APL is how non-programmers imagine what programming is, a kind of sorcery involving exotic symbols and elaborate ritual processes to create data and algorithms out of thin air. Like LISP, it's an eternal classic that every generation rediscovers and gets their mind blown as an educational experience about the strange nature of computing, logic, and language.

Notation as a Tool for Thought, the 1979 ACM Turing Award Lecture by Ken Iverson who developed APL, is insightful in understanding some of the thinking behind it.

APL was the first programming language I learned, I was obsessed with learning every intricacy of the language. I almost lost my mind when I had to switch to a procedural language. APL warped my mind in such a way that it took almost a month to feel comfortable using anything else.

At my very first professional EE position, in 1978, there was an older staff member, PhD, kind of a professor type, who was programming in APL. Naturally curious, I asked him about it and what the strange symbols on the keyboard meant. His explanation mostly flew right over my head but I asked a few questions, thanked him, and went away. In 43 years, he's the only person I ever saw using it.

Related. Others?

The APL Source Code (2012) - https://news.ycombinator.com/item?id=34064480 - Dec 2022 (58 comments)

I went out to talk with a guy, back in about 1995, in Chicago, fired up APL on his notebook, loaded the JPMorgan Risk Metrics data set, fiddled a bit, and reduced the thing to a one-line formula in an interactive window - like a Jupyter Notebook.

Well that opened a bit of a rabbit hole. Looking at the source code I saw this which made me chuckle:

    RNUMBER  DC    F'16807'            RANDOM NUMBER.

Surely we wouldn't have DailyWTF quality code here. Looking for how it's referenced we find the RANDOM function itself:

    *        RANDOM NUMBER GENERATOR.
    *        LEHMER'S METHOD, CACM JUNE '66, P 432.
    *        N(I+1) = P RES Q X N(I)
    *        P = (2**31)-1
    *        Q = 7**5

Following that lead to the original ACM article referenced:

https://dl.acm.org/doi/epdf/10.1145/365696.365712

Which further lead to this rather interesting article on the history of the ROLL function which relies on this random number generator:

https://www.jsoftware.com/papers/roll.htm

Neat!