Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

An Introduction to Functional Programming

Andreas Pauley – @apauley

Jozi JUG

July 28, 2014

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

@lambdaluminary

We meet once a month, on the second Monday of the month.

http://www.meetup.com/lambda-luminaries/

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Jemstep

Retirement portfolio analysis in Scala.

http://www.jemstep.com/

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Who likes FP?

“ And that’s why I think functionalprogramming is a natural successor toobject-oriented programming. ”

— Dave Thomas, Pragmatic Programmers [2]

“ Without understanding functionalprogramming, you can’t invent MapReduce ”

— Joel Spolsky [3]

“ ...it’s almost certainly true that functionalprogramming is the next big thing. ”

— Uncle Bob Martin [4]

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

“ No matter what language you workin, programming in a functional styleprovides benefits. You should do itwhenever it is convenient, and youshould think hard about the decisionwhen it isn’t convenient. ”

— John Carmack, ID Software [5]

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

But what exactly is

“Functional Programming”?

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

A simple programming example

Say I give you an array of integers, and I want everyone of them doubled.

How would you write it?

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

A simple programming example

I have this:

[1,3,6]

I want this:

[2,6,12]

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

A simple programming example

public static void doubleAll(int[] numbers) {

for (int i=0; i < numbers.length; i++) {

numbers[i] = numbers[i] * 2;

}

}

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

A simple programming example

Sorry, I forgot to mention that you cannot doin-place mutation on the list of numbers I give you.

Please try again.

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

A simple programming exampleOur first try had a void return type:

public static void doubleAll(int[] numbers) {

for (int i=0; i < numbers.length; i++) {

numbers[i] = numbers[i] * 2;

}

}

Now we have to return a result:

public static int[] doubleAll(int[] numbers) {

int[] nums2 = new int[numbers.length];

for (int i=0; i < numbers.length; i++) {

nums2[i] = numbers[i] * 2;

}

return nums2;

}

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

A simple programming example

Looking much better, but you can’t mutate anycollection, appending to the new array won’t work.

Also, you can’t vary your variables (i++).

Please try again.

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

A simple programming example

???

public static int[] doubleAll(int[] numbers) {

int[] nums2 = new int[numbers.length];

for (int i=0; i < numbers.length; i++) {

nums2[i] = numbers[i] * 2;

}

return nums2;

}

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Let’s try giving a definition of

Functional Programming

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Functional Programming, noun:

Functional Programming is a

list of things you CAN’T do.

[10]

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

You can’t vary your variables

1> X = 42.

42

2> X = X + 1.

** exception error:

no match of right hand

side value 43

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

No while/for loops. Sorry :-(

int i;

for (i=1; i<=3; i++) {

System.out.println(i);

}

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

You can’t mutate/change your data structures

Python

>>> list1 = [1,2,3]

>>> list2 = list1

>>> print list1.reverse()

None

>>> list1

[3, 2, 1]

>>> list2

[3, 2, 1]

Haskell

> let list1 = [1,2,3]

> let list2 = list1

> reverse(list1)

[3,2,1]

> list1

[1,2,3]

> list2

[1,2,3]

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

You can’t have any side e↵ects

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Are you kidding me?

How can anyone program like this???

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

GOTO 10

This sounds like

“You can’t have GOTO statements”

See Hughes and Dijkstra [1, 6]

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

It’s not about what we cannot

do.

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

We need a better definition of

Functional Programming.

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Programming Paradigms(Very Simplified)

Imperative Declarative

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Functional Programming, noun:

“ Functional programming is so called because aprogram consists entirely of functions. ”

— John Hughes, Why Functional Programming Matters [1, p. 1]

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

OK... so what exactly is a function?

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

An example function

f (x) = 2x2 � 2x + 3

�1 1 2 3

4

6

8

10

12

14

x

y

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Variables in functions

f (x) = 2x2 � 2x + 3When we evaluate the function:f(2) = 8� 4 + 3 = 7

• The value of x will not change inside the function body.

• Same input, same output. Every time. (ReferentialTransparency)

• We can call f multiple times without any side e↵ects.

• We don’t have to recalculate f(2), we can replace anyoccurrence of f(2) with 7.

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Variables in functions

f (x) = 2x2 � 2x + 3When we evaluate the function:f(2) = 8� 4 + 3 = 7

• The value of x will not change inside the function body.

• Same input, same output. Every time. (ReferentialTransparency)

• We can call f multiple times without any side e↵ects.

• We don’t have to recalculate f(2), we can replace anyoccurrence of f(2) with 7.

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Variables in functions

f (x) = 2x2 � 2x + 3When we evaluate the function:f(2) = 8� 4 + 3 = 7

• The value of x will not change inside the function body.

