The problem is that mainstream static types systems have two massive holes in them: nulls and pervasive mutability. Knowing that a list has length n at compile

The problem is that mainstream static types systems have two massive holes in them: nulls and pervasive mutability. Knowing that a list has length n at compile time isn't that useful if you're expecting the program to change the list's length at runtime. Or maybe there was never a list in the first place because it was null.