In this chapter, we looked at monads as a way of sequencing computations. We studied how the meaning of this sequencing changes among the different monads we've implemented. The Id just composes computations as is. The Option adds a possibility to stop with no result if one of the steps returns no result. Try and Either have semantics similar to Option but allow you to specify the meaning of no result in terms of an Exception or as a Left side of Either. The Writer makes an append-only log available for computation in the chain. The Reader provides some configuration to every computation step. The State carries a mutable state between actions.

We discussed how the two primitive methods defining a monad, unit and flatMap, allow you to implement other useful methods such as map, map2, and apply, thus proving that every monad is a functor and an applicative.

In terms of map and flatMap—as for-comprehensions—we defined some small business logic to steer a boat. We then demonstrated how this logic can be reused without changes, even if the implementation of the underlying monad was reshaped.