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 Chapter 20 Outline
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* 20.1 Foldable
* 20.2 The Foldable class
* 20.3 Revenge of the monoids

  * 20.3.1 And now for something different

* 20.4 Demonstrating Foldable instances

  * 20.4.1 Identity
  * 20.4.2 Maybe

* 20.5 Some basic derived operations

  * 20.5.1 toList
  * 20.5.2 null
  * 20.5.3 length
  * 20.5.4 elem
  * 20.5.5 maximum and minimum
  * 20.5.6 sum and product
  * 20.5.7 Exercises: Library Functions

    * 1
    * 2
    * 3
    * 4
    * 5
    * 6
    * 7
    * 8
    * 9
    * 10

* 20.6 Chapter Exercises

  * 1
  * 2
  * 3
  * 4
  * 5
  * 6

* 20.7 Follow-up resources

  * Jakub Arnold; Foldable and Traversable. (Blog post)