New Frontiers in Intensional Programming∗

Intensional Programming (IP) was born in 1986 at Arizona State—or at least that is when we came up with the name. We had actually been doing it for some time already. Tony Faustini and I had been discussing whether or not Lucid is really a “functional” programming language. This is quite an interesting topic, and I will have something to say about it a bit later. Suddenly, we asked ourselves why should we be begging to be let on board someone else’s bandwagon. Lucid really is different from SASL, Miranda et al. They’re extensional programming languages, and Lucid is an . . . “intensional” programming language. Intensional programming . . . what a concept! At first this coinage was almost tongue in cheek. But the more we thought about it, the more sense it made. There were already several people in the so-called “Lucid Community” working on systems which could hardly be called mere extensions of Lucid—for example, Chronolog, or Tao’s attribute grammar compiler compiler. What they all had in common, though, was that they were inspired by Intensional Logic (IL), the semantics and even the implementations were based on the IL notion of “possible world”, and finally intensional operators appeared in object-language expressions. This is fairly vague, of course, but clear enough to be the definition of a paradigm (after all, what exactly is OOP?). For that matter, we should not expect a paradigm to have too precise a definition—then it is a dead end as far as research is concerned. IP, like IL itself, is open-ended. IP is a relatively new paradigm but already it has produced some promising ideas, some of which are better than half-baked. In this paper we will try to give a survey of the various directions which are being followed, or should be followed. Some of these are familiar to you, others are more recent brainwaves which no-one has had the time (or been crazy enough) to pursue. I do not want to claim credit for all these ideas and in fact the better they are, the more people there are who contributed. I fear that some of what I say will be a (dare I say) rehash of ideas that appeared in the Lucid book or earlier. But many of these ideas are (like typical hash) a bit lumpy, and could use another trip through the grinder.