Here are a few concepts that weren’t discussed in the main text of this course but could still prove useful.
The Writer monad
The Writer monad is another classic monad. Whereas Reader was used to retrieve values at the right time, Writer is used to store them away. The most common example use case for Writer is gathering log messages.
The These monad
The These monad has some similarities with the Either monad in that it has a left and a right value. However, while Either doesn’t handle its left value, These processes both its left, its right, or both at the same time.