"Declarative programming" at its core seems to be about separating what the code says from what it does, and trying to put more "magic" in between the saying and the doing. But that's not a binary distinction for all kinds of reasons. What a code "says" is already a bit subjective to start with, but then we get into things like, is this "declarative"?
def SayHello():
print("Hello!")
SayHello()
After all, when I type "SayHello()", I'm not specifying
how I want Hello to be Said, just that I want it said. Yet, of course, if this is called "declarative" than the word is stripped of all meaning, and we do clearly want it to mean something.
Personally I'd set the line as being somewhere where by definition of the system, there is a barrier set up and you are no longer allowed to "see into" the implementation because it's going to do things so crazy to your declaration that allowing you to see into it would ruin it. e.g. in SQL when you send in your query the query engine is reserving the right to rewrite it in absolutely crazy ways, and in principle, you have no right to ask about what it actually did as long as you get the "right answer". This is because I then like to segue into how that barrier is somewhat illusory because it's always going to take some concrete actions and you may have need to see into those actions. But that isn't the only place to draw the line, it's just where I find it convenient to draw the line so I can ride a pet hobbyhorse. There isn't really a bright line, even though there is clearly some real difference we are trying to talk about.