This is the seminal work by John Backus that introduced the idea of functional programming style.

I met John when I was a mainframe developer at IBM. Although he didn't live to see the advent of AI agents, I think he'd push even harder for functional style now, in the age of AI assisted programming. While harder to write initially than imperative or object oriented code, functional is more bulletproof in practice and "frees us" from the problems of those other styles, as he tries to explain in the paper.

Buried in this paper is a mention of "Applicative State Transition Systems", a programming paradigm that never caught on.