24 July 2012

So I write compilers for a living now.

I’m a regular visitor of Stack Overflow, and have a Stack Overflow Careers profile. A month ago, I saw a job listing for Spaceport.ioa YouWeb-funded company that makes a tool for converting Flash applications to run natively on several different platforms. A key component of their technology is a compiler, written in Haskell, from ActionScript 3 to the Spaceport runtime.

Since I enjoy working in Haskell, and compilers and games are two of my favourite things, I readily applied. After phone and Skype interviews, I flew out to California for a day of in-person interviews. Much as I like flying, it’s no fun to spend more time in the air than at your destination. Lesson learned.

Soon afterward I received an offer, accepted, and in a few short weeks found myself moving across the country from southern New Hampshire to the Bay Area. It’s been a great adventure so far, with plenty of good people, good food, and endless opportunities to practice my Chinese. :)

Stack Overflow Careers

When it launched, I believed Careers would soon become the go-to place to hire developers. Unfortunately, none of the job offers I actually received were the right thing for me. Most were a mediocre, uninteresting kind of bad, not the hilariously terribad stuff of great blog posts.

But it was well worth the wait. I got a job on the basis of reputation and merit, not just a résumé. And now I can happily say I make my living off compilers, which is what I want to be doing for the foreseeable future. The coolest part is that I can apply much of the theory I learned while doing language research in college—to solve practical problems.

From the company’s side, Careers is a win as well. You get many good developers looking at your job listing, for a downright minuscule fraction of the financial and time cost of a recruiter. When you get an application from a developer, all of the information you could possibly want to know about their skills and history is right there in their public profile.

So, fellow devs, if I’m ever in a position to hire you, it will probably be through Stack Overflow Careers.

Professional Haskell

Since I started using it professionally, my opinions on Haskell have definitely evolved. It’s a well designed language, to be sure, but till coming to Spaceport I hadn’t really had the opportunity to explore a large project. Here are some of the things I’ve learned while using Haskell in production.

Haskell is Scalable

I was skeptical that the language could scale, not only to a large project but to a complex one. It’s not that I was doubtful, exactly. It’s just that languages that perform well for small code don’t necessarily work well for big code, and vice versa. You can’t know unless you test.

After working with a roughly 10,000-line codebase for several weeks, I’m now convinced that Haskell is excellent for coding in the large. With good unit tests in place—we have about four times as much test code as application code—I was able to productively make changes and feature additions starting on day one.

A Haskell codebase seems to remain readily comprehensible well beyond the size where, say, a C++ codebase starts becoming unwieldy; it’s maybe a 2:1 ratio. Functional languages are good at factoring, too, which is basically the essence of abstraction and the key to managing complex code.

Still, you must be diligent about it. Using Haskell does not automatically make you a better programmer, athough the people who learn Haskell well do seem to be better programmers for it—if only because Haskell is pretty progressive as languages go.

Haskell is Productive

Proponents of any language often talk about increased productivity. I do feel more productive in Haskell—in terms of the amount of code I have to write in order to add and test a feature—than in most any other language. But I can’t exactly quantify that, so I invite you to try it for yourself.

LOC is funny in that it’s simultaneously an important metric and a completely useless one. Language designers tend to be in agreement, more or less, that syntax matters—that good syntax does wonders for the effectiveness of language as a tool of thought. Naturally, they disagree about what actually constitutes “good syntax”. There might be something more to it, or it may just be a matter of taste that Python looks good to Alice and APL looks good to Bob.

Regardless, Haskell looks good to Jon. It’s pleasant to read—succinct, but not so dense that reading it is a chore. As is the case for any language, tutorial code looks nothing like production code. In Haskell you also see many academic papers where the style is very mathy—one-letter names, excessive abbreviations—and that style doesn’t scale at all beyond 200 lines. You need good names, no matter what.

Static Typing? More. Gimme.

Haskell’s type system is powerful enough that type signatures can tell you a good amount of useful information as to what a polymorphic function does. The “polymorphic” part is important: specific types naturally have much broader interfaces than simple type variables. You can easily say that a function of type (a → [a] → [a]) has only a few possible sane definitions. (Well, okay, countably infinitely many, but still.)

  1. f x xs = x : xs
  2. f x xs = xs ++ [x]
  3. f x xs = intersperse x xs

Whereas you know considerably less about what a function of type (Double → Double → Double) might do, because the set of floating-point numbers, and functions operating on them, is so large. The type system makes the trade-off explicit: the more domain-specific your functions are, the fewer guarantees you have about them.

In practice what this means is that, say, glancing at the type signature of a poorly-named function can give you a decent clue about what it ought to do. Or if you need a utility function, you can search Hoogle for a plausible type signature and usually find what you need. No other popular language’s API search comes close to matching that kind of utility. Thanks to Hackage, new libraries are very easy to discover and integrate into a project.

Haskell is Reliable

Where Haskell really does shine is in building reliable software. Static typing, unit testing with HUnit, and property testing with QuickCheck let you easily make a lot of guarantees that your code does what it’s supposed to. This is especially valuable for compilers, which are essentially (very large) pure functions, and thus very much amenable to such testing.

Testing definitely requires more up-front thought, but the end result is more reliable software—and that’s synonymous with better software, because good software is, for lack of a better term, obedient. Except maybe for AI.

Haskell lets us have a near-complete, robust AS3 compiler in under 10,000 non-blank lines of not-particularly-terse code. And that’s saying something: ActionScript is not what anyone would call a “pretty little thing”. And you don’t even know the half of it.


  • Compilers are fun and I will write them forever.
  • Haskell is good and I will use it for a good long time.
  • The Bay Area is a cool place to be, and I’m glad I moved here.

For those interested in my language work, an update is coming soon. Till next time.