Abstract

Abstract. The Scheme language report advocates language design as the composition of a small set of orthogonal constructs, instead of a large accumulation of features. In this paper, we demonstrate how such a design scales with the addition of a class system to Scheme. Specifically, the PLT Scheme class system is a collection of orthogonal linguistic constructs for creating classes in arbitrary lexical scopes and for manipulating them as first-class values. Due to the smooth integration of classes and the core language, programmers can express mixins and traits, two major recent innovations in the object-oriented world. The class system is implemented as a macro in terms of procedures and a record-type generator; the mixin and trait patterns, in turn, are naturally codified as macros over the class system. 1 Growing a Language The Revised 5 Report on the Scheme programming language [20] starts with the famous proclamation that “[p]rogramming languages should be designed not by piling feature on top of feature, but by removing the weaknesses and restrictions that make additional

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