Re ective Building Blocks for Modular

The formal deenition of any namespace device found in a programming language can be given in terms of transformations on a semantic environment. It is therefore worthwhile to consider the implications of incorporating environments as bona de data objects in a programming system. Because of their expressive power, environments can be easily abused. Reifying an environment can entail the capture of unwanted bindings, leading to potentially severe violations of lexical abstraction and locality. Reeecting a data structure into an environment may cause useful program transformations which rely on static scoping (e.g.,-conversion) to be no longer applicable. Proposals that have heretofore been suggested for manipulating environments as data objects, however, provide no mechanism to constrain the eeect (or extent) of the reiication or reeection process. In this paper, we propose a treatment of environments and the mechanism by which they are reiied and manipulated that addresses these concerns. The language described below permits environments to be reiied into data objects, and objects to be reeected into environments, but gives users great exibility to constrain the extent and scope of these processes. We argue that the techniques and operators developed deene a cohesive basis for building large-scale modular systems using reeective programming techniques.