The other thing this article seems to miss is the level of abstraction at which FP applications need to live. At the OS level, where state is being managed, using FP would be insane. I don't think anyone really would want to do that. The UNIX philosophy of piping along a stream, while it may have something in common with FP, it still exists at a higher level of abstraction than the kernel, which is still a giant, complex Turing machine.