OCaml did become popular, but via Rust, which took the best parts of OCaml and made the language more imperative feeling. That's what OCaml was missing!
True, but writing imperative code in OCaml feels quite right. Maybe it's just me, but it always feels significantly clunkier than the functional equivalent. I've had things I've been implementing and though "this works be easier to express imperatively", and almost invariably I've gone and rewritten the code to be functional because of how ungainly the imperative implementation ended up being. It feels a bit too much like somebody embedded a subset of Rexx or Pascal awkwardly into an otherwise perfectly reasonable functional language
Rust is very different from Ocaml. Rust is a low level language with a borrow checkers, traits, no first class module system, no GADT, no effects.
The similarities are fairly superficial actually. It’s just that Rust is less behind the PL forefront that people are used to and has old features which look impressive when you discover them like variants.
There is little overlap between what you would sanely use Rust for and what you would use Ocaml for. It’s just that weirdly people use Rust for things it’s not really suited for.
As someone who has worked professionally in both, programming in rust does not feel much like programming in OCaml. Working with traits in an imperative manual memory managed language is really different than working with proper modules in a mixed paradigm language with an awesome GC.
Expression-based, immutable by default, pattern matching, static typing, and result / option types are the mix of features that make Rust what it is at its core, and each of those you'll find in OCaml. There's a couple others but that's the heart of it.
I'm not saying that Rust feels like Ocaml as some are interpreting, I said Rust is more imperative feeling, they're not the same. The reason Rust has had success bringing these features to the mainstream where Ocaml has not, I believe, is because Rust does not describe itself as a functional language, where as Ocaml does, right up front. Therefore, despite Rust having a reputation for being difficult, new learners are less intimidated by it than something calling itself "functional". I see it all the time. By the time they learn Rust, they are ready to take on a language like Ocaml, because they've already learned some of the best parts of that language via Rust.
Note my comment about their similarities is not at the level of borrow checkers and garbage collectors.
OCaml did become popular, but via Rust, which took the best parts of OCaml and made the language more imperative feeling. That's what OCaml was missing!