Expression-Oriented evokes purity in that expressions have external facing properties which can be understood and even kept in the type system. If you use globa

Expression-Oriented evokes purity in that expressions have external facing properties which can be understood and even kept in the type system. If you use global mutation and you don't keep track of it in the type system you can't say much about those properties anymore. You can however use mutation locally and maintain your referential transparency which is a nice way to get around the limits of some not-so-functional-friendly algorithms. I'm planning on writing about that next time around because my tracking shows barely anyone gets to the end of an article beyond a certain length.