• HPFP §20: Foldable
• Consider writing Foldable for data Tree a = Leaf | Node (Tree a) a (Tree a). What happens if you change the order the elements are mappended in?

• HPFP §21: Traversable
• Arnold, Foldable and Traversable
• Jaskelioff, An Investigation of the Laws of Traversals

• HPFP §22: Reader
• O'Sullivan et. al., Real World Haskell: Programming With Monads
• McBride and Patterson, Applicative Programming with Effects
• Marlow et. al., Desugaring Haskell's do-Notation into Applicative Operations, particularly §1, 2, 5, & 6

• HPFP §24: Parser Combinators
• Billy Rhoades, Parser Combinators (Slides detailing practical examples of using parser combniators)
• Yorgey et. al., Typeclassopedia: Failure and choice: Alternative, MonadPlus, ArrowPlus
• Fenwick et. al., Confused by the meaning of the 'Alternative' type class and its relationship to other type classes - Stack Overflow
• Geli, Two kinds of backtracking

• HPFP §25: Composing Types
• HPFP §26: Monad Transformers

Lenses:

• Gabriel Gonzales, Program Imperatively using Haskell Lenses
• Aditya Bhargava, Lenses in Pictures
• Russell Matney, Haskell Lens Operator Onboarding
• Joseph Abrahamson, A Little Lens Starter Tutorial
• Artyom Kazak, lens over tea #1: lenses 101, traversals 101, and some implementation details
• Edward Kmett, Lenses, Folds, and Traversals

Codata:

• Michael Maloney, Data vs. Codata
• Downen et. al., Codata in Action, §1 & 2

Lists:

• Johannes Waldmann, When You Should Use Lists in Haskell (Mostly, You Should Not)

The amb Operator:

• Erik Kidd, McCarthy's Ambiguous Operator
• (Haskell Wiki), Amb
• John McCarthy, A Basis For A Mathematical Theory of Computation (see section on "Ambiguous Functions")
• Erik Kidd, Build Your Own Probability Monads

Halloween:

• Matt Noonan, Ghosts of Departed Proofs