Anything and everything, seriously! How easy is it to model in APL, how much did you enjoy coming up with array oriented solutions not possible in standard algol based languages? What type of field did you work in and did others in your group or company know APL? How easy was it to share/read code from/ collaborate with others? What were the pain-points? Do you now use something else that although less terse, you feel is better? If so, why? What were the coolest projects you worked on and do you think APL has a future?
While I’m learning kdb+, Dyalog remains my favourite language for hacking and is likely to remain so for much of the publishing work I do, because of its solid UTF-8 support.
Iverson was on to something with his iterators. I am still getting insights decades later. When I use control structures like for-loops (even in APL) it feels like talking with my thumb in mouth.
Can you go into more details please? How's your experience with kdb+?