In practice, C++ and Python codebases have insane amounts of churn. They're constantly moving targets and require a lot of overhead to keep in shape, from just about every perspective: performance, security, maintainability, implementation compatibility, etc. It's tolerated though because it's standard practice, and we developers get a paycheck anyway.

Languages like OCaml, Common Lisp, Haskell, and other niche but obviously practical languages continue to be indispensable to certain cohorts of companies and developers. While I do agree that Lisp specifically peaked in popularity decades ago, it still exists in the commercial domain with resounding staying power, even in environments where it could in theory be replaced by C++ or Python.

It's also abundantly clear there's a hunger for new (or at least replacement) languages for the stalwarts. Rust, Julia, Mojo, Swift, and OCaml are a few such languages that get attention in their commercial investment and development, often with the specific purpose of replacing Python or C++.

As a personal anecdote, I find professional Common Lisp programmers generally to be quite sensitive to matters of bloat. Some programmers go as far to chisel away needless cons cells (or allocation in general), and constantly be in touch with the compiled assembly—something frequently ignored by professional C++ programmers of comparable experience.

I would love to hear the "very sound technical reasons" explaining Common Lisp's obscurity. Given the vast and diverse biome of programming languages in use today, including and especially in professional settings, I'm quite interested in understanding what technical reasons uniquely relegate Lisp to obscurity.

SBCL has pretty good type inference at a keytstroke (not complete, but useful, for a Haskell on top of CL see Coalton), it's fast, different implementations, stability…

C++ and Rust programmers usually don't have to go down into the assembly. The language itself provides fine-grained control over when and how allocations take place, and how memory is laid out. It's a doddle to establish efficient allocation patterns and lay out memory to take advantage of locality of reference so you don't blow up your data cache. Lisp, not so much; in addition to the pointer-heavy semantics of Lisp objects you have the inherent nondeterminism of a GC. So not surprising that when a Lisp program stutters or hangs, the programmer has to go down to assembly level to determine the memory usage pattern that's causing the slowdown. With C++ or Rust, the memory usage patterns are (usually) obvious from the code itself.

> I would love to hear the "very sound technical reasons" explaining Common Lisp's obscurity.

* Lisp is dynamically typed. Yes, I know Common Lisp allows for type annotations, but the type system is just barely enough to enable speed hacks in the compiler; it is neither sound nor robust when compared to, say, fully parametric type systems (now commonplace in blub languages like C# and TypeScript). The lesson of the past couple decades or so is that strong, static typing is an unmitigated win, especially when building large systems; to choose dynamic typing in 2024 is to forgo early checking of all sorts of bugs.

* GC. You will incur a performance and memory consumption penalty using a GC'd language, an in this day and age with RAII and ARC, those are penalties you need not incur in order to safely, conveniently use memory within deterministic space and time constraints. With languages like Rust we can get the same kinds of advantages with static lifetimes that we do with static types. There is no need to defer object lifetime management to runtime; and to do so just wastes electricity and contributes to climate change.

* The syntax. Programmers are not just annoyed by all the parens. Today, they're used to being able to get a list of all the methods valid on a given object by typing '.' after the object's name. You need a certain kind of syntax to do that, plus static typing, and Lisp eschews both.

* Lispers wax nostalgic for the Lisp machines, which were pretty groundbreaking in their day... but as the sibling to my comment points out, the tooling for other languages has caught up if not surpassed the best Lisp development environments since. Microsoft has had edit and continue in its IDEs since decades ago; I believe Eclipse got something similar later. Modern refactoring tools. Project Lombok. There has been a lot of research done into how to improve the developer experience in practical languages. Meanwhile, Lispers are still recommending frickin' Emacs to beginners.

* Plus, the library support for practical languages like Java, JavaScript, Python, or C# -- or even C++ -- is just so much better than what's available for Common Lisp.

ARC is slower than good GC.

A distinguishing characteristic of languages like Lisp or Elixir is the metaprogramming. C++ and Java have nothing like their macro systems, and when an application grows and turns complex it's a very nice tool to keep developer facing code neat and readable. Usually it also keeps it more robust under change, there are fewer lines where typos or logic bugs can creep in when you write less boilerplate and use well designed code generation instead.

Then why are virtually all browser engines, high-performance games, and video codecs written in C++? Why is every massive-scale web application written in C++? The only one that comes close that's written in Lisp is QPX, and even then they had to write a lot of C++ because Lisp's I/O facilities just weren't up to the task of moving the huge amounts of data they had to move.

> C++ and Java have nothing like their macro systems

C++ has template metaprogramming, which covers like 90% of the cases where you'd actually need Lisp macros, and outputs code that's statically typechecked with deterministic memory and runtime constraints.

Path dependence. There are only two lineages of widely used browsers today: Chromium/Webkit (coming from Konqueror, initial release 2000) and Firefox/Gecko (coming from Netscape, initial release 1994). Nobody's rewriting them.

The same goes for video game engines and other high-performance software.

Sibling mentioned WhatsApp which was Erlang. Facebook is PHP. Pre-Musk Twitter was written in Scala.

You can have compile time type checking in Common Lisp if you want.

The contents of that data file was generated by lisp batch job, too.

But will good programmer be equally productive in all languages?

What if good programmer + good lang/ecosystem = 1.25x productivity of good programmer + worse language/ecosystem?

Kinda. Language-specific expertise should not be downplayed.

If you've spent a decade honing your craft writing C and assembly for exotic embedded platforms, you'll be a valuable asset for certain projects, but your experience won't translate to Scala or Haskell.

Well, I’ll rewrite it in Brainfuck then.

Consider an expert in C programming for embedded platforms, and an expert in Ada (or Forth) programming for embedded platforms. The surrounding expertise is similar, but the language-specific expertise shouldn't be dismissed.