Yeah, and---literally, not to be snarky---times change. FP isn't a technical definition. It's a cultural, family-resemblance definition that's driven by some m

Yeah, and---literally, not to be snarky---times change. FP isn't a technical definition. It's a cultural, family-resemblance definition that's driven by some mixture of fashion and genuine exploration into a niche of ideas in computing and formal languages. The best you could hope for would be a set of shared values of the FP culture and then justifications for why various things that call themselves FP feel that are upholding those values. Even with a definition like this though, the values will shift and waver over time. Here's my FP values: * Simple over easy. Rich Hickey's old egg. Favor abstractions with few moving parts, reduced interactions, and more parsimonious overall models over ones driven by metaphor or target use case alone. Result is tools which are perhaps harder to get started using but have better complexity scaling properties. * Mathematics has been doing this a long time. PLs are just formal languages and mathematicians have been working seriously on formal languages for at least 150 years: steal their ideas. From this we get ideas around comparative linguistics, semantics, various proof/reasoning mechanisms and types. * Readability counts, but not just in the sense of syntax. Really this means "legibility" or even "static legibility". It's another hint toward types, immutability, and general state space reduction. It's a strong push away from "emergent behavior" to the greatest degree possible. Things should endeavor to do what they say on the tin... and to to the greatest degree reasonable say things on the tin in such a way that the information is available from the get-go. * Tradeoffs between power-to-construct and power-to-analyze. This is everywhere in formal languages/PL, but it's especially strong in FP since there's a focus on (simple over easy) semantics. It opens the door for there to be mathematical semantics as opposed to just operational semantics and this makes for rich opportunities for on-the-tin static reasoning (equational reasoning, changing interpreters, embedded DSLs). I think these values more or less were in place back in the old Lisp-y FP days, but they really are taken to new places by "more modern" takes on FP.