Dmitry Petrov Back

On Common lisp

Though I have some experience with clojure which is lisp I always had a feeling that this experience is incomplete without trying other dialects, especially common lisp. This post describes my experience on starting with this language.

I’ve read a couple of books, namely Practical Common Lisp, Land of lisp, Full stack lisp and Let over lambda and they all are beautiful. Macros part is especially mind blowing because you simply don’t think that way in most other languages, and I still not used to them in a sense that I don’t write my own.

After reading them I decided to give it a try and write something. The result is my shiny new incomplete cl-journal client for webservice. I set the following requirements for the client:

While I found a solution for all these points, I was left with the feeling of very fragmented toolchain and ecosystem. If we compare it to clojure world, all project management and build tassk are done either with Leiningen or with boot, and is mostly enough to get all standard library documentation and finally most libraries have decent looking documentation pages. Every package has well-defined project file that contains configuration, dependency information etc.

It’s not like this in common lisp world (according to my limited knowledge). Dependency management is done with the help of Quicklisp, project scaffolding is done with Quickproject. Building is a whole different story since common lisp doesn’t build standalone binaries in usual sense, here the way is to dump current state of lisp process (including compiler and documentation) and define an entry point and run it whenever this image is executed. There are several projects that aim to simplify building and running applications, most notably Rosewell and Buildapp. Rosewell looks really great, my problem with it was that I haven’t found a way to install scripts from local repository, so I had to do manual symlinks here and there to make it work.

Since there is no central place for the documentation, I mainly used Practical Common lisp book, common lisp cookbook and quickdocs website.

One of other unsolved issued for me was password prompt. I didn’t find a proper nonhackish way to do it with common lisp itself, so I had to run external command for exactly this purpose.

For running commands I had to use uiop library, especially uiop/run-program and uiop/os packages. Package there is more like a namespace in clojure and clojure package is a system in common lisp terms. uiop/os namespace was especially useful because standard library doesn’t provide any functions to control current working directory.

Most of things described can be blockers only with first steps in the language and I believe that once all they fit in head programming in common lisp gets not harder than in any other language but with additional cool features.

The coolest one is of course an interactive development. Basically I’ve started repl just once or twice during development, all the features were written and tested in it and that had huge consequences for development process. If we take any usual language, you work there by continuously running the whole program or firing one-liners to get result from desired function. The drawback of former is that if initialization step is costly, your development speed can suffer from it a lot. The drawback of latter is that most of function cannot work in isolation - they need specific datastructures initialization of global data object etc, and if it’s not necessary, developer still has to take care of proper printing of the results. So, sometimes it’s just to much of work to run things in this way, so people revert to the first way. First way also suffers from the fact that instead of implementing business logic developer spends a lot of time on boilerplate code for passing parameters, entry functions etc. Interactive development eliminates all that hassle.

My client had to load local database with information of already published files before doing anything. Instead of loading it on every change, I did it once and then simply tested new functions that I wrote with it. And since I could do it, I did not implement boilerplate code till the very last stage when I already had working business logic in place.

Interactive approach is very different also in sence that bottom-up approach feels really natural there. E.g. In order to create a post I need to read a file. How do I do that? (= new function). After I read, how do I parse it (= new function). After I parse it ho do I understand that it’s not a draft (= new function predicate) etc etc. So I continued asking questions and putting my answers as functions there till I get to very top where I wrote simple function that really created the post.

As a result most functions are really small and easy to reason about. You can go bottom-up in other programming languages, but since you either have to write a lot of boilerplate code to get feedback or right a lot of tests which really slow you down when you’re just trying to find a way to solve the problem.

If you ever did any clojure programming you know that it provides similar level of experience. Where common lisp wins is a startup time for a built program (instant) and human readable stacktraces that are not polluted with java classes etc and excellent restart functionality instead of simple exceptions. Restarts are super cool, because they don’t just throw warnings, they also provide options to recover and sometimes error can be just worked around with this.

As a resume - language is definitely worth looking at and using for real things. I have very limited exposure to the language to talk about good parts and bad parts, but a scattered and some times very brief documentation adds to a challenge of learning the language. Many libraries that are widely used have there latest release a few years in the past, and documentation is not always nice (e.g. I had to read the code to understand how to use uiop packages properly. The whole ecosystem has a few active members which means that not much happens in a public space.

However there is a suprisingly big number of common lisp implementations that are really high-performant and battle tested. I do think that if language gets a proponent like Yehuda Katz, who will put effort into popularisation of the language and systematic toolchain enhancements, common lisp can get a great level of adoption.