I used to do this as I commuted on the train between school and my hometown in the early 2010s. As a way to learn and pass the time.

No access to internet so mostly hacking from memory. I could use man pages for C, but Haskell was a bit more tricky.

Sometimes I’d just end up sketching things out on paper, but eventually I could complete entire modules without looking anything up. Was always a bummer to be stuck on something that I knew could be answered online in mere seconds. Good times.