A Road to Lisp: Which Lisp

Most programming languages evolve as a single language. Python, Java, Javascript, C++, have new versions and standards, multiple implementations, but they still remain the same language. C and C++ can be compiled with GCC or Clang, Python can be compiled with CPython or PyPy, the same JavaScript runs in both Firefox and Chrome, and Java programs can run on JVM or GraalVM.

Lisp is not like that.

The Wikipedia page lists more than 20 different dialects, which means there are many variations to choose from. That’s because Lisp is a family of programming languages. They share the same fundamental syntax but differ in their operators, semantics, standard libraries, and language capabilities.

One of the main concerns Lisp beginners have is which dialect to learn first. I see this question frequently asked in online forums. The answer is that the dialect matters, but not as much as a beginner might think. Learning Lisp is about learning a new type of programming. A new way of thinking about problems using code. You will learn the fundamental concepts with any dialect. Then, once you have learned one, it will be relatively easy to switch to another.

I will briefly present the most relevant dialects that are actively used and maintained. I’ll try to highlight their strengths and weaknesses to help you overcome your indecision and pick one to start your Lisp journey.

If you are a beginner, many of the concepts in this article might be completely new to you. Don’t worry too much about them yet. When I was a beginner, I found many of these concepts fascinating and they pushed me to learn more about the Lisp world.

Common Lisp

Abbreviated as CL, it is the most mature and comprehensive of all Lisp dialects. It’s considered the old-school Lisp, since the language — I will call it a language from now on — was standardised in 1994 with a formal ANSI specification. Thanks to this standardisation, there are multiple implementations of Common Lisp targeting different platforms and use cases.

The most famous implementation is SBCL, which compiles directly to native code. It is fast, open-source, and compatible with modern hardware. With it, well-written Common Lisp code can achieve performance comparable to C and Rust. Because SBCL optimises heavily, it compiles a bit more slowly than other implementations but generates some of the fastest code in the Lisp family.

When I said above that Common Lisp is the most comprehensive dialect, I meant that it offers the broadest set of features among all Lisps. It provides a large amount of functionality out of the box, much of it defined directly in the standard. For example, CL has functions for controlling compilation and evaluation from within the language itself ( COMPILE, LOAD, EVAL, COMPILE-FILE, and others). This means that I can use these functions in my code or at the REPL to tell the Lisp process to compile a function, load a file, or evaluate some code. CL also provides DISASSEMBLE, which lets you inspect the machine code generated for a compiled function.

Common Lisp has a condition and restart system, which is one of the most powerful way I ever saw to debug and inspect programs. If a condition occurs during execution, the Lisp process might stop and let you inspect the state of the program and its variables at that point in time. You can then choose to restart the program and maybe retry the operation, or ignore the condition and continue execution. This is also possible because the Common Lisp REPL is deeply integrated with the running system. If your program fails on a remote server, you can connect to its REPL and inspect the live process to understand what went wrong.

Common Lisp supports all major programming paradigms, like functional, imperative, metaprogramming, and object-oriented programming. It offers one of the most advanced object systems, called CLOS, which supports features such as multiple dispatch and generic functions. This makes OOP more flexible than in common object-oriented languages such as Java or C++. It is also worth noting that Common Lisp is dynamically typed but has a rich and expressive type system with optional type declarations.

Standardisation means that the language is stable. The ANSI Common Lisp from 1994 is still the one in use today. Because of it, lispers rarely fall victim to backward incompatibility, and old Common Lisp code often still runs perfectly today. For example, if you go through old Lisp books such as PAIP by Peter Norvig, published in 1991, you’ll find that much of the code still runs on modern implementations. With Common Lisp, you encounter far fewer of the incompatibilities typical of languages like Ruby and Python. Even Common Lisp libraries that haven’t been updated recently will often still run fine on your system because they don’t need to be continually adapted to new versions of the language.

Common Lisp lacks some conveniences that have become common in newer languages, such as concise literals for a wider variety of data structures, persistent immutable collections, lazy sequences and built-in general-purpose pattern matching. Common Lisp was designed in the 1980s by consolidating several existing Lisp dialects into a single language. Its designers chose to retain much of the syntax and patterns of those older dialects, making sure that existing Lisp programmers could adopt the language without having to learn a completely new syntax.

Common Lisp doesn’t have a central direction or corporate sponsor. The community is relatively small and spread across several implementations, projects and communication channels. It’s mostly made up of volunteers who help keep the language alive. It can be a bit difficult to find guides and help: tutorials are often either too simple or too advanced, and documentation is not always easy to find.

It’s currently used in quantum computing at Rigetti Computing, and has been used at Grammarly for its core grammar service and also powers Google Flight Search. Worth mentioning is Kandria, an open-source video game released on Steam and written entirely in Common Lisp.

An interesting fact: Paul Graham originally wrote HackerNews in a custom Lisp dialect called Arc, based on Racket. Today, Daniel Gackle ( @dang ) reimplemented it in Clarc, a Common Lisp implementation of Arc that runs on SBCL ^1, serving around 10 million pages per day.

Where Common Lisp shines

CL compiles to native code on all major operating systems and can achieve fast execution, at the cost of slightly slower startup times, so it’s great for long-running processes. It offers one of the most powerful REPLs among Lisp dialects and can be considered, despite its steep learning curve, one of the fastest languages for writing solid software in a short time.

CL shines when you need to do research and prototyping with quick iterations, as in quantum computing, or when the specifications aren’t fully defined yet and change frequently, requiring the software to adapt quickly, as in startups. Paul Graham wrote a famous and inspiring article about how Common Lisp helped his internet startup beat the competition.

Learning resources

A good starting point is A Road to Common Lisp by Steve Losh, which inspired me to write this series. One of the most accessible books for beginners is Practical Common Lisp by Peter Seibel, which is available entirely online. It’s the best introduction to the language I’ve read, as the most important concepts are neatly separated into chapters. A more playful introduction is Land of Lisp, which uses funny characters and comics to teach programming through games. A more advanced book that I enjoyed is On Lisp by Paul Graham, which is entirely dedicated to macros.

The best IDE support for CL is Emacs + Sly / Slime ( Doom already has support for it) or Vim + VLime. A weaker alternative is VSCode + Alive.

One of the best resources for beginners is The Common Lisp Cookbook, which has many up-to-date tutorials. For reference…