19.6 Encodings vs. Primitive Objects

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19.6 Encodings vs. Primitive Objects

We have seen two contrasting approaches to the semantics and typing of simple object-oriented languages. In Chapter 18, we used combinations of features from the simply typed lambda-calculus with records, references, and subtyping to encode objects, classes, and inheritance. In the present chapter, we gave a direct account of a simple language in which objects and classes are primitive mechanisms.

Each of these approaches has its uses. Studying object encodings lays bare the fundamental mechanisms of encapsulation and reuse and allows us to compare them to other mechanisms with related aims. These encodings also help in understanding the way objects are translated into yet lower-level languages by compilers, and in understanding the interactions between objects and other language features. Treating objects as primitive, on the other hand, allows us to talk about their operational semantics and typing behavior directly; this kind of presentation is a better tool for high-level language design and documentation.

Ultimately, of course, we would like to have both views-a high-level language including primitive features for objects, classes, etc., with its own typing rules and operational semantics, plus a translation from this language to some lower-level language with just records and functions (or, indeed, an even lower-level language with nothing but registers, pointers, and instruction sequences), and finally a proof that this translation is a correct implementation of the high-level language, in the sense that the translation preserves the evaluation and typing properties of the high-level language. Many variants of this exercise have been carried out-for FJ itself by League, Trifonov, and Shao (2001), and for other object-oriented core calculi by Hofmann and Pierce (1995b), Bruce (1994, 2002), Abadi, Cardelli, and Viswanathan (1996), and others.

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