The APL Programming Language Source Code

Thousands of programming languages were invented in the first 50 years of the age of computing. Many of them were similar, and many followed a traditional, evolutionary path from their predecessors. What eventually became APL was first a mathematical notation, not as a computer programming language.

CHM
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.
I used it when I was in college for my Senior Project. That would have been 1978/1979. I had a keyboard with the APL symbols molded onto the keypad.