[m-users.] Refactoring Mercury/C FFi code

[emacstheviking]

Zoltan,

It's nice to meet somebody working on Mercury!

My current goal is to create a low-level wrapper around SDL2 to the point
where I can create applications with it. Maybe it was a bit ambitious for a
first project but what the hell, I started so I have to finish now. I need
it!!!

I understand your comments but it will take some time to turn it into code
for me.

Thanks,
Sean.


On Sun, 14 Jul 2019 at 23:43, Zoltan Somogyi <zoltan.somogyi at runbox.com>
wrote:

>
>
> On Sun, 14 Jul 2019 23:08:13 +0100, emacstheviking <objitsu at gmail.com>
> wrote:
> > My question then is what would be "the best way"
> > to handle this case. With two rectangles I have had to create four
> > predicates, with three it would be 8 etc etc and it feels like there is a
> > better way.
>
> The "best way" depends on information you haven't told us: whether
> most of the code working on rectangles is in Mercury or in C.
>
> If most of the code that manipulates values of a type is in C, then you
> should
> define the type in C, and write foreign_proc predicates and/or functions to
> export the needed functionality to Mercury code.
>
> If most of the code that manipulates values of a type is in Mercury, then
> you should
> define the type in Mercury, and write Mercury predicates and/or functions,
> exported
> to C, to provide the needed functionality to C code.
>
> If the code sample you gave is representative, then I would have a C
> declaration
>
> typedef SDL_Rect *SDL_RectPtr;
>
> and Mercury definitions
>
> :- type sdl_rect;
> :- pragma foreign_type(c, sdl_rect, "SDL_RectPtr").
>
>    % Return the x and y coords, width and height of a rectangle if and
> only if non-null.
>    %
> :- pred sdl_rect_is_real(sdl_rect::in, int::out, int::out, int::out,
> int::out) is semidet.
>
> Then you would need only one sdl_rendercopy predicate, which could do its
> own null/nonnull tests as needed.
>
> > From what I have learned so far I know that
> > parameters are passed in as MR_Word sized values so would it be safe for
> > example to cast an sdl_rect() declared as:
> >
> > *:- type sdl_rect ---> sdl_rect(x::int, y::int, w::int, h::int).*
> >
> > *typedef struct SDL_Rect{    int x, y;    int w, h;} SDL_Rect;*
> >
> > to an SDL_Rect which has the same size and shape?!?!?!?!
>
> No, that would not be safe. It may work with the current data
> representation
> scheme used by the Mercury compiler, but that scheme may change.
> I should know; I am working on such a change right now.
>
> Zoltan.
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mercurylang.org/archives/users/attachments/20190715/0f0347b2/attachment.html>