• Same input, same output. Every time. (ReferentialTransparency)

• We can call f multiple times without any side e↵ects.

• We don’t have to recalculate f(2), we can replace anyoccurrence of f(2) with 7.

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Variables in functions

f (x) = 2x2 � 2x + 3When we evaluate the function:f(2) = 8� 4 + 3 = 7

• The value of x will not change inside the function body.

• Same input, same output. Every time. (ReferentialTransparency)

• We can call f multiple times without any side e↵ects.

• We don’t have to recalculate f(2), we can replace anyoccurrence of f(2) with 7.

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Varying variables does not make sense

x = x + 1

x� x = 1

0 = 1

) x 6= x + 1

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Values are functions

Constant values are just functions with no input parameters

x = 42

Python function definition:

def x():

return 42

Haskell function definition:

x = 42

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Functions can be composed

h(x) = (f � g)(x) = f (g(x))

or

h = f � g

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Higher-order Functions

Functions can take functions as input.Functions can return functions as the result.

h(f, g, x) = f (x) + g(2)

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Higher-order FunctionsThe derivative of f(x) returns another function.

f(x) = 2x2 � 2x+ 3d

dx

f(x) = 4x� 2

�1 1 2 3

�5

5

10

15

x

y

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

A functional program consists entirely of functions

def main():

time = datetime.now()

args = sys.argv[1:]

print outputString(time, args)

def outputString(time, args):

return str(time) + " " + joinArgs(args)

def joinArgs(args):

return "-".join(args)

$ ./justfunctions.py Hello from Python

2014-02-15 10:36:42.062697 Hello-from-Python

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

A functional program consists entirely of functionsmain :: IO()

main = do

time <- getCurrentTime

args <- getArgs

putStrLn (outputString time args)

outputString :: UTCTime -> [String] -> String

outputString time args =

show(time) ++ " " ++ joinArgs(args)

joinArgs :: [String] -> String

joinArgs = intercalate "-"

$ ./justfunctions Hello from Haskell

2014-02-15 08:36:50.822728 UTC Hello-from-Haskell

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Some Haskell Syntax

Python:

def outputString(time, args):

return str(time) + " " + joinArgs(args)

Haskell:

outputString :: UTCTime -> [String] -> String

outputString time args =

show(time) ++ " " ++ joinArgs(args)

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Recursive function: Haskell

doubleAll :: [Int] -> [Int]

doubleAll [] = []

doubleAll (x:xs) = x*2 : doubleAll xs

Example use in the interactive interpreter:

Prelude Main> doubleAll [8,2,3]

[16,4,6]

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Recursive function expanded

doubleAll [] = []

doubleAll (x:xs) = x*2 : doubleAll xs

doubleAll [8,2,3]

16 : (doubleAll [2,3])

16 : 4 : (doubleAll [3])

16 : 4 : 6 : (doubleAll [])

16 : 4 : 6 : []

16 : 4 : [6]

16 : [4,6]

[16,4,6]

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Recursive function: Python

def doubleAll(numbers):

if numbers == []:

return []

else:

first = numbers[0]

rest = numbers[1:]

return [first * 2] + doubleAll(rest)

Example use in the interactive interpreter:

>>> doubleAll([8,2,3])

[16, 4, 6]

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Pattern Matching: Haskell

doubleAll :: [Int] -> [Int]

doubleAll [] = []

doubleAll (x:xs) = x*2 : doubleAll xs

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Higher-order Functions3 Basic List Operations

Map Convert each element of a list into some other value.

Filter Get a subset of a list based on some condition.

Fold Reduce a list of items to a single value.

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Higher-order Functions3 Basic List Operations

Map Convert each element of a list into some other value.

Filter Get a subset of a list based on some condition.

Fold Reduce a list of items to a single value.

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Higher-order Functions3 Basic List Operations

Map Convert each element of a list into some other value.

Filter Get a subset of a list based on some condition.

Fold Reduce a list of items to a single value.

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Map

Apply a function to each element of a list, and you get a new list.

a1 a2 a3 ...

an

b1 b2 b3 ...

bn

f(a1) f(an)

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Map

doubleAll :: Num a => [a] -> [a]

doubleAll = map (*2)

8 2 3

16 4 6

⇤2 ⇤2 ⇤2

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Account Data

data Bank = ABSA | Capitec | FNB | Nedbank | SBSA

data Account = Account {bank :: Bank,

accNum :: String,

owner :: String,

balance :: Amount}

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Account Data

[Account {accNum="4076814233",owner="J. Doe",balance=(Amount 123000.23),bank=ABSA},

Account {accNum="6868773585",owner="J. Black",balance=(Amount 5782347.99),bank=FNB},

Account {accNum="4055892156",owner="A. Kay",balance=(Amount 100),bank=ABSA},

Account {accNum="6584539813",owner="S. Jones",balance=(Amount 2937361.45),bank=FNB}]

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Map

Map on account data:

balances :: [Account] -> [Amount]

balances accounts = map balance accounts

acc1 acc2 acc3 acc4 acc5

bal1 bal2 bal3 bal4 bal5

balance(acc1) balance(acc5)

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Filter

acc1 acc2 acc3 acc4 acc5

acc1 acc3 acc5

in? in? in? in? in?

