As for J's cousin K, the official K interpreters are commercial (with a free workspace-limited demo version for tinkering), but there is also an actively mainta

As for J's cousin K, the official K interpreters are commercial (with a free workspace-limited demo version for tinkering), but there is also an actively maintained open-source implementation called Kona: https://github.com/kevinlawler/kona https://github.com/kevinlawler/kona I think K is a bit more pleasant to read than J because it relies far less on digraphs, but naturally it's a matter of taste.