> 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.

I worked at a betting exchange where the backend was written entirely in erlang. There were a lot of areas in the code where we had to deal with incredibly complicated business rules.

For example, we were not allowed to allow customers to gamble money they didn't have in their account but we did want to allow them to sell out of a position. This meant keeping track their minimum possible gains from all their current positions to figure out how much money they could safely be allowed to use. Money laundering regulations meant we also had to keep track of where different pools of money came from (eg different bank accounts, paypal etc) and these pools could not be allowed to mix (eg you can't deposit money into the site using paypal and then withdraw it into your bank account). Users are allowed to pick their own odds so we always had to be very careful whether money gets rounded up or down. And then there were promotions giving extra money for new members or for large deposits and any bets had to be taken from bonus pools before other money pools.

Functional programming is a very overloaded term so I will talk about specific things that made this easier. I personally find it much easier to construct complex data structures and maintain invariants on them when using algebraic types + pattern matching rather than objects. The use of immutable data structures made it much easier to verify that no operation ever changed the total amount of money in the system and to safely implement highly concurrent systems. First class functions were used throughout the code but we probably could have replaced most uses by objects with little change to the overall structure.

I think that in general OO conflates the management of code and the management of runtime state. In erlang modules are the unit of code management and processes the unit of state management. I would also point out that an entirely purely functional program is impractical with current software and probably not desirable in general.

Bear in mind that most of the advocacy for functional programming tends to from people who have only just encountered the idea and are extremely excited. There are much more nuanced viewpoints available if you dig a little eg Rich Hickey's ideas on the separation of state and identity: http://wiki.jvmlangsummit.com/images/a/ab/HickeyJVMSummit200...