Software evolution is one of the most important features in advanced computer systems, and the significance of its theoretical study is acknowledged. Software evolution is divided into two categories: static and dynamic evolution. Static evolution involves changes in a software system that occur before compilation; a typical example is a software update. On the other hand, dynamic evolution involves changes in the execution time of a software system. The dynamic library mechanism in operating systems and dynamic class loading in Java are types of dynamic evolution. An environment represents a mapping of variables onto values. We have studied the lambda calculus with first-class environments (called the environment calculus). With this, we can treat environments as first-class citizens: environment values can be passed as parameters and returned as resultant values. The first-class environments are formalized according to the idea of explicit substitutions. This paper proposes programmable environments, as a further extension of first-class environments, which provide a computational mechanism allowing first-class environments to be treated as functions mapping variables onto their bound values. Conversely, such functions can also be treated as first-class environments. Programmable environments allow us to operate meta-level name spaces directly, and they enable us to model the dynamic evolution mechanism
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