[mattgreenrocks]

I know this is written as OO, but it is very functional in style. Keeping IO as far away from entities and business rules goes a very long way in easing maintenance. However, sometimes you need to do IO in your business rules, which necessitates interfaces for mocking.

IMO, this general idea is very powerful for most code. I believe algebraic effect systems will popularize it further by making it a natural pattern. While OO should be written in this way, we are currently in this weird period of programming history where we don't talk about practices and design as seriously as we should.

But, I'm glad you mentioned this approach. It is not hard to understand, it is elegant, and it is as simple as can be. Just requires a tiny amount of glue and forethought.