As a Lancashire lad, I can confirm that, sadly, they do indeed.
(for those unfamiliar with this side of the pond, Lancashire and Yorkshire are mortal enemies right up until a Southerner tries it on at which point Northern Solidarity kicks in and we take them out first -then- go back to bickering ;)
The fact that people were doing this kind of malarkey in Emacs Lisp even before I started using Emacs over 25y ago just goes to show what a powerful, flexible, and general-purpose language Emacs Lisp is -- and it's not even my favorite Lisp by a long shot!
Emacs Lisp is nobody’s favorite lisp, yet it is the most widely used lisp by a wide margin.
Emacs proves that having a free and open source cross platform GUI editor where you can introspect and modify the editor’s state, and where a visual step debugger and lots of documentation are available, far outweigh the niceties that other lisps provide.
Emacs is oftened remarked to be a whole operating system to itself, fulfilling Dan Ingalls' definition[0]:
"An operating system is a collection of things that don't fit into a language. There shouldn't be one."
People use Emacs for anything and everything, so by extension they write elisp for all kinds of things too. Any why not tbh? It's a general purpose language that's arguably no worse than python or ruby (subjectively more fun for me instead). Yes in specific cases like data science or web backend/frontend you should prefer something else, but everyday text processing, scripting is elisp's forte, so given that it's like a cross platform VM that's always there it makes a lot of sense to use it as general purpose tech to marry logic with ergonomic frontend inside Emacs.
Yes, the backend for Google Flights, Orbitz, Kayak etc. That seems rather critical.
While it's not clear to me how much of that backend is still in Common Lisp, when Google bought ITA Software it was primarily a Lisp shop and I understand that it largely stayed that way for at least a long time.
At the time of the ITA acquisition, Google had a strict four-language policy: C++, Java, JavaScript, or Python. Nothing else was allowed.
The ITA acquisition made them rethink that a little bit, as they noticed that all the smart people working on QPX, the kind of people Google wanted to keep more of around, really loved Lisp and had built something great in it. And then Go came on the scene and Dart and... well, so much for the four-language rule.
QPX defied bitwize's Corollary to Greenspun's Tenth Rule: "Any sufficiently complicated Common Lisp or Scheme program will eventually be rewritten in C++, Java, or Python." (These days I would add JavaScript, TypeScript, and Rust.) It was still being maintained in Lisp as of the late 2010s and probably still is today. If Google ordered a Big Rewrite of QPX in another more marketable programming language, they risk losing the good people who worked on it to begin with. So... maybe next decade.
I think they tried to rewrite it in C++ and failed, so it’s still mostly in CL last I heard. I think they have an internally-maintained fork of sbcl and a core maintainer of sbcl on payroll.
> it's not clear to me how much of that backend is still in Common Lisp
Yes, that's why "suspect" and "obscure": I thought that well-known system is already rewritten in Google's usual languages. Turns out it isn't publicly known (?), but plausible: Google isn't afraid to rewrite their software whenever they find legacy hard to maintain, I even heard of a project that got rewritten 5 times in ~10 years.
I did something like that 20+ years ago, a subset of Flavors with some graphics classes, running inside of Emacs. My school deleted my account with all the code in it at the end of a semester. Gack.
Not sure what you mean about other languages. At the time (as a snotty undergrad) I thought what I was doing was exciting and important, but I can't say for sure that it would have turned into anything interesting. The OO stuff worked ok but the amount of user functionality that I got working wasn't that great.
I don't think I invented much with that code. I implemented a subset of Flavors based on the Flavors manual, as an add-on to Emacs Lisp. I guess it was cool in some ways, but it is long gone now. I've hacked on other compilers and interpreters but mostly about implementation, rather than inventing languages. IMHO we have plenty of languages and we don't need more ;).
https://www.gnu.org/software/emacs/manual/html_node/eieio/