I learned Java and C at school, I learned Python by myself and it was a relief. After 8 years working and doing side projects in Python and JavaScript (mostly web dev, Django/Flask/AngularJS/Vuejs), I am not satisfied anymore by the overall experience so I’m making Common Lisp my language of choice.

I am not here to compare languages themselves, but their inherent workflow and their ecosystem. This is the article I wish I had read earlier, when I was interested in Lisp but was a bit puzzled, because the Lisp way always seemed different, and I couldn’t find many voices to explain it. The Python way may not be the most practical or effective, Common Lisp might not be a dead language. I find many “workflow fixes”, overall improvements and hackerish possibilities on the CL side, even if sometimes the Python tooling is superior.

Let’s dive in.

and thanks to the proofreaders.

Table of Contents

Development process


In Python, we typically restart everything at each code change, we use breakpoints: this takes some time, I find it too repetitive and boring, it requires to re-manipulate data to re-reach the state we were at to analyze and debug our program. We might figure out a non-standard, more interactive way, but still: a web server needs to restart, object instances don’t get updated after a class definition. We can get a prompt on an error (-m pdb), some tools include it (Werkzeug): a sign that it is a good thing to have. Unfortunately, it is not built-in, as in CL.

In Common Lisp, everything is much more interactive in the REPL. Even developing web apps. On an error, we get an interactive debugger with the stacktrace in our editor, we press v and voilà, we are at the problematic line. We can of course catch errors to avoid the debugger, or disable it with global settings. We can resume the program execution from any stackframe. No process needs to restart. The variables that we define on the REPL stay here. If we change a class definition (say, we remove a field), existing instances get (lazily) updated.

The Lisp REPL is part of the development process, it is not only used for exploration and debugging. It’s fun, it’s a productive boost, and it allows to catch errors earlier, both because we try functions earlier, and because we get type warnings when we compile the file or the current function (yes, we can compile a single function).

Now, the cost is that one must learn to play with this live data. We might come to a state that doesn’t reflect the code anymore, so we’ll write our own “reset” functions or just restart the lisp image.

Here’s a video where the developer defines a dummy interface, makes it fail, develops it, and tests it, all quickly by interacting with the REPL.

In this video I show how we can interactively fix a function and resume the program from this precise point in the call stack, without re-starting the whole, time-consuming operation.

Debugging a complex stacktrace in Python VS Common Lisp (author unknown)

Editing code

Python: we edit code line by line, paragraph by paragraph. We can try out half-backed editor plugins to edit code by semantic units. Sometimes we must even pay attention to add a couple whitespace there, remove one there. We are far from the immediate interactive feedback of the hacker’s vision “Inventing on Principles”.

Common Lisp: we edit code by semantic units. I love emacs’ lispy mode, which is weird at first of course, but so convenient. We can navigate to expressions back and forth, we can delete a whole “if” expression with a keypress, indentation is automatic, etc. There are other emacs plugins. Parinfer is appreciated in other editors too.

Actually, we edit code by parenthesis units, which doesn’t carry as much meaning as an Abstract Syntax Tree. For a real AST, we’d need a code walker (like Concrete-Syntax-Tree). But since Lisp’s syntax is based on parenthesis, in practice the experience is similar.

I had a try on writing a little plugin to help editing Python code by manipulating the AST (red4e). We first need an AST parser. There was a couple for Python 2, another one for Python 3 without type annotations, eventually one emerged a couple years later: these are signs of an unstable language and ecosystem, and it is more work required by the developer. I went the simple way by calling each function into a new Python process, which is of course too slow. traad is a better project, it can do much more but still, it’s difficult to answer questions like cross-referencing: “who calls this function” or “who does this function call”, which are built-in in SLIME. SLIME is like the Language Server Protocol for Common Lisp in Emacs, its backend Swank being editor-agnostic.

Maybe other editors and proprietary ones come with a better experience, at the cost of freedom, money, configuration time and memory and CPU resources. If I have the choice, I prefer to not go this route, and choose a better platform from the start.

Traad is built around a client-http server approach, this is the idea behind LSP… this reminds me of the architecture of SLIME! It has a backend, Swank, and a client (SLIME for Emacs, SLIMA for Atom,…). It thus has a modern architecture since its inception :) It is moreover based on a stable language whose syntax can not rot and has decades of development behind it, so we can be confident about the tool. Saying this because it’s hard to grasp what SLIME is at the beginning.

