In the previous post, we looked at the Reader/Writer monads and comonads, and discussed in general what comonads are and how they relate to monads. This time around, we’re going to look at some more comonads, delve briefly into adjunctions, and try to get some further insight into what it all means.

Since a comonad has to have a counit, it must be “pointed” or nonempty in some sense. That is, given a value of type W[A] for some comonad W, we must be able to get a value of...