Archive for September, 2009

SequenceL : Declarative Language Design

Wednesday, September 16th, 2009

I faithfully scan the tables of contents for IEEE Computer every time it comes out, and every now and then there’s a paper that I find both interesting and well written.  Today I found one called: “Taking Parnas’s Principles to the Next Level: Declarative Language Design” by Daniel E. Cooke and J. Nelson Rushton, at Texas Tech University.  I’d never heard of Parnas, but the paper discusses a programming language called SequenceL, which addresses some of the issue I have with programming language design.  Specifically, both functional and procedural languages make it necessary to “wire down” an implementation too much, and don’t allow enough leeway for an intelligent compiler or an intelligent library to offer a generic software service.  The language discussed in the paper revolves around an increased reliance on declarative syntax to specify a richer set of functionality. You might also describe the approach as heavily polymorphic, but I don’t think that description really captures the elegance apparent in the examples.

The emphasis in the paper seems to be on the ability to create an algorithm implementation that is very close to a natural language specification of the requirements.  In fields where that’s an issue (space shuttle software, in the article) that seems like a good measure of success. But I think the increased level of data hiding and abstraction also lends itself to supporting much more freedom in the compiler, libraries and runtime system to choose implementation details that are best suited to executing the program on particular hardware or in a particular operating environment. Unsurprisingly, when I googled for SequenceL I also found papers with names like: “Automatic parallel control structures in SequenceL” and “Automatic Concurrency in SequenceL”.

Anyway, it was a very stimulating paper. Unfortunately, it’s a restricted download on the IEEE site. If you’re a Sun employee, you can access the IEEE site without a special login, the instructions are available on the Sun internal network at the Sun Learning Services web site. I assume you can also purchase it online from the IEEE site, but I didn’t verify that.

Update: Very interesting.  Two days after I read this paper, I scanned through some slides being presented at Sun by Kunle Stanford Pervasive Parallelism Laboratory.  He discussed the future of highly parallel app development, and his vision includes more emphasis on Domain Specific Languages, using Scala as a base platform for the DSLs.  More support for the idea that current programming languages require you to put too much implementation knowledge into the application level. You can find a set of slides very similar to what he presented on the Scala web site.