I once had a thought that it would be interesting to see a treatment of category theory in APL. Then i went back and read Iverson, and he pretty much had it already. That guy was so far ahead, the rest of computing still hasn’t caught up.
This is just a gorgeous HTML-ification of a classic paper/talk. I wish it had a little note for folks like me who just clicked into it. I’ll have to set some time aside to read this. Great work. Also it would be interesting to see where/how this connects to J, which also I just now found out about.
J is probably most directly influenced by another Iverson paper, “A Dictionary of APL”  as described in “An Implementation of J.”