I use a lot of Haskell — it’s probably my best language and the one that I think clearest in. It’s a strong statically typed FP language in the ML lineage that been around since roughly 1991.
The core language has some unfortunate design decisions that make it very powerful but famously difficult for newcomers. I wish that there were more widely agreed upon ways of doing things that learners could copy into their applications. Once you have a few tricks down like Tagless Final, it’s a really lovely language to work in.
Fission Codes had a false-start originally trying to write the server in Rust before it had decent support for web development. Since I was the main developer and wanted the guard rails while I was building, I chose Servant expecting that it would be eventually rewritten. It served us extremely well — very stable, plenty fast, easy to do large-scale refactors or replicate huge chunks of broken code from dependencies in other languages. The big problem was that the langue’s reputation proceeded it, and it has a serious learning curve that requires a lot of mentorship in the first few weeks to get over (though honestly no more than Rust, but that language has clear career & community benefits).