I have tried to learn J, it seems to be one of the most well documented.

And Ike itself is not a bad environment to play around with K https://johnearnest.github.io/ok/ike/ike.html

It has a repl as well in: https://johnearnest.github.io/ok/ you can find source here: https://github.com/JohnEarnest/ok and manual here: https://github.com/JohnEarnest/ok/blob/gh-pages/docs/Manual....

If this is the entirety of K - https://kparc.com/k.txt it does fit on a single full-screen page.