At some point you have to start being precise about what you mean by memory leak -- how explicit does your code have to be in how it burns its way through memory? A strictly evaluated functional language (if not a language that lacks dynamic allocation entirely) will have no cycles, but you could still blow through memory with infinite recursion. Suppose you prohibit non-tail-call recursion. You could still write code that grows a list as it executes other logic. But it's hard to do so "accidentally."