About Slate and GUI

[Tony Garnock-Jones]

Hi Paul,

Paul Dufresne wrote:
> But now that I
> know that GTK+ is LGPL, have signals much like QT, is now using CAIRO
> graphics library, my
> choice is made, and I prefer GNOME (GTK+).

Wow, I really strongly prefer Qt - but not because Qt is good, although 
when I've coded for it it hasn't been too bad, but because GTK+ is 
staggeringly awful.

> But I now feel that SDL is a bit too
> low-level to serve as
> the level under DUIM-morphic code. I now have the feeling GTK+ would be
> more appropriate.

I really disagree! GTK+ has one of the most horrible object-models I've 
ever used, and it's not at all shy about forcing you into it. I wrapped 
both GTK+ 1.2 and 2.0 for the Chicken Scheme compiler, and it was quite 
a painful process. (Mostly because GTK+ is very inconsistent and 
incomplete in the metadata it exposes.)

Cairo, on the other hand, seems pretty good.

> It is not so obvious to me (as it seems to be for Brian) why the
> concurrency have to be
> implemented in Slate, before working on the GUI code.

Yes, this is the main problem. Without a framework for (something 
resembling) multi-threaded I/O, there is no way of mediating the many 
(pseudo-)parallel operations that together make up a functioning GUI.

The main piece missing is an outer driver loop that calls select() to 
wait for I/O and timeouts, and dispatches I/O events and timeout 
callbacks. By itself, this isn't so tricky - but integrating with SDL's 
event sources as well adds a few wrinkles.

Of course, given such a driver, having actual real concurrency is kind 
of optional, although having call-with-current-continuation would 
obviously be very nice :-)

A few months ago (almost a year!) I implemented, in Scheme, a Slate-like 
language, and I got as far as implementing an SDL + SDL_net based user 
interface before deciding I'd learnt enough. The key idea is to avoid 
using SDL's SDL_WaitEvent, and instead to roll one's own from:

  - SDL_PeepEvents
  - a Slate-implemented Timer queue (heap)
  - a customisable "delay" function

The delay function is called when SDL-proper decides nothing has 
happened. The time-to-next-timer-event is retrieved from the timer 
queue, and passed in to the delay function. In simple cases, the delay 
function can be SDL_Delay, but to integrate with SDL_net, the delay 
function should be the piece of code that checks for socket activity, 
i.e. that calls select().

Tony