[GUI] Example for thought

[Axel Simon]

On Thu, Apr 03, 2003 at 08:53:54AM -0800, David Sankel wrote:
> --- David Sankel <camio@yahoo.com> wrote:
> > What would a haskell solution look like?  In my
> > opinion this functionality would be very desirable
> > if
> > not required.
> > 
> Here is one solution using normal MVars.  It is IMHO
> uglier than the c++ version.  I think creating an
> extention of MVars to allow callbacks on change and
> syncronized variables would be very nice with this
> example problem.  I would also suggest removing the
> empty MVar status.
Your solution might be dirty but it works. It might be cleaner if we had 
state variables which trigger a callback when they change (I guess this 
functionlity is close to Manuel's Port library). But in case we need to 
synchronize three entities we might run into problems with augmented state 
variables. But maybe it's always sufficient...

I wouldn't claim that features like linking state variables and having
callbacks on changing these is completely high level, I still think it is
beyond the *needs* of an initial CGA and, thus, shouldn't be implemented 
right now. Two observations:

a) It would be nice to have useful abstractions like linkable state
variables, state variables with callbacks on change, streams, filters,
etc.

b) At this early stage we can only guess how useful/restrictive these
features are in practice. If these additions have to be desinged in from
the start (i.e. are not orthogonal to the CGA), then we even risk making
high level APIs on top of CGA impossible.

I would like to start with something simple and verify in practice which
concepts in a) are useful. Otherwise we run into the risk of cluttering
the API with unused functionality. I don't mind breaking the CGA between 
version 1.0 and 2.0, but I don't want to go to version 2.0 right away 
since it takes much longer and it's harder to get it right.

Axel.