when you specify
the .h on the c2hs command line.)

Manuel