That FP subtracts from everything else can be switched around pretty easily. If you, as a client, can prove that you do not mutate the data structure that I han

That FP subtracts from everything else can be switched around pretty easily. If you, as a client, can prove that you do not mutate the data structure that I hand back to you, then I don't have to worry about you corrupting my alias of it, and I don't have to worry about giving you a deep-copy instead of a direct reference. Pure functional programming subtracts something from your expressive power, but it also subtracts from the assumptions you have to make about code. Less expressive power is a bad thing, but fewer assumptions are a good thing. The inequality of assumption versus conclusion is right there in the functional arrow. I'm not sure I can easily work the vegetarian analogy into that. Vegetarians notoriously frustrate catering, while I think people who agree never to mutate data are probably easier for programmers to cater for.