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AI Memo No. 453.

May 1978

The Art of the Interpreter
or, The Modularity Complex
(Parts Zero, One, and Two)


Guy Lewis Steele Jr.[*] and Gerald Jay Sussman[**]


We examine the effects of various language design decisions on the programming styles available to a user of the language, with particular emphasis on the ability to incrementally construct modular systems. At each step we exhibit an interactive meta-circular interpreter for the language under consideration. Each new interpreter is the result of an incremental change to a previous interpreter.

We explore the consequences of various variable binding disciplines and the introduction of side effects. We find that dynamic scoping is unsuitable for constructing procedural abstractions, but has another role as an agent of modularity, being a structured form of side effect. More general side effects are also found to be necessary to promote modular style. We find that the notion of side effect and the notion of equality (object identity) are mutually constraining; to define one is to define the other.

The interpreters we exhibit are all written in a simple dialect of LISP, and all implement LISP-like languages. A subset of these interpreters constitute a partial historical reconstruction of the actual evolution of LISP.

Keywords: abstraction, actors, applicative order, bindings, control structures, debugging, dynamic scoping, environments, fluid variables, FUNARG problem, functional objects, interactive programming, lambda-calculus, lexical scoping, LISP, modularity, procedural data, recursion equations, referential transparency, SCHEME, side effects, static scoping, structured programming

This report describes research done at the Artificial Intelligence Laboratory of the Massachusetts Institute of Technology. Support for the laboratory's artificial intelligence research is provided in part by the Advanced Research Projects Agency of the Department of Defense under Office of Naval Research contract N00014-75-C-0643.

^  NSF Fellow

∗∗^  Jolly Good Fellow

© Massachusetts Institute of Technology 1978