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176 :: C bindings and structs with unions

• exhu (orginal) [2013-07-03T15:54:03+02:00] view original

  I'm definitely doing something wrong, trying to use xlib wrapper, because nimrod code looks ugly. This is the original C source:

  
  while(1) {
          XNextEvent(dpy, &e);
          if(e.type==Expose && e.xexpose.count<1) {
              XDrawString(dpy, win, gc, 10, 10, "Hello World!", 12);
              paint(cs);
          }
          else if(e.type==ButtonPress) break;
          else if(e.type==ClientMessage && e.xclient.data.l[0] == wmDeleteMessage) {
              break;
          }
      }
  
  Run

  This is how I translated it into Nimrod:

  
  while true:
          discard XNextEvent(dpy, addr(e))
          if e.theType == Expose and cast[PXExposeEvent](e.addr).count < 1:
              discard XDrawString(dpy, win , gc, 10, 10, "Hello World!", 12)
              paint(cs)
          
          elif e.theType == ButtonPress:
              break
          elif e.theType == ClientMessage and
              cast[ptr cint]((cast[PXClientMessageEvent](e.addr).data.addr
                  ))[] == cast[cint](wmDeleteMessage):
                      break
  Run

  Awaiting your suggestions on how to improve the code in nimrod way.

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• gradha (orginal) [2013-07-03T20:32:03+02:00] view original

  Since the original code doesn't check return values of XNextEvent() and others you could add the {.discardable.} pragma to them to avoid having to write it explicitly in the calling code.

  I know nothing of xlib and its types, but it seems that checking for the ClientMessage type and extracting the first entry of that nested array is sort of predefined. I would wrap that code in something similar to:

  
  proc ClientMessage(e: TypeOfE): probablyEnumType =
    foo
  Run

  Where foo would be a check of e.theType to be ClientMessage and then you apply the casts. With the advantage of adding runtime checks or maybe a when release for quick access and otherwise raise errors if something goes wrong, like the array not being there or whatever you think is a common error. Then your later code would look similar to:

  
  elif e.ClientMessage == wmDeleteMessage:
    bar
  Run

  Not sure if the ClientMessage proc would collision with the type, maybe find a slightly different name/convention for such wrapping procs. And maybe make them a template for speed.

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• Araq (orginal) [2013-07-03T21:51:41+02:00] view original

  Most likely many types of Nimrod's xlib wrapper are inconvenient to work with. Please fix them instead of hacking around. That said unions are always ugly to access. Best practice is to define some templates that hide the messy details:

  
  template checkEv(ev, val: expr, body: stmt) {.immediate, dirty.} =
    if ev.theType == val:
      let ev = cast[`PX val Event`](addr(ev))
      body
  
  checkEv(e, Expose):
    if e.count < 1:
      discard XDrawString(dpy, win, gc, 10, 10, "Hello World!", 12)
  Run

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• exhu (orginal) [2013-07-03T22:49:13+02:00] view original

  Ok, i'll fix it as soon as get back to code and issue a pull request.

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• exhu (orginal) [2013-07-04T09:49:43+02:00] view original

  After I push it, i'd better create a new repo for xlib/cairo bindings, because forking and pushing nimrod repo takes a day (git push nimrod fork it's 5 KiB/s here) :(

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• fowl (orginal) [2013-07-05T22:17:27+02:00] view original

  In the CSFML binding, for TEvent, I was able to use a type like

  
  type
    TEvent {.pure.} = object
      case kind*: TEventKind
      of MouseButtonDown, MouseButtonUp: button: TMouseButtonEvt
      etc
  Run

  However, I tried to do this with SDL's TEvent and was unsuccessful because SDL_Event's alignment is "packed" (IIRC)

  Good luck

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• exhu (orginal) [2013-07-07T21:57:26+02:00] view original

  @Araq, that kind of template significantly changes code, i think the following although unsafe would be better:

  
      proc xclient*(e: PXEvent): PXClientMessageEvent =
          ## Treats XEvent as XClientMessageEvent
          return cast[PXClientMessageEvent](e)
      
      proc xclient*(e: var TXEvent): PXClientMessageEvent =
          return xclient(PXEvent(e.addr))
  
  Run

  BTW is there a better way to treat both var and pointer type the same here, in order to have only one such proc?

  Then the user code resembles the original C code:

  
  else if(e.type==ClientMessage && e.xclient.data.l[0] == wmDeleteMessage) {
              break;
          }
  Run

  Translated into nimrod:

  
  elif e.theType == ClientMessage and
              cast[PAtom](e.xclient.data[0].addr)[] == wmDeleteMessage:
                      break
  
  Run

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• Araq (orginal) [2013-07-08T10:56:57+02:00] view original

  Shrug your way is fine too; whatever accomplishes your goal. It'll always be ugly because xlib is ugly.

  Regarding ptr vs. var: It's yet-another planned feature to offer auto-deref so only the var version would be necessary. For now you need both or define a converter.

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