Chapter 6 Summary

In this chapter you finally got in touch with the notion of monad:

• Several implementations of the K-means clustering algorithm have been presented, starting with a handwritten one, then refining it using our own combinators, and finally creating versions using theState and RWS monads,
• Gathering information from incomplete data lead to define combinators for working with Maybe values in an easier way,
• From the similarities in the code using Maybe and using State the notion of monad was devised: a way to combine computations with some special characteristic, such as being able to fail or having an internal state,
• Monads are one of the most important constructions in Haskell and come with a custom syntax, called do notation, which you have studied in depth. This is the most used style of writing monadic code,
• You have seen several other monads: Reader, which holds a read-only context; Writer, which outputs a write-only value that is combined using a monoid structure; RWS, which combines the threeReader, Writer, and State monads; and ST, which implements controlled mutable variables,
• Apart from monads, in this chapter lenses have also been introduced as a way to query and update data structures in a common and powerful way.