I like to learn new tools and concepts by experimenting with small projects whose sole purpose is to help me grasp something better. Working on a larger project in a team with other people is invaluable, but doing things on a smaller scale on my own offers a different perspective.
Having recently started to write a lot more C at work than I used to, I felt that it was a good idea to do a smaller project in the language. It turned out Build Your Own Lisp came out just when I needed it, so I had a go at it.
I cannot recommend this book enough if you feel like you would enjoy building a dynamic language in C from scratch (using just a parser library). It is one of the best tutorial-style books I have ever read. It achieves a perfect balance between being didactic and leaving freedom to the user.
My Lisp ended up being different from the one described in the book in several minor ways. For instance: it has more types, builtins take expressions instead of values, values are based on a union (which saves memory), memory management works differently, the source more organized... In that respect the book delivers exactly what it promised: the reader is encouraged to build "his own" Lisp.
In the end I got just what I wanted from this experience: I clearly improved my C and had the satisfaction to write a working programming language in a few hours scattered over three weekends.
Oh, just in case you were wondering, nobody paid me to write that blog post!
Next, I will probably be learning Go. Go is a weird language in that about half of the technical people I follow and look up to like it a lot, and the other half hates it. The latter tend to be users of languages with stricter type systems...
I have to admit that, if I had looked solely at the technical merits of both languages, I would probably have learned more Rust instead. I already know some Rust, but not enough to use it productively. Its main advantages are its (arguably) better type system and the fact that it can run without a GC or a frontend, making it suitable to write dynamic libraries.
However, Rust doesn't look completely stable yet, whereas Go is already used in production by several serious companies. Also, I am a distributed systems programmer, and most of the interesting codebases I see popping up around in that field are written in Go.Moreover, Go appears to be a simpler language than Rust. I suspect I can learn enough of it to read code "fluently" and write some much faster. So Go it is, and Rust will probably be next.