Re: Some questions about the Esterel primer...

Hi everyone,

I have quite some familiarity with the synchronous approach, so i can
try to contribute on these questions:


Herman Bruyninckx wrote:
> - p.14 ``As of now, there is no completely satisfactory way of
>   unifying the data- flow styles of Lustre and Signal and the control
...
>   Where can I find more information about these differences?

data-flow (i.e., block-diagrams, equations) and control (i.e.,
imperative sequencing, FSM) are quite different structures for
programming constructs, and work quite differently: so mixing them
involves choices for which there is no obvious criteria. An interesting
proposal, accompanied by an interesting insight on that question, is the
work on Mode Automata by Florence Maraninchi e.a.
(http://www-verimag.imag.fr/~maraninx/MATOU/index.html)
They and I have a paper in CDC01 about these issues.

> - ``Reactive systems, also called re ex systems, continuously react to
...
>   I somewhere found a description of Statecharts as ``a modelling
...
>   _and_ controllers in one integrated modelling language)?

Reactive systems are mainly about Discrete Event systems; they can
describe "continuous" dynamics once it is discretized as an endless loop
sampled implementation. A data-flow reactive language can then handle
these control equations as calls to elaborate numerical computation
functions (themselves written in another langage, e.g. C, for
efficiency).
StateCharts is a hierarchical-parallel-FSM language, a bit different
from the synchronous languages (cousin rather than brother/sister), so
it handles sequential control well. One of its dialects is StateFlow in
Mathlab/Simulink.


> - p.25 ``Valued Signals Versus Variables''
>   I learn from this Section that it is not a good idea to make events into
>   objects that can carry data: if two processes generate the same event
>   but with different data in the ``event object'', this will sooner or
>   later lead to indeterministic behaviour because different processes
>   will handle the events in different order. Is this a correct conclusion?

Actually, it will not be non-deterministic: if you have two source
producing values for the same variable or signal, you will have to
define how they should be combined in order for the global result to be
coherent, which seems a good thing.

> Can you explain in what sense it can harm? Can you share some of your
> experience and thoughts on this Maestro approach?

IMO, Maestro is meant for end-users not willing to go into detaild event
treatment in Esterel; for designers, Esterel is a bit more complex and
technical, but then it offers the right techniques to handles precisely
those details.

> Indeed. And my impression is that the efforts to extent current UML
> with real-time features boils down to re-inventing FSM/Petri-nets; do

I think UML is more fashionable, but less mature, concerning solid
technical results for real-time.
Reactive languages have precise design, concrete compilers, and analysis
tools available.

> maybe there happens to be a similar event (organised at your
> lab or somewhere else) in the not so far future

We would be happy to present Orccad at an Orocos meeting, and interested
people are certainly welcome to come and visit our robotics lab where it
is used.


best regards,


Eric


-- Eric Rutten                  INRIA Rhône-Alpes                projet
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