Functional programming is characterized by immutable data and functions without side effects. It uses techniques like first class functions, pattern matching, and higher order functions. In a purely functional style, functions have no side effects and return the same output for the same input. Common data structures like lists are immutable and operations on them return new instances rather than mutating the original. Scala supports both immutable and mutable collections and Scala 2.8 added features like default and named parameters to make functional programming more convenient. Strive for purity when possible for benefits like easier parallelization, concurrency, debugging and testing.

1. Functional programming

2. “Functional programming”
First class functions
Pattern matching
Higher order functions

3. Functional programming
Purity
Mathematical functions have no side effects
f(x) = 2x + 3
y = sin(x)
Only immutable objects (and object graphs)

4. In practice
All field must be immutable
All methods must return something
No side-effects from method calls

5. List
head :: tail
Prepends to the head of the list
Other operations (filter, map, remove, partition...)
returns a new List instance

6. scala.collection.immutable
...or scala.collection.mutable
Pick your poison!

7. Scala 2.8
Case classes get copy methods
Constructors and methods get
default parameters
named parameters

8. Strive to be pure
Parallelization and concurrency (no synchronization)
Usually easier to find errors (a lot less state)
Easier to test