Positivism's actually quite interesting from a game design point of view --- because you can only see in front of you, you can't tell where your snake is or how big it is; so you're playing blindly, waiting for that moment when you get long enough that you start glimpsing parts of your own tail coming out of the darkness... it genuinely adds new interest to a really tired old game.

(Although, UI-wise, I'd like the play area to distinguish between spaces which you can see but don't contain anything and spaces which you can't see.)