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Filter

Filter on account data:

topAccounts :: [Account] -> [Account]

topAccounts accounts = filter isRich accounts

isRich :: Account -> Bool

isRich acc = balance acc >= (Amount 1000000)

Output:

*Main> topAccounts accounts

[FNB 6868773585 (J. Black) R5782347.99,

FNB 6584539813 (S. Jones) R2937361.45]

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Fold/Reduce/Inject

foldl (+) 0 [8,2,3]

13

0 8 2 3

8 2 3

10 3

13

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Fold/Reduce/InjectbalanceSum :: [Account] -> Amount

balanceSum accounts = foldl (+) 0 (balances accounts)

0 bal1 bal2 bal3

sum

bal2 bal3

sum

bal3

sum

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Function CompositionbalanceSum :: [Account] -> Amount

balanceSum accounts = foldl (+) 0 (balances accounts)

0 bal1 bal2 bal3

sum

bal2 bal3

sum

bal3

sum

s2 :: [Account] -> Amount

s2 = balances |> (foldl(+)0)

h(x) = (f � g)(x) = f(g(x))

h = f � g

s3 = f1 � balances

s3 :: [Account] -> Amount

s3 = (foldl(+)0) . balances

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

balancesPerBank

foldl insertBalance Map.empty accounts

{} acc1 acc2 acc3

map

acc2 acc3

map

acc3

map

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Fold

type BankMap = Map Bank Amount

balancesPerBank :: [Account] -> BankMap

balancesPerBank = foldl insertBalance Map.empty

Output:

*Main> balancesPerBank accounts

fromList [(ABSA,R123100.23),(FNB,R8719709.44)]

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Fold

type BankMap = Map Bank AmountbalancesPerBank :: [Account] -> BankMapbalancesPerBank = foldl insertBalance Map.empty

insertBalance :: BankMap -> Account -> BankMapinsertBalance bankmap account =

Map.insert key value bankmapwhere key = bank account

value = addBalance bankmap account

addBalance :: BankMap -> Account -> AmountaddBalance bankmap account =

case (Map.lookup (bank account) bankmap) ofNothing -> balance accountJust bal -> (balance account) + bal

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Programming Paradigms(Very Simplified)

Imperative Declarative

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

In imperative programming:

• Your variables can vary any time!

• You have to use locks to be thread-safe!

• You have to write your own loops for the most basic listoperations!

• Your data structures are mutable!

• You have to defensively make copies of data to prevent bugs!

• You have to defensively check for null values!

• You have to think about implicit state! (this, self)

• Code is generally riddled with side e↵ects!

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

In imperative programming:

• Your variables can vary any time!

• You have to use locks to be thread-safe!

• You have to write your own loops for the most basic listoperations!

• Your data structures are mutable!

• You have to defensively make copies of data to prevent bugs!

• You have to defensively check for null values!

• You have to think about implicit state! (this, self)

• Code is generally riddled with side e↵ects!

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

In imperative programming:

• Your variables can vary any time!

• You have to use locks to be thread-safe!

• You have to write your own loops for the most basic listoperations!

• Your data structures are mutable!

• You have to defensively make copies of data to prevent bugs!

• You have to defensively check for null values!

• You have to think about implicit state! (this, self)

• Code is generally riddled with side e↵ects!

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

In imperative programming:

• Your variables can vary any time!

• You have to use locks to be thread-safe!

• You have to write your own loops for the most basic listoperations!

• Your data structures are mutable!

• You have to defensively make copies of data to prevent bugs!

• You have to defensively check for null values!

• You have to think about implicit state! (this, self)

• Code is generally riddled with side e↵ects!

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

In imperative programming:

• Your variables can vary any time!

• You have to use locks to be thread-safe!

• You have to write your own loops for the most basic listoperations!

• Your data structures are mutable!

• You have to defensively make copies of data to prevent bugs!

• You have to defensively check for null values!

• You have to think about implicit state! (this, self)

• Code is generally riddled with side e↵ects!

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

In imperative programming:

• Your variables can vary any time!

• You have to use locks to be thread-safe!

• You have to write your own loops for the most basic listoperations!

• Your data structures are mutable!

• You have to defensively make copies of data to prevent bugs!

• You have to defensively check for null values!

