On-screen tablet/phone keyboards seem perfect for apl to me.

There’s an iOS port of J but it’s no longer available on the App Store.

J is unclear on the concept, it's a hack to port APL to ASCII. I'm typing on a ZMK keyboard now where an APL unicode layer would be trivial. There are modern APLs. One can code a iOS keyboard with AI help.

While a native APL would be nice, the cloud solution would be robust access to a professional product. Build the chain via existing tools to bridge the iPad to APL running on a desktop machine.

I looked into this recently; but I've decided on Lean 4 as the successor "best language in existence" for my needs.

My recent language comparison: https://github.com/Syzygies/Compare

My APL one-liner story: https://news.ycombinator.com/item?id=27460887