Functional style does not allow for 'evolving' a value in multiple steps. In some sense this is the heart of the difference between functional style and other

Functional style does not allow for 'evolving' a value in multiple steps. In some sense this is the heart of the difference between functional style and other styles of programming. Language features which are only possibly used in a non-functional style make me want to say that describing the language as 'functional' is either wrong or saying a language is 'functional' is a meaningless claim. If every language is functional, saying it is completely redundant. I like your idea of saying FP style is idiomatic in that language might qualify the language as being described as 'functional', which I think agrees with my intuition that mutable strings being built in is evidence that calling that language 'Functional' is misleading or silly.