> Pure Functions are an implementation strategy of FP, not the core differentiator.
"Functional Programming" is not a well-defined term. But I would argue that if you are not using pure functions then you have more of a hybrid imperative/functional language, which is fine, especially if you are careful with how you structure your effects. Modern FP languages use monads (Haskell) and/or algebraic effects (Eff) to enable pure functions to describe effects. The pure function is what enables mathematical reasoning and safe function composition.
I don’t think there is a boolean pureness flag, rather a spectrum.
Haskell can also introduce arbitrary side effects in any code with some unsafe escape hatches, but sure, the common case doesn’t employ that, so it is a bit more pure than Clojure.
"Functional Programming" is not a well-defined term. But I would argue that if you are not using pure functions then you have more of a hybrid imperative/functional language, which is fine, especially if you are careful with how you structure your effects. Modern FP languages use monads (Haskell) and/or algebraic effects (Eff) to enable pure functions to describe effects. The pure function is what enables mathematical reasoning and safe function composition.