The Agora Programming Language

Motivation

Agora is a reflective prototype-based object-oriented programming language that is entirely (and solely!) based on message passing. Message passing is considered as one of the fundamental characteristics of object-oriented programming languages. It lies at the heart of late-binding polymorphism which is the key feature that makes incremental modification of programs possible. However, prototype-based languages often go beyond message passing. Deep down in their implementation, many things such as delegation, encapsulation, cloning and object concatenation can be found as explicit operations on objects. By making such languages reflective, these implementation operators also become visible to the programmer. As such, the programmer can easily bypass the message passing paradigm just by 'going meta'. Also, very often, these operators are an explicit feature in the language, even when reflection is not a language feature. We claim that these explicit operators on objects is the source of encapsulation problems, over-flexibility and the reason why it is so difficult to make abstractions in (reflective) prototype-based languages. We therefore designed Agora as a prototype-based object-oriented programming language whose only operation on objects is message passing. Agora shows that even within these limits, it is possible to build a full-fledged object-oriented language that features inheritance, cloning and reflective operators. The idea is that an object is fully encapsulated and can only be subject to message passing. But seen from the inside of the object, the object knows all about it's own structures. It is therefore perfectly capable of cloning and extending itself. This is accomplished by special methods we call cloning methods and mixin-methods.

Ongoing work

Currently, we are finalizing Agora98, an implementation of Agora in Java. Agora98 allows full access to all Java API's and allows applets to be created from within Agora98. From the language point of view, Agora98 is a considerable simplification of previous versions of Agora.

Papers

Agora98: Reflective Programming in a Web Browser Submitted to ECOOP98
Agora: The Story of the Simplest MOP in the world - or - The Scheme of Object-Orientation (Working paper to be published in a new book on prototypes),
A Marriage of Class- and Object-Based Inheritance Without Unwanted Children. In Proceedings of ECOOP'95, Springer-Verlag 1995. abstract+ftp,
Agora: Message Passing as a Foundation for Exploring OO Language Concepts.SIGPLAN Notices, 29(12):48-57, December 1994. abstract+ftp
Modular Inheritance of Objects Through Mixin-Methods. JMLC'94 Proceedings. abstract+ftp
Nested Mixin-methods in Agora. In Proceedings of ECOOP93, Springer-Verlag, 1993 abstract+ftp

Artifacts

The main artifacts of the Agora project are:

A Smalltalk implementation of Agora94, written by Dr. Patrick Steyaert, Wim Codenie and Koen De Hondt.
A C++ implementationof Agora96, written by Wolfgang De Meuter.
A Java implementationof Agora98, written by Wolfgang De Meuter.
A Scheme implementationof Agora-S, written by Prof. Dr. Theo D'Hondt
A few implementations of Minimix (MiniMix is the theorical variant of non-reflective Agora), written by Wolfgang De Meuter.

Conferences, Workshops, etc.

Presentations at Conferences

ECOOP'93 Conference, Kaiserslautern, Germany:
Nested Mixin-methods in Agora
by Patrick Steyaert. (see papers)
JMLC'94 Conference, Ulm, Germany:
Modular Inheritance of Objects Through Mixin-methods
by Carine Lucas (see papers)
ECOOP'95 Conference Aarhus, Denmark:
A Marriage of Class- and Object-based Inheritance Without Unwanted Children
by Patrick Steyaert. (see papers)