http://stackoverflow.com/questions/2078978/functional-programming-vs-object-oriented-programming

• Object-oriented languages are good when you have a fixed set of operations on things, and as your code evolves, you primarily add new things. This can be accomplished by adding new classes which implement existing methods, and the existing classes are left alone.
• Functional languages are good when you have a fixed set of things, and as your code evolves, you primarily add new operations on existing things. This can be accomplished by adding new functions which compute with existing data types, and the existing functions are left alone.

http://www.codenewbie.org/blogs/object-oriented-programming-vs-functional-programming

In all programs, there are two primary components: the data (the stuff a program knows) and the behaviors (the stuff a program can do to/with that data). OOP says that bringing together data and its associated behavior in a single location (called an “object”) makes it easier to understand how a program works. FP says that data and behavior are distinctively different things and should be kept separate for clarity.

1. where to store the data:
   1. oop - object
   2. fp - hashmap, array of arrays
2. This idea of not changing the contents (or “state”) of a variable once it’s been created is called immutability and is another key aspect of FP. In the OOP version, the contents of the employees array changes over time: before change_salary is applied the salaries are 100K and 125K, but afterwards they are 110K and 135K! This means we don’t always know which value we’ll have at any given point in the program: we have to walk through the program flow and find out if change_salary has been called or not. This can be difficult as the program increases in complexity. The FP version avoids this because employees represents the “before” state and happier_employees represents the “after” state. Their values are always the same over their entire existence.

Our example is “data about people” -- since we’re changing the salary directly -- so the FP version is shorter and simpler. If we described a feature like “employee requests time off” that requires a more-complicated interaction with the data -- likely creating a “time-off request” object attached to the employee -- then OOP might be a better fit.