If you start learning about functional programming, it won't be long before you come across the idea of pure functions. And as you go on, you will discover functional programmers appear to be obsessed with them. “Pure functions let you reason about your code,” they say. “Pure functions are less likely to start a thermonuclear war.” “Pure functions give you referential transparency”. On and on it goes. And they have a point. Pure functions are a good thing. But what do you do with the impure bits of your code?