> the only real reasonless difference is string functions are haystack / needle, whereas array functions are needle / haystack

There's a trick to this that isn't too hard to remember. Searching for a value in an array is much more akin to searching for a needle in a haystack.

So, for array functions, it's like searching for a needle in a haystack. For string functions, you're searching in a haystack for a needle.

I'm not really commenting on whether one should have to remember this, only that it helps if you need to remember it.