SLIME itself is tied to Emacs, and thus a newcomer can find the UI unpractical. Swank though can be used outside of Emacs, and it is for example for Atom’s SLIMA, which now has all the most important SLIME features: REPL, integrated debugger, jump to definition, autocompletion, interactive object inspection, and more.

Running, testing programs

Python: the default workflow is to run commands in the terminal. Scroll, read the output, copy-paste manually (or use the non-UX-optimal termux or a terminal inside emacs), go back to your editor. Type commands without completion, type the whole path to a single unit test (pytest path/to/test.py::foo), or configure your editor and find a good plugin that is compatible with your test runner (I can’t use the excellent nose-mode :( ).

Common Lisp: the default workflow is to do everything interactively into the REPL, but some people still use a write-compile-run approach. Consequently there is built-in completion for everything. We don’t have to use the shell (except from once in a while to run global tests or build the system) and that’s a good thing. There is an interactive debugger. We can interactively fix and re-run code and tests.

Here’s a quick demo on how to interactively fix failing tests:

Running and debugging on a remote server: in Python, we usually rsync sources and run tests manually, or start vim/emacs under tmux on the server. We have to kill the app to reload it. In Common Lisp, we can connect to the running, remote instance, write changes from the comfort of our editor locally, hit C-c C-c on a function to compile it and see changes on the remote image. CL has more hackerish capacities here, no doubt, and I find it attractive :)


Python: we catch a lot of type errors in production, and/or we have to write a lot more unit tests. Hope we agree on this.

Now we can improve the situation somehow with type annotations, however it has the cons of being an after-thought: it is not stable (differences between Python versions), not well integrated (we have to run another command, choose between mypy, the new typing module, pyre), it is not interactive, we need to configure our IDE, it adds a start-up penalty (which might or might not be important).

In Common Lisp, particularly with SBCL, we get a lot of type errors or warnings at compile time. We can compile a single function, and thus have an immediate feedback. We’re closer (not there, just closer) to the “if it compiles, it works” situation (we know it runs, since we constantly compile and try the functions). We can also create our compound types and add type declarations to variables and functions. It’s great, though it doesn’t do as much static checks as a real typed language.

Adding type declarations in well chosen places such as inner loops also allows to gradually speed up the program where needed.


Python: we can’t refactor code as we want. Decorators, context managers: they have an interface and they are limited to what they offer. You can’t do things a bit differently, you must comply to the interface. That might be a feature, but I prefer not being restricted. In my experience, this leads to code repetition whereas in CL, we can refactor how we want, and we get a cleaner code.

Common Lisp: there are similar patterns than in Python, but we can escape them. We can use macros, be concise and do what we want. We can have the decorator syntax with the cl-annot library, and any other by writing our reader macros (they can bring triply-quoted docstrings, string interpolation, infix notation, C syntax…). It’s not only macros though. The polymorphism of the object system (or generic dispatch) helps, and Lisp’s “moldability” in a whole allows us to refactor code exactly how we want, to build a “Domain Specific Language” to express what we want. Other language features than macros help here, like closures or multiple values (which are different, and safer for refactoring, than returning a tuple).

Now, speaking about refactoring tools, they are better Python side. I don’t know of a Lisp tool that allows to change all the code-base according to the AST, maybe in a proprietary editor. There are utilities to make local transformations, like “extract this expression into a let variable at the top of the function”, “transform a function to a lambda equivalent” or the contrary, etc.

(edit January, 2020: the language-agnostic tool Comby will be useful here. I used it for syntactic manipulation, for example to replace a (if (…) (progn …)) by a (when (…) …)). See Colisper (a POC).


Library management

pip: use virtual environments (virtualenv, virtualenvwrapper, tox, anaconda,… or install per-user), pin dependencies (pip-tools, pipenv, poetry, pyupdate,…). Debug problems due to a third party library that didn’t pin its dependencies strictly enough (happens at the wrong moment).

quicklisp: think of it like Debian’s apt, shipping releases that work together (that load together), and that we upgrade together, when we want. If needed, we can still clone projects into ~/quicklisp/local-projects/ for a system-wide installation, or have project-local dependencies with Qlot. Quicklisp is very slick. Libraries are installed at runtime, during our REPL session. We don’t have to reload the Lisp process.

