Lightweight Languages

What happens if you get a bunch of academic computer scientists and implementors of languages such as Perl, Python, Smalltalk and Curl, and lock them into a room for a day? Bringing together the academic and commercial sides of language design and implementation was the interesting premise behind last weekend's Lightweight Languages Workshop, LL1, at the AI Lab at MIT, and I'm happy to say that it wasn't the great flame-fest you might imagine.

While there were the occasional jibes from all sides, (and mainly in our direction) it was a good-natured and enjoyable event. The one-day workshop began with a keynote by Olin Shivers, associate professor of computing at Georgia Tech, on how "Lambda" was the ultimate little language. He was, of course, teaching us about the Joys of Scheme. Olin showed us how we could use Scheme as a basis for implementing domain-specific languages, such as awk, and by embedding these languages in Scheme, one would have not just the power of the original language, but also the implementation language available as well. The alert reader will remember that this is something Larry spoke about in the State of the Onion this year with regard to Perl 6, and something that Damian Conway and Leon Brocard have been working on in Perl 5.

Olin demonstrated the power of his "embedding" technique by calling from Scheme to Awk and from Awk back into Scheme in the same routine. He's even written a Scheme shell along the same lines, but there was some disagreement as to whether or not being able to do shell-like things with Scheme syntax means you have a Scheme shell. Olin also brushed aside suggestions that writing Awk, Scheme and Shell in the same syntax in the same function might be potentially confusing, saying that it had never tripped him up.

Next came an enlightening panel discussion on the merits of the Worse-Is-Better philosophy; that is, the idea, that doing it right is not as important as doing it right now. I spoke first, explaining how Parrot development has to strike a balance between being correct, maintainable and fast, and ended by asking why doing the right thing is almost always the opposite of doing the fast thing; Jeremy Hylton, one of the Python developers and a really nice guy, replied that Python did not have this problem - for them, correctness and maintainability were more important than performance, giving them more flexibility in terms of doing it Right.

Dan Weinreb piped up with the idea that programming implementation tends to be driven by managerial paranoia; thankfully, this is something that both the open-source language implementation community and the academic community are pretty much immune from. Nevertheless, the pressure to release can have adverse affects on future performance, as illustrated by Guy Steele, one of the designers on Java, who told us a sad story about the dangers of Worse Is Better - by putting some vital component of Java (I think it was tail recursion, but it might have been something else) off in order to get release 1.0 out of the door, it made it much harder to implement later on. This resonated with the experience that Python implementors had adding lexical scoping. This rapidly caused a degeneration into whether a language without lexical scoping or the lambda operator can be said to be properly thought out.

Joe Marshall gave a short talk on one element of the initial implementation of the Rebol programming language. Rebol 1.0 was slightly different from the "network programming language" that Rebol is today - it was essentially Scheme with the brackets filed off. He explained that in his original Scheme implementation, he used a terribly complicated trick to avoid outgrowing the stack; something that was completely rewritten in the next implementation. Again we found that maintainability was more important in the long run.

Jeremy Hylton then gave his presentation, on the design and implementation of Python. It was amusing but, at least to me, not entirely surprising, that Jeremy was the one developer at the conference that Dan Sugalski and I had most in common with. The implementation teams of Perl and Python are facing very much the same problems and approaching them in only very slightly different ways. That is, compared with many of the other approaches we saw presented, at least.

Then it was our turn. We were running very late by this point, so I skipped over most of my talk; Dan explained what a lightweight language usually needed from its interpreter, and how Parrot was planning to provide that. David Simmons, one of the developers of SmallScript, a "scripting" Smalltalk implementation, challenged us on the viability of JIT compiling Parrot code, something we were interested in after looking at the Mono project's JIT compiler.

We had been intrigued all day by the title of Shriram Krishnamurthi's talk, "The Swine Before Perl." We didn't know whether that referred to us or to Jeremy, who was talking before us. In the event, Shriram delivered a wonderfully interesting presentation about Scheme, and how he could apply "laziness, impatience and hubris" to Scheme programming. He mentioned a Scheme Web server that served dynamic pages many times faster than Apache/mod_perl, and a way of manipulating Scheme macros to provide a pre-compiled state-machine generator. He countered criticisms that Scheme is just lots of insane silly parentheses by demonstrating how XML was just lots of insane silly angle brackets. A fair point well made.

Waldemar Horwat lead us through some of the more important design decisions involved in JavaScript version 2.0. No, don't laugh, it's a much more fully featured language than you'd think. His main point was, unsurpisingly, something else that Larry had picked up on in his State of the Onion talk; that modules need to provide ways of encoding the version of their API so as to protect their namespace from subclasses. That's to say, if there's a Perl module Foo and you subclass it to Foo::Advanced by adding a frobnitz method, then what happens when the original author of Foo produces the next version of Foo that already has a frobnitz method? Waldemar's solution was to use version-specific namespaces that only expose certain subsets of the whole API if a particular version number is asked for. This allows dependent modules to be future-proofed.

Christopher Barber gave a talk on Curl, a bizarre little language that fills the same sort of niche as Flash. David Simmons spoke about SmallScript and his work porting Smalltalk to the Microsoft CLR; he had high hopes for the way Microsoft .NET was panning out. Jonathan Bachrach, one of the AI Labs researchers gave a clever talk, which I missed due to being out in the corridors debating the finer points of threading and continuations with a Scheme hacker. I returned to see him explain a efficient but highly difficult to follow method of determining whether two classes are the same; he also said that his language compiles down to C and the C compiler is executed on the fly, since it's faster to call GCC than to start up your own virtual machine. Paul Graham rounded off the talks by talking about his new dialect of Lisp, which he called Arc. Arc is designed to be a language for "good programmers" only, and gets away without taking the shortcuts and safeguards that you need if you're trying to be accessible. His presentation was very entertaining, and I have high hopes for his little language.

As I've indicated, the interest of the workshop was as much what was going on outside the talks as well; Dan and I got to meet a load of interesting and clever people, and it was challenging for us to discuss our ideas with them - especially since we didn't always see eye to eye with our academic counterparts. Sadly, few people seemed to have heard much about Ruby, something they will probably come to regret in time. Dan seemed to have picked up a few more interesting technical tips, such as a way to collect reference count loops without walking all of the objects in a heap. Oh, and we found that you should pour liquid nitrogen into containers first rather than trying to make ice cream by directly pouring it into a mix of milk and butter. And that the ice-cream so produced is exceptionally tasty.

But seriously, what did we learn? I think we learned that many problems that we're facing in terms of Perl implementation right now have already been thoroughly researched and dealt with as many as 30 years ago; but we also learned that if we want to get at this research, then we need to do a lot of digging. The academic community is good at solving tricky problems like threading, continuations, despatch and the like, but not very interested in working out all the implications. To bring an academic success to commercial fruition requires one, as Olin Shivers puts it, "to become Larry Wall for a year" - to take care of all the gritty implementation details, and that's not the sort of thing that gets a PhD.

So the impetus is on us as serious language implementors to take the time to look into and understand the current state of the art in VM research to avoid re-inventing the wheel. Conferences such as LL1, and the mailing list that has been established as a result of it, are a useful way for us to find out what's going on and exchange experience with the academic community, and I look forward intently to the next one!

O'Reilly & Associates was proud to be a co-sponsor of the Lightweight Languages Workshop.

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