Currently getting up to speed with Reason (ReasonML). The latest version (3) brings Javascript-like syntax, and after many years of dynamically typed languages, having a compiler infer and check my types for me (and catch many errors before they reach the browser) is a good feeling. It's built using Bucklescript which brings a great toolkit of its own, and also means I'm pretty much learning OCaml at the same time.
I'm learning Haskell too for home/side projects, but ReasonML means I can do functional programming for work whilst I learn all about FP again.
Perl6. I have been using perl5 forever and I remember going to the second Perl Conference in 1998 and coming home feeling so cool showing off my new Perl Mongers t-shirt. The amount of new perl that I write has steadily dropped over the years (replaced by a mix of c++, python, and R), but I going to make a resolution to try and get up to speed on perl6 this year.
I might inherit a project written in Julia. If I do, I guess I'll have to learn it ;-) I'm in a university research group that does ecological modeling, for which so far I've been using Python. But from what I hear, Julia might actually be a better fit (especially performance-wise), so I wouldn't mind being forced to learn it.
Javascript/Typescript I've dabbled with it but after avoiding web dev for going on 7 years of professional development I figure it is time. I have some ideas I want to do that are web based and would like to do the front end on my own.
Erlang. I spent 2017 learning Elixir and would like to learn Erlang too.
Java/Scala. I did a lot of Java and some Scala for my graduate studies but haven't touched it in awhile. It seems like there are a lot of interesting jobs in the Chicago area that are JVM based and I would like to move away from .NET.
Kotlin. I'd like to work to build a multiplatform project, sharing as much of the code as possible. It's a young enough technology, and changing rapidly enough, that it might be good fodder to build a blog around.
It is in the same boat as Ruby was with Rails. One could argue that Rails made Ruby popular but mostly popular as a "web language". Phoenix is a great web framework but Elixir can do so much more by taking advantage of the OTP. I've used Elixir to get up and running with back end services that process data. Using OTP features such as GenStage it becomes easy to handle data as a pipeline. That said, most people who will move to Elixir will probably only use it with web dev and Phoenix.
It's general purpose but well suited for web, soft real time services, distributed data processing, control plain etc. Erlang VM has unique featureset that no other platform really can match so if you need to build HA fault tolerant apps/services it's pretty good match. Coming from a different language one of the nicest things is preemptive scheduler so you don't have to worry about blocking event loop or process consuming all the CPU etc.
I've followed some videos on f#, rust is just one of those things i keep hearing about and want to look at why it keeps coming up in forums and i actually assumed it was just another functional programming language. Dropping into https://www.rust-lang.org/ I see that's not the case but im still interested, I look forward to getting rid of my ignorance in that area.
I understand that given you are listing Rust alongside Solidity, your interest might be in cryptocurrencies, but I still thought this might be worth to ask.
I'm very experienced in C++98, less so in C++11 and successors as I spent most of my time on other languages this decade (OCaml, Scala and R). C++11 is much more modern and appealing, so I'm wondering whether I should update my C++ skills, learn Julia, learn Rust, or a combination of those.
I'm learning Haskell too for home/side projects, but ReasonML means I can do functional programming for work whilst I learn all about FP again.