We are not even limited to Quicklisp any more (it can be limiting because of its one month release cycle). The Ultralisp distribution builds every 5 minutes. clpm is a package manager with a traditional approach. One can publish his own Quicklisp distribution, to provide a set of packages that are known to work together.

State of the libraries

CL might have more libraries than you think, see the Awesome CL list, http://quickdocs.org/ or do a quick search on the net. I know I am constantly surprised.

But sure, the Python ecosystem is huge. A few remarks on the differences:

  • Quicklisp has around 1500 packages, PyPI over than 170 000. It’s hard to imagine that there are a hundred times more useful libraries :D Even in CL we have duplication of libraries with a dozen of test frameworks.
  • Quicklisp is a curated distribution, PyPI is not. That means that libraries that don’t compile anymore are rejected (after a notice to the maintainers), and that orphan projects’ URL can be updated to point to a community maintained one.
  • Anybody can easily publish a library to PyPI on its own. Less so with Quicklisp, one must open an issue (Ultralisp doesn’t have this limitation).
  • numcl is a Numpy clone.
  • if needed, you can use py4cl and more to interface with Python.

An important remark, is that Common Lisp is a stable language, and that the libraries play this game (I saw a deprecation feature staying for 12 years). We can still run code that was written in the early 90’s.

Lisp’s simpler, non-rotting syntax plays a good role on stability. Caution: that doesn’t mean the implementations don’t evolve, quite the contrary.

In his appreciated article A Road to Common Lisp, the author writes:

as you learn Common Lisp and look for libraries, try to suppress the voice in the back of your head that says “This project was last updated six years ago? That’s probably abandoned and broken.” The stability of Common Lisp means that sometimes libraries can just be done, not abandoned, so don’t dismiss them out of hand.

hacker news comments


Ever had headaches with Jinja ? Ever fought against Django templates ? Ever abandoned to clean up a big mess of html and templating code ? Used Jinja macros to factorize code ? Maybe you turned to the good looking Jade (Pug). So you read another documentation, you install tools to integrate it into your project. And now damn, no more cross-files macros. You edit blocks of code whitespace by whitespace. And in the end, your html may still not be valid…

You might use Mako templates, but there’s something you can’t do.

In CL, we can also use a Django-like templating engine, Djula templates (despite its modest number of stars, it is one of the most downloaded projects on Quicklisp). The Mako equivalent would be Ten. However, we can alternatively just use plain old Lisp, for example with Spinneret. As a consequence, we can factorize code as we always do (with spinneret functions or lisp macros). We manipulate code as we always do. It even warns on malformed html and has some neat features (it is clever about headers levels, it can embed markdown, etc).

Stuff like this is less possible with Python, because the language is less flexible. The components libraries I have seen use strings inside Python code.

Deployment, Shipping

Shipping an app, even more a web app, in Python (and JS) is tedious. There are no default way to ship a self-contained executable. Current projects aiming at fixing that can work… and may not.

So the current solution is to turn to containers. They’re the Big Thing, but we still need to spend hours on reading resources, building the Docker file, the deployment pipeline, fixing bugs, updating the stack, accepting security holes, etc. Hours we could put on our app. With Docker though, users still can’t download a binary.

In Common Lisp: we (re)discover the joy of a compiled language. We compile our program to machine code, the binary embeds the run-time, the debugger, the web server, the static assets, and we ship it. We run it on the server and we can access it from the outside straight away.

An SBCL image of a non-trivial web project will weight ± 20 to 30MB (with core compression). For a lighter binary (not that I care personally), we could try ECL (that compiles to C), or use tree-shakers of proprietary implementations (LispWorks, Allegro).

We can still benefit from Docker if needed, of course.

Deployment process in Python: install Python and pip, install pip dependencies and their system requirements and be prepared for errors (or try non-standard tools, like Platter), configure a server for static files (nginx, whitenoise), run a WSGI web server,…

Deployment in CL: build your binary, send it to the server, run it. Configure nginx eventually. We can compile and include assets into the image (see Rock).


Python is notoriously slow, and passed the hobby project you quickly realize that.

Python has a Global Interpreter Lock.

SBCL compiles to machine code and is garbage collected [1].

We can fine-tune the types in our Lisp programs for the compiler to make the consequent optimizations. We can run in “debugging first” or in “speed first” modes. We can inline code to gain in the cost of function calls.

As a consequence, you may not need memcached in your Lisp project yet.

[1]: and rest assured, Google improves the GC


I hope I killed some FUD and showed you new ways to make stuff. May that inspire you!

