The author introduces the concept of the “impureim sandwich,” which is a pattern in functional architecture where an impure entry point calls a pure function and uses another impure action to act on the result. They use the metaphor of a sandwich to explain how impure actions enable you to handle a pure function without getting your fingers greased. They provide examples in F# and C# to illustrate the pattern. The author acknowledges that they have been searching for a better name for this pattern but have settled on “impureim sandwich,” which can be pronounced as “impurium sandwich.” The author believes this pattern is essential in functional architecture.
https://blog.ploeh.dk/2020/03/02/impureim-sandwich/