This. I love getting stuck into new-to-me technologies that are tried, tested, and still around. Is this a good idea from a career perspective? I'm not sure. While I knew some SQL before, I'm deep diving now and I'm sure that'll be useful in snagging future jobs in the data space. But learning Lisp? The principles and perspectives I pick up will no doubt be applicable elsewhere, but perhaps not the language. But then again, not everything is about career progression — I'm really doing it for fun and on that front it's certainly delivering!