An extensible interpreter

• Authors: Stephen A. Schuman
• Venue: Proceedings of the international symposium on Extensible languages
• Year: 1971

The purpose of this brief paper is to propose an alternative approach for the implementation of an extensible language. The idea outlined here will be referred to as an extensible interpreter. In this particular case, the term itself constitutes an exceptionally concise description of the basic concept, even though there exists another, equally valid way of viewing the same notion, namely as a strategy for integrating an optimizing compiler into an essentially interpretive system. Regardless of which point of view is preferred, however, both are indicative of the fundamental assumption which underlies the present proposal: that the “semantics” of a programming language must ultimately be defined by means of an interpreter. This is to say that the definition of a given language must finally be formulated in terms of an “abstract machine” which is capable of carrying out, in an appropriate environment, the “unit operations” comprising the semantic primitives that language. Thus, an implementation which reflects the definition of the language in question must necessarily include a realization of this abstract machine, whether it is present implicitly, in the processing performed by the compiler, or whether it appears explicitly. In the latter case, the abstact machine may be represented in the form of a conventional software interpreter, or more directly, by means of a suitable set of micro-instructions, the only distinction being the number of different “layers” of interpretation which are actually interposed between this machine and the supporting hardware. In the context of an extensible language, one can immediately identify the natural instruction set for such an abstract machine: it corresponds to the repertoire of primitive operations provided by the base language. Since it is axiomatic to extensibility that new linguistic elements are constructed by systematic composition of previously defined and/or primitive constructs, the “meaning” of every operation in any subsequent extended language is ultimately defined by its derivation in terms of the operations belonging to the base language. Given that it is, conceptually, the definition of the language which is expected to evolve as a consequence of extension, there are strong motivations for explicitly representing the definition itself in any implementation of that language. The logical point of departure, therefore, is to implement an interpreter for the base language, thereby providing a direct realization of the underlying abstract machine.