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 Chapter 21 Outline
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* 21.1 Traversable
* 21.2 The Traversable type class definition
* 21.3 sequenceA
* 21.4 traverse

  * 21.4.1 mapM is traverse

* 21.5 So, what's Traversable for?
* 21.6 Morse code revisited
* 21.7 Axing tedious code
* 21.8 Do all the things

  * 28.8.1 Strength for understanding

* 21.9 Traversable instances

  * 21.9.1 Either
  * 21.9.2 Tuple

* 21.10 Traversable Laws

  * traverse

    * Naturality
    * Identity
    * Composition

  * sequenceA

    * Naturality
    * Identity
    * Composition

* 21.11 Quality Control
* 21.12 Chapter Exercises

  * 21.12.1 Traversable instances

    * Identity
    * Constant
    * Maybe
    * List
    * Three
    * Pair
    * Big
    * Bigger
    * S

  * 21.12.2 Instances for Tree

    * hints

* 21.13 Follow-up resources

  * Jakub Arnold. Foldable and Traversable
  * Jeremy Gibbons and Bruno C. d. S.
    Oliveira. The Essence of the Iterator
    Pattern.
  * Conor McBride and Ross Paterson.
    Applicative programming with effects.