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 OverflowDeveloper Survey 2015

Stack OverflowDeveloper Survey 2015

Is this valid?

x = x + 1

x = x + 1

if x is 5, then:

5 = 5 + 15 = 6

no, 5 ≠ 6

Purity

// impure var a = 1def incr() = { a += 1}

// puredef 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?

ImmutabilityValue In, Value OutEmphasis on data flowDescriptive, not imperative

Immutability

OO makes code understandable by encapsulating moving parts.

FP makes code understandable byminimizing 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 valuefinal 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

Demo

A Detour - Functional Lists

List(1,2,3)

A Detour - Functional Lists

1 :: 2 :: 3 :: Nil

synonym for empty list

A Detour - Functional Lists

1 :: 2 :: 3 :: Nil

pronounced “cons”

A Detour - Functional Lists

1 :: 2 :: 3 :: Nil

A Detour - Functional Lists

1 :: 2 :: 3 :: Nil

A Detour - Functional Lists

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

A Detour - Functional Lists

1 :: 2 :: 3 :: Nil

head

tail(also a List)

A Detour - Functional Lists

1 :: 2 :: 3 :: Nil

head

tail

A Detour - Functional Lists

1 :: 2 :: 3 :: Nil

head

tail

A Detour - Functional Lists

1 :: 2 :: 3 :: Nil

A nice overview of Scala

If you want to learn FP - short

FP for OO folks

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.

Questions?