My answers to the challenges in Bartosz Milewski's excellent Category Theory for Programmers
Chapter 1 - Category: The Essence of Composition
Chapter 2 - Types and Functions
Chapter 3 - Categories Great and Small
Chapter 4 - Kleisli Categories
Chapter 5 - Products and Coproducts
Chapter 6 - Simple Algebraic Data Types
Chapter 9 - Function Types: no challenges
Chapter 10 - Natural Transformations
Chapter 11 - Declarative Programming: no challenges
Chapter 12 - Limits and Colimits