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Dave Thomas: The legacy of Simula 67

Early computers were designed to solve difficult numerical problems; to the extent that models entered the picture, they were limited to mathematical equations describing those problems. The difficulty of programming algorithms in machine code quickly led to the development of scientific and business programming languages. Languages required that machines work with symbols, however, the models remained mathematical or sort/merge/match/update (CRUD). Early AI researchers created symbolic list processing languages in to model knowledge representation and reasoning. These languages were then used for symbolic mathematics -- again to support mathematical models. But then a profound change occurred. Increased processing power enabled the use of computers to simulate real world systems and processes. This led in turn to a family of languages for building simulations. Simula 67 took a unique anthropomorphic viewpoint “to compute is to model, to model is to compute”. The model was placed front and center as the essence of the computation. Simula expressed models as a society of communicating “actors” interacting with classes and their instances. Objects could be defined and specialized via refinement – known as Object-Orientation in Europe. Dahl and Nygaard put great thought into how a language should describe both work flow and social interaction, in addition to the computation, enabling models to be directly expressible for the first time. In this talk we describe the industrial impact of the “Scandinavian school” perspective on models and computation, so elegantly articulated in Simula and Beta. OO Languages which followed have dominated the computing landscape for almost 4 decades. Analysis and Design has also been profoundly influenced by object modeling - OOAD, OMT, RDD, ORM, Design Patterns and UML/SysML all trace their roots to Simula. We discuss the impact of model as language and object-orientation as they have been applied in industry, both in terms of lessons learned and more importantly of lessons still to be learned.