Environments as first class objects

  • Authors:

  • D. Gelernter;S. Jagannathan;T. London

  • Affiliations:

  • Yale University;Massachusetts Institute of Technology;AT&T Bell Laboratories

  • Venue:
  • POPL '87 Proceedings of the 14th ACM SIGACT-SIGPLAN symposium on Principles of programming languages
  • Year:
  • 1987

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Abstract

We describe a programming language called Symmetric Lisp that treats environments as first-class objects. Symmetric Lisp allows programmers to write expressions that evaluate to environments, and to create and denote variables and constants of type environment as well. One consequence is that the roles filled in other languages by a variety of limited, special purpose environment forms like records, structures, closures, modules, classes and abstract data types are filled instead by a single versatile and powerful structure. In addition to being its fundamental structuring tool, environments also serve as the basic functional object in the language. Because the elements of an environment are evaluated in parallel, Symmetric Lisp is a parallel programming language; because they may be assembled dynamically as well as statically, Symmetric Lisp accommodates an unusually flexible and simple (parallel) interpreter as well as other history-sensitive applications requiring dynamic environments. We show that first-class environments bring about fundamental changes in a language's structure: conventional distinctions between declarations and expressions, data structures and program structures, passive modules and active processes disappear.