I’ve been intermittently working on a common lisp library to produce a binary encoding of arbitrary objects, and I think I’m going to be abandoning the project. This is an explanation of that decision and an reflection on my experience.

Why Common Lisp?

First, some background. I’ve always thought that Common Lisp is a language with a bunch of cool features and selling points, but unsurprisingly, I’ve never really had the experience of writing more than some one-off bits of code in CL, which isn’t surprising. This project was a good experience for really digging into writing and managing a conceptually larger project which was a good kick in the pants to learn more.

The things I like:

  • the implementations of the core runtime are really robust and high quality, and make it possible to imagine running your code in a bunch of different contexts. Even though it’s a language with relatively few users, it feels robust in a way. The most common implementations also have ways of producing fully self contained static binaries (like Go, say), which makes the thought of distributing software seem reasonable.
  • quicklisp, a package/library management tool is new (in the last decade or so,) has really raised the level of the ecosystem. It’s not as complete as I’d expect in many ways, but quicklisp changed CL from something quaint to something that you could actually imagine using.
  • the object system is really nice. There isn’t quite compile time-type checking on the values of slots (attributes) of objects, though you can opt in. My general feeling is that I can pretty easily get the feeling of writing statically typed code with all of the freedom of writing dynamic code.
  • multiple dispatch, and the conceptual approach to genericism, is amazing and really simplifies flow control in a lot of cases. You implement the methods you need, for the appropriate types/objects and then just write the logic you need, and the function call machinery just does the right thing. There’s surprisingly little conditional logic, as a result.

Things I don’t like:

  • there are all sorts of things that don’t quite have existing libraries, and so I find myself wanting to do things that require more effort than necessary. This project to write a binary encoding tool would have been a component in service of a much larger project. It’d be nice if you could skip some of the lower level components, or didn’t have your design choices so constrained by gaps in infrastructure.
  • at the same time, the library ecosystem is pretty fractured and there are common tools around which there aren’t really consensus. Why are there so many half-finished YAML and JSON libraries? There are a bunch of HTTP server (!) implementations, but really you need 2 and not 5?
  • looping/iteration isn’t intuitive and difficult to get common patterns to work. The answer, in most cases is to use (map) with lambdas rather than loops, in most cases, but there’s this pitfall where you try and use a (loop) and really, that’s rarely the right answer.
  • implicit returns seem like an over sight, hilariously, Rust also makes this error. Implicit returns also make knowing what type a function or method returns quite hard to reason about.

Writing an Encoder

So the project I wrote was an attempt to write really object oriented code as a way to writing an object encoder to a JSON-like format. Simple enough, I had a good mental model of the format, and my general approach to doing any kind of binary format processing is to just write a crap ton of unit tests and work somewhat iteratively.

I had a lot of fun with the project, and it gave me a bunch of key experiences which make me feel comfortable saying that I’m able to write common lisp even if it’s not a language that I feel maximally comfortable in (yet?). The experiences that really helped included:

  • producing enough code to really have to think about how packaging and code organization worked. I’d written a function here and there, before, but never something where I needed to really understand and use the library/module/packaging (e.g. systems and libraries.) infrastructure.
  • writing and running lots of tests. I don’t always follow test-driven development closely, but writing lots of tests is part of my process, and being able to watch the layers of this code come together was a lot of fun and very instructive.
  • this project for me, was mostly about API design and it was nice to have a project that didn’t require much design in terms of the actual functionality, as object encoding is pretty straight forward.

From an educational perspective all of my goals were achieved.

Failure Mode

The problem is that it didn’t work out, in the final analysis. While the library I constructed was able to encode and decode objects and was internally correct, I never got it to produce encoding that other implementations of the same specification could reliably read, and the ability to read data encoded by other libraries only worked in trivial cases. In the end:

  • this was mostly a project designed to help me gain competence in a programming language I don’t really know, and in that I was successful.
  • adding this encoding format isn’t particularly useful to any project I’m thinking of working on in the short term, and doesn’t directly enable me to do anything in particular.
  • the architecture of the library would not be particularly performant in practice, as the encoding process didn’t deploy a buffer pool of any kind, and it would have been harder than not to back fill that in, and I wasn’t particularly interested in that.
  • it didn’t work, and the effort to debug the issue would be more substantive than I’m really interested in doing at this point, particularly given the limited utility.

So here we are. Onto a different project!