Dyalog APL was my first exposure to functional programming outside of being in awe of Project Euler solutions in the forum back in the day.

I spent a lot of time on https://tryapl.org/ before eventually moving on to running it on a raspberry pi ~10 years ago with my family TV as a monitor.

A really great way to spend an evening or two. The syntax is wild in retrospect, and I cannot see where it could be used in a production environment today (if anyone knows, please tell me!) but it holds a nostalgic place for me :)