Scottish Programming Languages Seminar

I went to the Scottish Programming Languages Seminar the other week. I wasn’t very sure beforehand if a) I’d be welcome, as a non-academic or b) if it’d all be way above my head. An email to one of the organizer revealed that I would be more than welcome to attend, and looking at the programme revealed that I could at least understand the title of all the talks.

It turned out to be an enjoyable day. These get-together seminars are evidentally the oil which keeps the academic world moving. They are sociable occasions for those who know other attendees, and lots of information and tidbits flow around between participants. I made a pretty good go of striking up conversations with strangers – made easier by the fact that a) they were all academics, and b) they were attending a programming languages seminar, which makes for easy conversation openers. Some people were surprised (but happy) that someone who works in industry would want to come along, and it was fun to listen to the talks and discussions from my (possibly) more pragmatic and less theory-driven point of view.

Richard Connor proposed trying to design a somewhat rigourous experiment to see whether there is a productivity difference between static and dynamic languages. While I approve of the broad idea, I think that in making the experiment practical (constraining everything except what you are measuring) you’d throw the baby out with the bathwater. When you use a dynamic language, it’s not because you have a masochistic enjoyment of finding statically-findable bugs by hand. It’s because you enjoy a much more flexible overall programming experience – different toolset, and better support for “exploratory programming” as you learn about the problem domain. So, by merely contrasting language variants without following through on the implications of those language choices (ie. if you had everyone using notepad for source editing, as opposed to eclipse for the static-typed case and squeak for the dynamic-typed case) you end up measuring something which isn’t very interesting. But, having said that, Richard made the point several times that if you don’t simplify things down you can’t measure anything useful. Then again, perhaps you need to go meta and ask exactly what is being measured, and whether it is possible or worthwhile to measure it.

Anne Benoit’s talk was about improving the performance of parallel programs running on a distributed hetreogenous network. It would appear that people write these programs in two levels. The lower level are (conceptually sequential) blackboxes of functionality, such as a blur filter in a video-processing application. The higher level is a declarative description of how these boxes should be combined – eg. in a pipeline, or as fan-out or fan-in patterns (almost like Design Patterns?). To deploy your applications, you map your high-level design onto the physical machines which are available to you. You could have all your blackboxes on one physical machine, communicating via shared memory. Or you could have each blackbox on a different physical machine, communicating over the network. A good mapping can be calculated programatically, but it’s not necessarily a one-shot operation. If your application is running on a shared cluster, machine load and node-to-node communication latency might vary throughout the run. At some point, it may be beneficial to reconfigure your application. For example, if latency is rising, it may be worthwhile moving your application to fewer CPUs, even though that would decrease the number of cycles available to you. So, by using frameworks like the Network Weather System you can monitor your application and make these kinds of decisions.

Conor McBride (Mr Dependent Types) talked about abstracting out a pattern which he’d found himself writing several times in source code. Monads are a good starting point here. Monads are the distillation of a commonly occuring pattern whereby the results of computations are combined by some fixed policy. If you realise that some part of your program fits this mould, then you start getting lots of nice stuff for free. Think of them as design patterns with formal algebraic properties. If a mathematician wakes up one day and realises “hey, the Integers (under addition) are an abelian group” then he can get out his list of “properties which all abelian groups have” and get all that for free. Anyhow, Conor’s pattern (an Idiom) isn’t quite a monad – it’s slightly weaker. Which, on one hand, means that there is less pretty structure to get for free. But on the other hand, it means that the entry requirements are easier to meet and so at least you get some pretty structure for free.

The formal side of this talk was a little bit beyond me. I have to concentrate when people discuss Monads formally. So when the talk moved on to contrast Monads, Idioms and Arrows (an even more general form) it was a level of abstraction beyond what I can cope with just now. Still, I wasn’t completely lost and it’s motivated me to learn instead of scaring me off.

I’ll skip the feature-oriented programming talk. It was interesting enough, but it wasn’t an area particularly close to my heart.

The final talk by Sven-Bodo Sholz was very good. The purpose of static type systems is to identify “good” and “bad” programs, for some definition of good/bad. Good programs can be typed, whereas bad programs fail to typecheck. That’s the ideal anyway. In practise, there are always a few good programs which are fail to type. There are also a few bad programs which pass the type checker (divide by zero, out-of-bounds array access) and require runtime checks. Innovations in type systems minimize that number of miscategorized programs, but types systems rapidly become undecidable when you go too far. Sven-Bodo works on an array-based language and for him the problem of catching out-of-bounds array accessing is much more pressing than for your average programmer – he deals with arrays all the time, of varying shape and length. The proposed solution is an interesting hybrid of static and dynamic typechecks. If the compiler can infer enough information to construct a type like “int array with 3 rows and 4 columns” it will do so. If it can only manage “int array with unknown shape” it’ll make do with that. At the end of the day, the compiler will statically check everything it can, and leave the remaining checks until runtime. So you have this interesting situation whereby a compiler doesn’t just say “yes, it typechecked” or “no, it didn’t typecheck”. Instead, the compiler acts more like a student who’s been told to do some proofs as homework. It says “well, I managed to prove all these properties in the first half perfectly, but I could only prove some vague and less concrete properties in the second half”. And then you can either go back and tweak your source code to provide more information, or you can go ahead and run your program, knowing that some properties have been checked statically (ie. they are true for all possible runs of the program) and some will be checked dynamically (ie. the program will terminate if a properties is discovered to be false). What’s interesting is the flexibility for static types to be more concrete or more vague, and the relationship between these (“int array of unknown shape” is a supertype of “int array with three rows and two columns”), and how to deal with partial information.

Anyhow, I’m glad I went along. I’m interested in this stuff, but I have very few people to discuss it with (since they mostly live in academia). It was nice to know that I wasn’t totally out of my depth too.