Slides from Introduction to Functional Programming with Scala talk by Derek Morr at the 2015 PSU WebConference

1. Derek Morr, Penn State
@derekmorr
Functional Programming with

2. Demo code is at
https://github.com/derekmorr/webconf2015

3. What’s your background?
● Java? C#? C++?
● Python, Ruby, JavaScript?
● Haskell, ML, Erlang, Lisp?
● F#? Scala? Clojure?
● Anything else?

5. Stack Overflow
Developer Survey
2015

6. Stack Overflow
Developer Survey
2015

7. Is this valid?
x = x + 1

8. x = x + 1
if x is 5, then:
5 = 5 + 1
5 = 6
no, 5 ≠ 6

10. Purity
// impure
var a = 1
def incr() = {
a += 1
}
// pure
def incr(a: Int) = {
a + 1
}

11. An example
find . -type f | grep foo | sort | uniq

12. What’s a side effect?
A lot of stuff we do now:
● Reassigning variables
● Modifying data in-place
● Throwing exceptions
● Interacting w/ the external environment

13. What’s it like?
Immutability
Value In, Value Out
Emphasis on data flow
Descriptive, not imperative

14. Immutability
OO makes code understandable by
encapsulating moving parts.
FP makes code understandable by
minimizing moving parts.
— Michael Feathers

15. An Example
def firstLarger(data: List[Int], threshold: Int): Int = ???

16. An Example
def firstLarger(data: List[Int], threshold: Int): Int = ???
val numbers = List(3, 2, 4, 7, 9, 8, 1)
firstLarger(numbers, 5) // returns 7
firstLarger(numbers, 10) // returns ?

17. Method Signatures & Error Signaling
public int firstLarger(final int[] data,
final int threshold) throws NoSuchElementException

18. Method Signatures & Error Signaling
/**
* ...
* @return null on error
*/
public Integer firstLarger(final int[] data, final int threshold)

19. “My Billion Dollar Mistake”
“I call it my billion-dollar mistake. It was the invention of the null
reference in 1965... This has led to innumerable errors,
vulnerabilities, and system crashes, which have probably
caused a billion dollars of pain and damage in the last forty
years.”
- Sir Tony Hoare (2009)
Null References: The Billion Dollar Mistake

20. Use the type system
Option[T]
Some(value: T) None

21. Option
sealed abstract class Option[T]
// wrapper around a value
final class Some[T](value: T) extends Option[T]
// Singleton marker for no value
// Not null. Safe to dereference.
object None extends Option

22. Scala Map
val people = Map("Frank" -> "N. Furter",
"Lady" -> "Gaga")
val frank = people.get("Frank")
frank: Option[String] = Some(N. Furter)
val bob = people.get("Bob")
bob: Option[String] = None

23. Signature
val numbers = List(3, 2, 4, 7, 9, 8, 1)
def firstLarger(data: List[Int], threshold: Int): Option[Int] = ???
firstLarger(numbers, 5) // Some(5)
firstLarger(numbers, 10) // None

24. Demo

25. A Detour - Functional Lists
List(1,2,3)

26. A Detour - Functional Lists
1 :: 2 :: 3 :: Nil
synonym for empty
list

27. A Detour - Functional Lists
1 :: 2 :: 3 :: Nil
pronounced “cons”

28. A Detour - Functional Lists
1 :: 2 :: 3 :: Nil

29. A Detour - Functional Lists
1 :: 2 :: 3 :: Nil

30. A Detour - Functional Lists
1 :: 2 :: 3 :: NilList(1,2,3) =

31. A Detour - Functional Lists
1 :: 2 :: 3 :: Nil
head
tail
(also a List)

32. A Detour - Functional Lists
1 :: 2 :: 3 :: Nil
head
tail

33. A Detour - Functional Lists
1 :: 2 :: 3 :: Nil
head
tail

34. A Detour - Functional Lists
1 :: 2 :: 3 :: Nil

35. A nice overview of Scala

36. If you want to learn FP - short

37. FP for OO folks

38. If you want to learn FP - long

39. Two good books
First 11 chapters online for free. Some chapters online for free.
40% off at
manning.com
w/ code tsfp14

40. Pattern Matching
Slides: Czech Scala Enthusiasts: Pattern
Matching and Case Classes (1h45)
See also their blog.

41. Questions?