<figure><img src="image_1.png" /></figure>

HAL
open science

# Gul Agha, A Natural Born Actor

Jean-Pierre Briot

► To cite this version:

Jean-Pierre Briot. Gul Agha, A Natural Born Actor. 2025. hal-05193633

HAL Id: hal-05193633

https://hal.science/hal-05193633v1

Preprint submitted on 30 Jul 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

Public Domain
# Gul Agha, A Natural Born Actor

Jean-Pierre Briot

Sorbonne Université, CNRS, LIP6, F-75005 Paris, France
& PUC-Rio, Rio de Janeiro, RJ 22451-900, Brazil

Jean-Pierre.Briot@lip6.fr

## Abstract

This short text<fnref n="1" /> is a quick summary of the scientific impact of Gul Agha's contributions on my research, as well as some anecdotes about the wonderful human being that I am very happy to know.

## 1 Gul's Scientific Impact

In the early 80's, I started my journey into computer science during my PhD, working on the concepts of object-oriented programming. I also participated in applying them to the modeling and processing of musical processes, a research project headed by Pierre Cointe at IRCAM [CBS87], the research institute about music, acoustic and computing in Paris. This was a great experience, because object-oriented programming was burgeoning a lot, and also thanks to the opportunity to meet scientists and musicians from all over the world. There, I met with Henry Lieberman and started discovering the concept of actors, and also met with Akinori Yonezawa, respectively, collaborator and past PhD student of Carl Hewitt, the great visionary about actors and other great ideas. As time and concurrency of events are fundamental in musical processes, I got much interested into actors. In 1985, soon after defending my PhD, I was welcomed by Akinori Yonezawa for a postdoc in his lab in Tokyo, while he was starting to work on the design and implementation of an actor-based language named ABCL [YBS86]. This was another great scientific and human experience.

There, I discovered the thesis [Agh86] and first articles by Gul Agha. I was really impressed with the concision and clarity of his specification of actors. Notably, his concept of behavior replacement was a key contribution to clarify and unify various issues:

<footnote n="1">¹To appear as a chapter, in "José Meseguer, Carlos A. Varela, and Nalini Venkatasub-ramanian, editors, Concurrent Programming, Open Systems and Formal Methods: Essays Dedicated to Prof. Gul Agha to Celebrate his Scientific Career, Lecture Notes in Computer Science. Springer Verlag, 2025."</footnote>
* internal concurrency of the processing of incoming messages (as soon as the behavior replacement is defined),

* (as well as) their synchronization (until the behavior replacement is defined),

* (and furthermore) regaining pure functional properties.

This clean foundation of actor programming gave me the inspiration to implement and classify various models of actors and concurrent objects in the Smalltalk environment [Bri89, Bri96]. It also served as a foundation for implementing agents [GB99], as multi-agent systems were now burgeoning. Further works by Gul and his team, e.g., about coordination languages [FA93], meta-level programming [VA99], dynamic adaptation [VA01], fault-tolerance [SA94], etc., were also very influential for my own projects, e.g., about fault-tolerant multi-agent systems [MBS+07, GBFM10]. Gul and I met on various occasions, in USA, France and other locations. I had the chance to visit Gul's lab at UIUC for a few months. And it was a great resourcing experience.

## 2 Gul's Personality

Through our encounters, I discovered Gul's amazing personality. He is always calm, positive, opened, generous and has a nice and peculiar sense of humor. He is also very serious about veganism<fnref n="2" />, while being a bon vivant. We also have some mutual interests in topics such as culture, music and spirituality.

Some nice experience was after the PhD defense of Salima Hassas, in Lyon University in 1992. Some of the members of the defense committee, Gul, Les Gasser, Peter de Jong and me, made some gastronomy/oenology short tour in the close Beaujolais region. We had the experience of a marvelous lunch (and wine :) at the famous and Michelin starred Auberge du Cep, in Fleurie (a village in the Beaujolais region, as well as likely the most elegant Beaujolais appellation). Coming back to Lyon though the little roads was also unforgettable, the roads were purple, as harvesting had just happened, the air full of grape perfumes, and the winery buildings filled with the delicate noise of the grape juice ongoing fermentation. Hopefully (for us), Peter de Jong did not drink alcohol and thus happily took the responsibility for driving!

An unexpected encounter was in Rio de Janeiro during the Middleware 2003 conference. I had a paper accepted with Frédéric Peschanski [PBY03], who after a PhD with me was doing a post-doc in Tokyo in Akinori Yonezawa's lab. We both arrived in Rio de Janeiro, him from Tokyo, me from Paris. I tried to call him at his hotel. The lobby connected me to some room. I guess that I said something like "Salut Frédéric", expecting to have him on the phone. But the person who replied me was definitely not Frédéric. At the same time, I thought "I know this voice!". Moreover, I had the feeling that this person was

<footnote n="2">²This was a shock to me when I discovered the existence of a Meat Science Laboratory at UIUC.</footnote>
also reflecting about my own voice. Until, after a few seconds of silence, both
Gul and I simultaneously realized about us<fnref n="3" />!

