Marcel Weiher wrote:
> >I am still not quite sure for what specific problems large numbers
> >of processes are a good solution.  Computing propagation patterns
> >on-the-fly and interleaved with the computation carried out by the
> >message payload may be one case, but many/most practical cases seem
> >to be solvable without the need for concurrency.
>
And Alan Kay responded:

> It's not so much about concurrency as about environments. And it's
> not so much about solution as realization.
>

Many things can be programmed without concurrency, just like all things can
be programmed without objects. Concurrency is not just a way to deal with
networked or parallel computers but is also a way of thinking about
computation. When I show young children how to build a Pong-like game in
ToonTalk it seems so natural to think of the ball, paddle, and score keeper
as concurrent processes. I think it is even natural to have internal
concurrency in those concurrent objects. E.g. the ball has a bouncing
behavior that is running at the same time as a behavior that makes a sound
when it touches anything.

And I am talking about true concurrency not some approximation. So the game
can have a second paddle that is running on another computer as a way to get
the game to work over a network.

I'm one of those radicals that prefer programming languages that only
support concurrent computations. Sequentiality isn't hard to program in a
concurrent language while concurrency can only be simulated in a sequential
language. And I prefer simplicity to having a hybrid language that
primitively supports both.

The main justification for object-oriented programming is that it gives
programmers a way to think about computation that has some
cognitive/psychological advantages. Those advantages derive from the fact
that we nearly always think about the world around us as objects. (There are
exceptions like clouds, swarms, and waves.)  Similarly the world around us
is concurrent. When I look out my window I see trees swaying the wind. I
know the trees are growing and doing things like photosynthesis. I hear a
dog barking, birds flying, and cars driving past. And I know that there is
lots of internal concurrency in the car: the wheels turning, the various
processes inside the engine, etc.

Where does call/return fit into this picture?

Best,

-ken kahn ( www.toontalk.com )