> To write the code, one looks at what data will go into the software, and what data will need to come out.

What if your program has no output? The applications I write seldom has. Instead they save down stuff or send it away here and there during the lifespan of the program. When they're done they just exit without any output. FP seems to be focused on one input going through transformations ending up in one output.

(I'm not a good programmer and barely know how to do OOP, much less FP.)