practice

Doi: 10.1145/1498765.1498781

The ecosystem of purpose-built languages
is a key part of systems development.
BY miKe ShAPiRo
Purpose-Built
Languages

In my college computer science lab, two eternal debates flourished during breaks from long nights of coding and debugging: “emacs versus vi?” and “what is the best programming language?” Later, as I began my career in industry, I noticed that the debate over programming languages was also going on in the hallways of Silicon Valley campuses. It was the 1990s, and at Sun Microsystems many of us were watching Java claim significant mindshare among developers, particularly those previously developing in C or C++.

I have always found the notion of best language to be too subjective and too dependent on the nature of the programming task at hand. Over my career, however, I have spent significant time pondering two related questions that I think are more fundamental. First, is software engineering at large being done on fewer languages over time? That is, is the set of computer languages converging Second, what makes a particular language “better” or useful or more rapidly adopted for a particular task?

In examining these questions I have found it
particularly interesting to look not at the battle of
the heavyweights, but rather at their less well-studied
offshoots, the purpose-built languages. These languages

36 communicAtionS of the Acm | APriL 2009 | voL. 52 | no. 4

sprout like weeds alongside the road of mainstream language development, and they exhibit properties and a history that lead one to reconsider instinctive answers to the fundamental language questions. Considering purpose-built languages, programming language development is not converging at all, and utility seems to have little to do with traditional notions of structure or properties that are empirically “better” from a lan-guage-design perspective. Purpose-built languages even defy a strict definition worthy of a prescriptive compiler grammarian: they somehow seem “smaller” than a full-fledged programming language; they are not always Turing-complete; they can lack formal grammars (and parsers); they are sometimes stand-alone but often a part of a more complex environment or containing program; they are often but not always interpreted; they are typically designed for a single purpose but often (accidentally) jump from one type of use to another. And some are even nameless.

Most significantly, purpose-built languages have often formed an essential part of the development of larger software systems such as operating systems, whether as a part of developer tools or as glue between distinct pieces of a larger environment. So it is particularly interesting to unearth some of these lesser-known creations and look at their connections to our larger language insights. In my career, while working on several commercial operating systems and large software components, I have come to conclude that not only are new languages developing all the time, but they are also often integral to the growth and maintenance of larger-scale software systems.

The Unix environment, with its philosophy of little tools that can be easily connected, was an ideal greenhouse for the growth of purpose-built languages. A cursory scan of Unix manuals from the early 1980s shows more than 20 little languages of vari-

illustration by john hersey