Intensionality

The intensional programming language, Lucid, described in Chapter 1 is based directly on intensional logic, a family of mathematical formal systems that permit expressions whose value depends on hidden contexts or indices. Our use of intensional logic is one in which the hidden contexts or indices are integers or tuples of integers. Intensional logic, as used to give semantics to natural language, uses a much more general notion of context or index. Of course, intensional logic is hardly the first example of a formal system of interest to both logicians and computer scientists. The language LISP (invented by McCarthy and others in the early sixties [34]) was originally intended to be an adaptation of the lambda calculus, although it diverged in its treatment of variable-binding and higher-order functions. Shortly after, however, Landin produced ISWIM, the first true functional language [30]. These “logical” programming languages such as ISWIM are in many respects vastly superior to the more conventional ones. They are much simpler and better defined and yet at the same time more regular and more powerful. These languages are notationally closer to ordinary mathematics and are much more problem-oriented. Finally, programs are still expressions in a formal system, and are still subject to the rules of the formal system. It is therefore much easier to reason formally about their correctness, or to apply meaningpreserving transformations. With these languages, programming really is a respectable branch of applied mathematical logic. These logic-based (or declarative) languages at first proved difficult to implement efficiently, and interest in declarative languages declined soon after the promising initial work of McCarthy and Landin. Fortunately, the advent of large scale integration and new compiling technology reawakened interest in declarative languages, and brought about a series of new “second generation” declarative languages, such as Prolog [12] and Miranda [44]. Lucid itself was one of these second generation declarative languages. Lucid is based not so much on classical logical systems as on the possible worlds approach to intensional logic—itself a relatively new branch of logic [43] which reached maturity during the period (1965-75) in which declarative programming languages were in eclipse.