1. Strategy Pattern

2. Strategy Pattern Intent Also known as “Policy” Motivating example
Define a family of algorithms, encapsulate each one,and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.
Also known as “Policy”
Motivating example
Different line break algorithms for varying types of text

3. SimUDuck example
Joe works for a company that makes a highly successful duck pond simulation game, SimUDuck. The game can show a large variety of duck species swimming and making quacking sounds.

4. Inheritance based design

5. Inheritance based design
Now RubberDuck can fly!

6. What about using Interface?

7. What about using Interface?
Duplicated code!
Now try changing fly().

8. Encapsulate Behaviors
Separate them out of the client
Define as interfaces

9. Delegate behaviors

10. Strategy Pattern

11. Design Principles
Identify the aspects of your application that vary and separate them from what stays the same.
Program to an interface, not an implementation.
Favor composition over inheritance
Has-A is better than Is-A

12. Strategy Pattern in FP
Different strategies are just different functions
Higher order functions (HOF)
Function is first -class, can live alone and be passed around
In OOP, method is not first-class, must be encapsulated in a class (“world of nouns”)

13. What about polymorphism?
Dispatch on type
Calling different functions according to the data type
In Clojure
Custom data type: defrecord
Custom behavior groups (interface): defprotocol
Don't support inheritance