When I asked how he was and when he arrived in Rio, Gul answered me that he was just coming back from a walk on Copacabana beach (where his hotel was located), and that his watch had just been stolen. He added that he just had bought a new watch from some informal seller on the beach, adding that it was likely that this watch had been recently stolen too! Anyway, this was the opportunity to have a great dinner in Rio for the three of us. And soon after, Gul welcomed Frédéric for a wonderful short research stay in his lab at UIUC.

3 Conclusion

It has been a great chance for me to know Gul's scientific work and also him
as a person. Congratulations Gul! Thank you for all your contributions, open-
ness and generosity! Thank you for inspiring so many colleagues and students!
Thank you for these great shared moments! All my best wishes for you and
your family! Jean-Pierre

References

[Agh86] Gul Agha. *Actors – A Model of Concurrent Computation in Distributed Systems*. Series in Artificial Intelligence. MIT Press, 1986. ISBN: 978-0-262-01092-4.

[Bri89] Jean-Pierre Briot. Actalk: a testbed for classifying and designing actor languages in the Smalltalk-80 environment. In Steve Cook, editor, *European Conference on Object-Oriented Programming (ECOOP'89)*, British Computer Society Workshop Series, pages 109–129, Nottingham, U.K., July 1989. Cambridge University Press. ISBN: 0-521-38232-7.

[Bri96] Jean-Pierre Briot. An experiment in classification and specialization of synchronization schemes. In Kokichi Futatsugi and Satoshi Matsuoka, editors, *Object Technologies for Advanced Software (ISO-TAS'96)*, number 1049 in LNCS, pages 227–249, Kanazawa, March 1996. Springer.

[CBS87] Pierre Cointe, Jean-Pierre Briot, and Bernard Serpette. The Formes language: A musical application of object-oriented concurrent programming. In Akinori Yonezawa and Mario Tokoro, editors, *Object-Oriented Concurrent Programming*, Computer System Series, pages 221–258. MIT Press, 1987. ISBN: 0-262-24026-2.

<footnote n="3">³It is worth mentioning that I had no a priori information about the venue of Gul to the conference (and I guess vice versa).</footnote>
[FA93] Svend Frølund and Gul Agha. A language framework for multi-object coordination. In Oscar M. Nierstrasz, editor, *ECOOP '93: Proceedings of the 7th European Conference on Object-Oriented Programming*, number 707 in LNCS, pages 346–360. Springer, 1993.

[GB99] Zahia Guessoum and Jean-Pierre Briot. From active objects to autonomous agents. *Concurrency*, 7(3):68–76, July–September 1999. Special Series on Actors and Agents, edited by Dennis Kafura and Jean-Pierre Briot.

[GBFM10] Zahia Guessoum, Jean-Pierre Briot, Nora Faci, and Olivier Marin. Towards reliable multi-agent systems - an adaptive replication mechanism. *Multiagent and Grid Systems (MAGS) – An International Journal*, 6(1):1–24, March 2010.

[MBS+07] Olivier Marin, Marin Bertier, Pierre Sens, Zahia Guessoum, and Jean-Pierre Briot. Darx – a self-healing framework for agents. In Fabrice Kordon and Janos Sztipanovits, editors, *Reliable Systems on Unreliable Networked Platforms*, number 4322 in LNCS, pages 88–105. Springer, 2007.

[PBY03] Frédéric Peschanski, Jean-Pierre Briot, and Akinori Yonezawa. Fine-grained dynamic adaptation of distributed components. In *Middleware 2003 - ACM/IFIP/USENIX International Middleware Conference, Rio de Janeiro, Brazil, June 16-20, 2003, Proceedings*, number 2672 in LNCS, pages 123–142. Springer, June 2003.

[SA94] Daniel C. Sturman and Gul Agha. A protocol description language for customizing failure semantics. In *Proceedings of the 13th International Symposium on Reliable Distributed Systems*, pages 148–157, Dana Point, CA, USA, 1994. IEEE.

[VA99] Carlos A. Varela and Gul Agha. A hierarchical model for coordination of concurrent activities. In Paolo Ciancarini and Alexander L. Wolf, editors, *Coordination Languages and Models*, number 1594 in LNCS, pages 166–182. Springer, 1999.

[VA01] Carlos A. Varela and Gul Agha. Programming dynamically reconfigurable open systems with Salsa. *SIGPLAN Notices*, 36(12):20–34, December 2001.

[YBS86] Akinori Yonezawa, Jean-Pierre Briot, and Etsuya Shibayama. Object-oriented concurrent programming in ABCL/1. In *Conference on Object-Oriented Programming Systems, Languages and Applications (OOPSLA '86). SIGPLAN Notices*, 21(11):258–268, November 1986. ISBN: 0-89791-204-7.