I really need to sit down and write that essay, but I always have better things to write and design essays are difficult to sell. A lot of the FP programming that goes on is just weak OOP with some immutability, closures, and list comprehensions. The real hardcore FP avoids not only mutation, but also aliases and identifiable objects. Object thinking is impossible in that context because nothing can have a name (beyond rolling your own GUIDs); it really is different.