There's a widening generational gap in what "functional programming", in a practical sense, means. "Back in the day" a language got called 'functional' if it had anonymous closures, upwards funargs, tail calls, you could implement a decent `reduce`, etc. Today it means purity and immutability, monadic comprehensions, and type systems to support all that.

Go's the first kind of functional language. But so is almost everything else, even Java, now.