Intro to Functional Programming with Scala - #psuweb

Derek Morr, Penn State
@derekmorr
Functional Programming with

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

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

Stack Overflow
Developer Survey
2015

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

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

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

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

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

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

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

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 ?

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

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

“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

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

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

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

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

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

1 :: 2 :: 3 :: Nil
synonym for empty
list

1 :: 2 :: 3 :: Nil
pronounced “cons”

1 :: 2 :: 3 :: Nil

1 :: 2 :: 3 :: NilList(1,2,3) =

1 :: 2 :: 3 :: Nil
head
tail
(also a List)

1 :: 2 :: 3 :: Nil
head
tail

If you want to learn FP - short

If you want to learn FP - long

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

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

Intro to Functional Programming with Scala - #psuweb

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