1. What is Functiona Programming?
   1. Referential transparency: same input - same result
   2. Information about possible inputs and outcomes
2. Why functional programming?
3. Immutability
4. How to Deal with Side Effects
5. Exceptions and Readability
   1. 分支主題 1
   2. Validations not equal to Exceptional situation
   3. Always prefer return values over exceptions
   4. Primitive Obsession
      1. Wrap primitive types with separate classes
6. The Billion-dollar Mistake
   1. return null
7. Mitigating the Billion-dollar Mistake
   1. Maybe<T>
8. Summary