Lisp Hackers: Daniel Barlow

Daniel Barlow was one of the most active contributors to the open source Lisp ecosystem, when its development took off in the early 2000s. Together with Christophe Rhodes he was the first to join SBCL hacking, after the project was started by William Newman. He also had created a lot of early Lisp web tools, like Araneida HTTP application server, and built on it the first version of cliki.net, which served the Lisp community for almost 10 years (the second version went live earlier in 2012). Studying Cliki source was a kind of zen experience for me, as it did so much, yet in a very simple way.


But his largest contribution is, probably, ASDF, regarding which, likewise Cliki, there are controversial opinions among Lisp programmers. And Dan explains his attitude in the interview.

In the mid 2000s his involvement with open-source Lisp gradually diminished, as he stopped working as a consultant and got a full-time job. Yet he remains fondly remembered in the community.

Tell us something interesting about yourself.

I don't do interesting. Um, improvise. Hacker, skater, cyclist, husband, father to a seven-month-old son as demanding as he is adorable, computing retro-grouch who uses Linux on the desktop.

I have a metal plate and some pins in my right forearm where I broke it a couple of months ago, inline skating in the Le Mans 24 hour relay event. I have now regained more or less complete range of motion in that hand and can advise anyone doing the event next year that the carpet in the pit boxes is unexpectedly treacherous when it's been waterlogged by a sudden thunderstorm.

Still, we came fifth in category, which makes me very happy.

What's your job? Tell us about your company.

I started a new job about three months ago, in fact. I'm now working at Simply Business in London, busily disrupting the business insurance market. Which is to say, writing web apps for the online sale of business insurance.

It's not quite as buzzwordy as it sounds, actually. Business insurance is traditionally sold by brokers, who are humans and therefore although really good at dealing with complex cases and large contracts where the personal touch is required, tend to be a trifle expensive for straightforward policies which could be much more economically sold online. The industy is ripe for disintermediation.

What brought you to Lisp?

A combination of factors around the time I was at university: the UNIX-Haters Handbook, which I bought to sneer at and ended up agreeing with; Caml Light, which I used in my fourth year project; Perl - specifically my horror to learn that it flattens (1,2,3,(4,5,6)) to (1,2,3,4,5,6) - yes, I know about references, but I think it's a bug not a feature that the sensible syntax is reserved for the silly behaviour - and meeting some people from Harlequin (as was) at a careers fair.

It took me another couple of years or so to find CMUCL - in fact, I think it was another couple of years or so before it was ported to the x86 architecture, so it wouldn't have done me much good if I had known about it earlier - but looking back I suppose that was where the rot set in.

Do you use Lisp at work? If yes, how you've made it happen? If not, why?

No, we're primarily a Ruby shop, with a sideline in large legacy Java app which we're working to replace. I think there've been a couple of uses of Clojure in hack days, but that's as far as it goes.

Why not? Well, apart from the point that I've only been there since July ... It's The Ecosystem, I suppose. Ruby as a language has pretty good support for OO paradigms and a whole bunch of free software libraries for doing web-related things: Ruby as a community is big on Agile and TDD and maintainable design. And it's at least possible (if not exactly easy in the current bubble) to engage a regular recruitment agent and task him with finding competent programmers who know it. I'm not saying Lisp is exactly bad at any of that, but it's at least questionable whether it's as good along all of those axes, and it's certainly not better enough to make the switch sensible.

What's the most exciting use of Lisp you had?

SBCL was probably one of the most fun projects I've ever worked on. Working with people who were mostly smarter than me or had better taste than me or both, on a project whose goal was to make it possible for people less smart than me to hack on a Lisp system. And context switching between the CL with all its high level features and (e.g.) Alpha assembly was a real kick - it's a bit like I imagine building a Lisp machine would be, except that the goal is achievable and the result is generally useful.

What you dislike the most about Lisp?

I don't really use it enough any more to react to that with the required levels of venom. I should probably say ASDF, everyone else does :-)

I guess if you force me to an answer, it'd have to be its disdain for the platform it lives on - take, for example, CL pathname case conversion rules. Whoever decided that Unix systems could reasonably be said to have a "customary case" had, in my view, not looked very hard at it.

As far as I can tell, you're currently mostly doing work in Ruby. What's the pros and cons of Ruby development compared to Lisp?

The transpose-sexps function in Emacs does nothing useful in Ruby mode. rails console is a poor substitute for a proper toplevel. Backtraces don't show the values of parameters and local variables. (Yes, pry helps a lot). And the garbage collector (in MRI, anyway) is sucky to the point that even 1990s Java GC could probably beat it in a fair fight.

On the other hand, libraries.

Here's an interesting thought experiment, though: there's a clear difference between the Lisp workflow where you change the state of your image interactively to get the code into working shape very quickly (and then later try to remember what it was you did) and the more scripted approach of test-driven development in Ruby where you put everything (code, test setup, assertions) in files that you reload from disk on each run. How would you meld the two to get repeatable and fast iterations? A lot of people are doing things like Spork (which forks your application for each test it runs, throwing the child state away after the test has run) but they never seem to me to be more than 80% solutions. My intuition is that you'd want to stick to a much more functional design and just make state a non-problem.

Among software projects you've participated in what's your favorite?

SBCL was a lot of fun, as I said earlier. ASDF is a candidate too, just because it must so obviously fill a need if people are still cursing it - as they seem to be - ten years later :-)

Describe your workflow, give some productivity tips to fellow programmers.

It's taken me the best part of three months to get this interview back to Vsevolod, I'm the last person anyone should be asking about workflow or productivity. :-)

Um. I've been doing a lot of TDD lately. Given that as recently as two years ago I was castigating it as a religion this might be seen as a capitulation or as a conversion, this might be perceived as a change of mind. What can I say? Actually, pretty much now what I said then: the value of TDD is in the forces it exerts on your design — towards modularity, functional purity, decoupling, all those good things - not so much in the actual test suite you end up with. Process not product. These days everyone thinks that's obvious, but back then it was either less widely known or less explicitly stated or else I was just reading the wrong blogs.

(Of course, the tendency in Lisp to write code interactively that can be tested ad hoc at the repl probably has a very similar effect on coupling and functional style. My personal experience is that TDD doesn't seem to be nearly as valuable in repl-oriented languages, but YMMV)

More generally: go home, do some exercise, get some sleep. Sleep is way underrated.

Anything else I forgot to ask?

Some day I will write an apology for ASDF.

Pedants will note that the word "apology" not only means "an expression of remorse or regret" but also "a formal justification or defence", and may infer from that and my general unwillingness to ever admit I was wrong that I'm not about to actually say I did a bad thing in writing it. Seriously, go find a copy of MK-DEFSYSTEM and try porting it to a Lisp implementation it doesn't support.

In 2002 I presented a paper at the ILC (about CLiki, not ASDF) that said essentially "worse is better than nothing", and - unless the "worse" has the effect of stifling a potential better solution from coming along later - I still stand by that

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