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We examined 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 exhibited an interactive meta-circular interpreter for the language under consideration. Each new interpreter was the result of an incremental change to the previous interpreter.

We started with a simple interpreter for LISP recursion equations. In order to capture certain abstractions we were forced to introduce procedural data. This in turn forced consideration of the meanings of free variables in a procedure, for the simplest extension unexpectedly introduced dynamic scoping of variables.

We were compelled to turn from dynamic scoping to lexical scoping to preserve the integrity of procedural abstractions. The referentially transparent language thus obtained is richer than expected. It allows the definition of procedures which construct other procedures by instantiation of a prototype. Unfortunately, we found that complete referential transparency in a language makes it impossible to construct an interactive interface to the interpreter. But such an interface is necessary to satisfy another requirement of modular construction: that parts of a program can be independently defined, replaced, and debugged. We were forced to give up absolute referential transparency to admit an interactive interface.

The problems of the interactive interface led us to consider the notion of state as a dimension of abstraction. Just as we didn't want to have textually monolithic programs, we wanted to avoid programs which manipulate a monolithic representation of the state. The decomposition of the state of a system into several independent parts induces the notion of a side effect. Side effects only make sense relative to a definition of equality on the space of data objects. But the definition of equality itself depends simultaneously on the notion of side effect. Only a few of the choices of equality predicate and side effect notion are consistent with the requirements of modular construction.

The introduction of side effects is inconsistent with referential transparency. But since both are important to support modular construction we must accept an engineering trade-off between them. We were led to look for controlled patterns of side effects which can be easily understood and safely applied. We discovered that one such pattern is equivalent to the use of dynamically scoped variables we discussed earlier. We investigated how to construct a system which integrates lexical and dynamic scoping in a smooth way.

There are many issues yet to be explored. The introduction of side effects raises questions about order of evaluation. An interesting order provided by Algol 60 is call-by-name. This discipline, so unlike LISP's, is induced from a different notion of procedure, expressed as the "copy rule". This idea is a syntactic one, and so differs in flavor from the