Very interesting article. As a primarily OO programmer that's been playing more and more with functional concepts due to javascript, I definitely see the theoretical advantages mentioned in the article.

However, I also see that there are very hard parts to purely functional as well. For instance, to be entirely side-effect free your language has to be able to enforce immutability (which JS lacks). It's also hard (or at least not obvious) how to do certain types of logic flows in a purely functional.

For small domain spaces I can visualize how purely functional bottom-up programming effectively causes you to write a DSL for your problem.

However I don't have the experience/wisdom to understand if this holds true as the size of the domain increases. Basically, I have concerns that purely functional solutions don't scale (not from a performance perspective, but from an organizational/architectural one).

My experience with complex business logic makes me worried that complex new business rules would be difficult to implement in purely functional way and would cause frequent, painful refactoring.

I would love to hear from someone that's done a purely functional application with a large surface area that has been subjected to a real-world business environment to comment on the subject.