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Facebook uses it, as ReasonML and also in Infer (a static analyzer https://fbinfer.com).

I believe OCaml is the main language in Jane Streets tech stack rather than just being used for dev tools like Infer. I saw they were hiring compiler engineers recently so, assuming this is for OCaml, they are pretty deep into the language.

Yes, they make billions of dollars of trades daily using OCaml.

https://www.janestreet.com/programming/

Yaron Minsky introduced OCaml into Jane Street almost 20 years ago:

https://blog.janestreet.com/author/yminsky/

He’s working on a new version of his book: http://dev.realworldocaml.org/

Used by Red Hat for miscellaneous virt tools.

Same name every OCAML or F# post.

They are very involved with the ecosystem. They sponser a lot of the events, conferences, etc. In fact I'm attending OPLSS this year and even that is sponsered by them.

I even saw someone compare OCaml build methods as 'Jane Street Way' vs 'Other Ways'. It's hard to ignore such an influential company like that when the question about who uses them comes up!

What's wrong with that?