In chapter 11 of our book, we talk about monads in Scala. This finally names a pattern that the reader has seen throughout the book and gives it a formal structure. We also give some intuition for what it means for something to be a monad. Once you have this concept, you start recognizing it everywhere in the daily business of programming.

Today I want to talk about comonads, which are the dual of monads. The utility of comonads in everyday life is not quite as immediately obvious as that of...