Appendix: FAQ

Some info every Python programmer will come across eventually. Saves you some googling.

Are there no iterators ?

In practice, we mostly rely on closures, but there are libraries to create iterators.

See https://stackoverflow.com/questions/32956033/is-there-a-straightforward-lisp-equivalent-of-pythons-generators

Can I define my own + operator like in an OO language ?

By default, no, because the Common Lisp Object System (CLOS) came after the language specification and thus everything isn’t object-based. However there are libraries like generic-cl and, in practice, we quickly forget about this. Different operators is also a means for performance, good type inference and error messages.

To which extent can Lisp be compiled, with all its dynamic nature, garbage collection, macros and what else ?

Many Lisp compilers compile to machine code (SBCL, CCL, CMUCL,…).

Full answer: https://stackoverflow.com/questions/913671/are-there-lisp-native-code-compilers/914383#914383

But what is Common Lisp good for, really ?

We have a ready-to-use citation :)

Please don’t assume Lisp is only useful for Animation and Graphics, AI, Bio-informatics, B2B and Ecommerce, Data Mining, EDA/Semiconductor applications, Expert Systems, Finance, Intelligent Agents, Knowledge Management, Mechanical CAD, Modeling and Simulation, Natural Language, Optimization, Research, Risk Analysis, Scheduling, Telecom, and Web Authoring just because these are the only things they happened to list. – Kent Pitman

Kent Pitman


See also http://random-state.net/features-of-common-lisp.html

First, some reminders:

  • popularity doesn’t equal quality, and popularity is hard to measure.
  • some success stories: http://lisp-lang.org/success/ Aircraft analysis suits, Missile defense, ICAD, music composition, algebra systems, bulk importer for PostgreSQL, grammar checking, 3D editor, knowledge graphs,…
  • did you know that pgloader was re-written from Python to Common Lisp? (for a x30 speed gain, among other benefits)
  • CL was used in a spacecraft (and the REPL was used to debug the system live from the earth)
  • some companies still use and pick CL: https://github.com/azzamsa/awesome-lisp-companies, companies provide professional support (Platform.sh).
  • Google’s ITA Software still powers airfare search on Orbitz or Kaya.com,
  • reddit v1 was written in CL! JavaScript was written sketched in CL! (see here, with lisp code still visible in the repository: here)
  • CL was number 2 on the Tiobe index for years in the 80s!

That being said, my 2 cents since you ask:

  • I think the CL world missed the web bandwagon for some time (common-lisp.net was horrible for some years), but that’s been fixed.
  • an enormous code-base existed before GitHub.
  • we missed visually nice, practical content on the web, even though there are many books. It’s a bit better now.
  • CL missed a package manager for some time behind other languages, that’s now fixed.
  • I reckon CL is still quite hard for the web, it doesn’t have a killer web framework (though maybe Weblocks, CLOG or ISSR soon©, all isomorphic web frameworks), hence no hype.
  • CL seems to be used for big, non-trivial projects, hence it gets no easy hype.
  • CL has no entity doing marketing today. We saw the Common Lisp foundation pairing with sponsors recently. It did receive a lot of financial and institutional support from the MIT, the NASA, Xerox, Carnegie Mellon University (CMUCL), Lisp vendors (Symbolics, Lucid, Franz…),…
  • CL worked well with Emacs, Vim, CCL’s built-in editor on macOs, LispWorks’ editor (which has a free version), but this doesn’t satisfy the masses. We now have more options, including Atom (very good support), VSCode (okay support) and Eclipse (basic support).
  • other reasons: it may be hard (or harder than the concurrence) to grasp and getting started with, Lisp isn’t for everyone, it gets a lot of FUD, and has a so-called Lisp curse!

but that’s all debatable, I wouldn’t focus much on this. Times are good for implementations, that’s what counts.

How do I get started?

You need to install sbcl and run it in your terminal. From there you can write code in the basic REPL (use rlwrap sbcl for a readline wrapper). You can write code to a file and use (load "myfile.lisp") to load and execute it. Then you’ll want to install Quicklisp and to set up a true editor. Here’s a video to get you started in no speed:

However, make you a favor and learn some Lisp for good! I am creating a video course on the Udemy platform, where I teach the language for practical purposes, and where I make you avoid all the gotchas that I faced during my journey.

It’s here: Common Lisp Programming, from novice to effective programmer.