Ocaml
Software / AppMentioned in 2 videos
A functional programming language, which DHH admits he hasn't used in a 'real' project but is curious about.
Videos Mentioning Ocaml

DHH: Future of Programming, AI, Ruby on Rails, Productivity & Parenting | Lex Fridman Podcast #474
Lex Fridman
A functional programming language, which DHH admits he hasn't used in a 'real' project but is curious about.

Guido van Rossum: Python and the Future of Programming | Lex Fridman Podcast #341
Lex Fridman
An obscure language mentioned as being very good for writing static type checkers, and used by Facebook to develop Pyre.