• You have to think about implicit state! (this, self)

• Code is generally riddled with side e↵ects!

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

In imperative programming:

• Your variables can vary any time!

• You have to use locks to be thread-safe!

• You have to write your own loops for the most basic listoperations!

• Your data structures are mutable!

• You have to defensively make copies of data to prevent bugs!

• You have to defensively check for null values!

• You have to think about implicit state! (this, self)

• Code is generally riddled with side e↵ects!

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

In imperative programming:

• Your variables can vary any time!

• You have to use locks to be thread-safe!

• You have to write your own loops for the most basic listoperations!

• Your data structures are mutable!

• You have to defensively make copies of data to prevent bugs!

• You have to defensively check for null values!

• You have to think about implicit state! (this, self)

• Code is generally riddled with side e↵ects!

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Are you kidding me?

How can anyone program like this???

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Unit tests

Start writing the unit tests for

your existing code in a

functional programming

language

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Join the anti-for campaign

Less loops, more

map/filter/fold

http://weblogs.asp.net/podwysocki/archive/2009/06/26/

the-anti-for-campaign.aspx

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Treat side e↵ects as a first-class concern

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Learn a functional language

“ A language that doesn’t a↵ect the way you thinkabout programming, is not worth knowing. ”

— Alan Perlis[8]

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Disclaim your inheritance

Write non-trivial code without

using objects and inheritance.

Get re-usability with higher-order functions.

Try to minimise moving parts instead ofencapsulating moving parts. [7]

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Join our group@lambdaluminary

We meet once a month, on the second Monday of the month.http://www.meetup.com/lambda-luminaries/

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Get out of your comfort zone

Functional Programming is unfamiliar territory formost.

“ If you want everything to be familiar you willnever learn anything new. ”

— Rich Hickey, author of Clojure[9]

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Companies In South Africa

Jemstep, Sandton Using Scala for Fund Analysis

Allan Gray, Cape Town Using Scala for backend logic and systemintegration.

Yuppiechef, Cape Town Using Clojure for their WarehouseManagement System.

Cognician, Cape Town Using Clojure to create coaching/learningmodules.

Eldo Energy, Johannesburg Using Clojure for automated meterreading and intelligent monitoring of consumerenergy.

Rheo Systems, Pretoria Using Clojure for supply chain integration.

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Companies In South Africa

Pattern Matched Technologies, Midrand Using Erlang for allsystems, eg. processing high volumes of financialtransactions.

E↵ective Control Systems, Kyalami Using Erlang for printermanagement.

Mira Networks, Somerset West Using Erlang for billingadministration and mobile development.

Kotive Using Scala for designing workflow processes.

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Online Courses

Functional Thinking by Neal FordO’ Reillyhttp://shop.oreilly.com/product/0636920030393.do

Functional Programming Principles in ScalaEPFL Universityhttps://www.coursera.org/course/progfun

School of HaskellFP Completehttps://www.fpcomplete.com/school

Programming LanguagesUniversity of Washingtonhttps://www.coursera.org/course/proglang

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

Books

Miran LipovacaLearn You a Haskell for Great Good!http://learnyouahaskell.com/

Fred HebertLearn You Some Erlang for Great Good!http://learnyousomeerlang.com/

Yaron Minski, Anil Madhavapeddy, Jason HickeyReal World OCamlhttps://realworldocaml.org/

Paul Chiusano, Runar BjarnasonFunctional Programming in Scalahttp://www.manning.com/bjarnason/

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

References I

John HughesWhy Functional Programming Mattershttp://www.cs.kent.ac.uk/people/staff/dat/miranda/

whyfp90.pdf

Dave ThomasProgramming Elixir, A Gentle Introductionhttp://theprosegarden.com/part-1-of/

Joel SpolskyCan Your Programming Language Do This?http:

//www.joelonsoftware.com/items/2006/08/01.html

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

References II

Robert MartinFunctional Programming Basicshttp://pragprog.com/magazines/2013-01/

functional-programming-basics

John CarmackFunctional Programming in C++http://www.altdevblogaday.com/2012/04/26/

functional-programming-in-c/

Edsger W. DijkstraGo To Statement Considered Harmfulhttp://www.u.arizona.edu/~rubinson/copyright_

violations/Go_To_Considered_Harmful.html

Introduction Definition Function Recap Common Idioms Imperative Comparison Challenges! Industry Use Now what?

References III

Tweet by Michael Feathershttps://twitter.com/mfeathers/status/29581296216

Alan Jay Perlishttp://www.cs.yale.edu/quotes.html

Rich Hickeyhttp:

//www.infoq.com/presentations/Simple-Made-Easy

Andreas PauleyAn Introduction to Functional Programminghttps://github.com/apauley/fp_presentation