Software evolution through iterative prototyping

The process of developing and evolving complex software systems is intrinsically exploratory in nature. Some prototyping activity is therefore inevitable in every stage of that process. Our program development and evolution methodology is predicated upon this observation. In this methodology, a prototype software system is developed as an approximation to an envisioned target system by compromising along one or more of the following dimensions: system performance, system functionality, or user interface. However, the prototype is not the end-product of the pro- cess. Instead, we support iterative evolution of the prototype towards the envisioned system by gradually dealing with the three general areas of compromise. This paper describes the methodology of using this al- ternative lifecycle; to wit, the programming language concepts and related implementation technology that support practice of the suggested methodology. We summarize the lessons we have learned in building and using this technology over the last several years.