As a Rust newbie and seasoned Go dev, I'm pretty interested to knownwhere would people experienced in both OCaml and Haskell, would put it in the spectrum.
Languages have features/constructs. It's better to look at what those are. And far more importantly: how they interact.
Take something like subtyping for instance. What makes this hard to implement is that it interacts with everything else in your language: polymorphism, GADTs, ...
Or take something like Garbage Collection. It's presence/absence has a large say in everything done in said language. Rust is uniquely not GC'ed, but Go, OCaml and Haskell all are. That by itself creates some interesting behavior. If we hand something to a process and get something back, we don't care if the thing we handed got changed or not if we have a GC. But in Rust, we do. We can avoid allocations and keep references if the process didn't change the thing after all. This permeates all of the language.
My team still has 8 year old Haskell systems in production. We pivoted away from the language a few years ago, which I discuss here a bit: https://news.ycombinator.com/item?id=37746386
I personally love ML languages and would be happy to keep developing in them, but the ecosystem support can be a bit of a hassle if you aren't willing to invest in writing and maintaining libraries yourself.
OCaml has some high profile use at Jane Street which is a major fintech firm. Haskell is more research oriented. Both are cool, but wouldn't be my choice for most uses.
