______ _ _ _ | ____| | | (_) | | | |__ _ _ _ __ ___| |_ _ ___ _ __ __ _| | | __| | | | '_ \ / __| __| |/ _ \| '_ \ / _` | | | | | |_| | | | | (__| |_| | (_) | | | | (_| | | |_| \__,_|_| |_|\___|\__|_|\___/|_| |_|\__,_|_| _____ _ | __ \ (_) | |__) | __ ___ __ _ _ __ __ _ _ __ ___ _ __ ___ _ _ __ __ _ | ___/ '__/ _ \ / _` | '__/ _` | '_ ` _ \| '_ ` _ \| | '_ \ / _` | | | | | | (_) | (_| | | | (_| | | | | | | | | | | | | | | | (_| | |_| |_| \___/ \__, |_| \__,_|_| |_| |_|_| |_| |_|_|_| |_|\__, | __/ | __/ | |___/ |___/

____ __ __ __ __ __ ____ __ _ _ __ ____ __ ____ __ __ _ ( _ \ / _\ ( ) / _\ _( )( ) ( \ / _\ ( \/ ) / \( \ / _\ ( _ \ / _\ ( ( \ ) _ (/ \/ (_/\/ \/ \) \ )( ) D (/ \/ \/ \( O )) D (/ \ ) // \/ / (____/\_/\_/\____/\_/\_/\____/(__) (____/\_/\_/\_)(_/ \__/(____/\_/\_/(__\_)\_/\_/\_)__)

http://eli.thegreenplace.net/wp-content/uploads/2008/05/tc_000.png

http://sites.google.com/site/andrewsporfotilio/_/rsrc/1226818616654/Home/math_400.jpg

Functional Programming is a style whose

underlying model of computation is the function.

f(x)

Referential Transparency Higher Order functions Lazy Evaluation Pattern Matching

http://picasaweb.google.com/damodaran.balaji/DevCampChennaiJuly2010#5493004664487949618

Lambda Calculus Continuations Monads Type Inference

Referential Transparency Higher Order functions Lazy Evaluation Pattern Matching

Referential Transparency “Equals can be replaced

with equals”

http://2.bp.blogspot.com/_DS3VD3b2euI/TRIcIUzduDI/AAAAAAAAAGA/Xp2E6kPxZCc/s1600/Pascal+the+chameleon+from+tangled+rapunzel+disney+cartoon.jpg

Referential Transparency “Equals can be replaced with equals”

f(x) = g(x) + 5 h(x) = x * f(x)

Referential Transparency “Equals can be replaced with equals”

f(x) = g(x) + 5 h(x) = x * f(x) h(x) = x * (g(x) + 5)

public class ReferentiallyOpaque { private int x = 0; static int f(int y) { return x += y; } public static void main(String[] args) { ReferentiallyOpaque ro = new ReferentiallyOpaque(); System.out.println(ro.f(5)); System.out.println(ro.f(5)); } }

public class ReferentiallyOpaque { private int x = 0;; static int f(int y) { return x += y; } public static void main(String[] args) { ReferentiallyOpaque ro = new ReferentiallyOpaque(); System.out.println(ro.f(5)); System.out.println(ro.f(5)); } }

5 10

public class Example { public static void main(String[] args) { StringBuffer buffer = new StringBuffer("Hello World"); System.out.println(buffer.reverse()); System.out.println(buffer.reverse()); } }

public class Example { public static void main(String[] args) { StringBuffer buffer = new StringBuffer("Hello World"); System.out.println(buffer.reverse()); System.out.println(buffer.reverse()); } }

dlroW olleH Hello World

public class Example { public static void main(String[] args) { StringBuffer buffer = new StringBuffer("Hello World"); StringBuffer s1 = buffer.reverse(); StringBuffer s2 = buffer.reverse(); System.out.println(s1 + " " + s2); } }

public class Example { public static void main(String[] args) { StringBuffer buffer = new StringBuffer("Hello World"); StringBuffer s1 = buffer.reverse(); StringBuffer s2 = buffer.reverse(); System.out.println(s1 + " " + s2); } }

Hello World Hello World

Varying Variables Global State

http://paulafanclub.com/wp-content/uploads/2010/12/confused.jpg

• Looping • File I/O • Modify Data • Do more than one thing?

Looping

static int factorial(int number) { int f = 1; for (int i = number; i > 0; i--) f = f * i; return f; }

http://www.shrink4men.com/wp-content/uploads/2010/11/dog-chasing-its-tail.jpg

Recursion

Looping

static int factorial(int number) { return (number == 1) ? number : number * factorial(number - 1); }

static int factorial(int number) { int f = 1; for (int i = number; i > 0; i--) f = f * i; return f; }

Modifying data

static List<Integer> addToList(List<Integer> integers, int a) { integers.add(a); return integers; }

Mutable data structures

Modifying data

static List<Integer> addToList(List<Integer> integers, int a) { integers.add(a); return integers; }

let addToList list a = list @ [a]

> let g = [1;2;3;];; val g : int list = [1; 2; 3] > addToList g 4;; val it : int list = [1; 2; 3; 4] > g;; val it : int list = [1; 2; 3] >

Immutable

http://4.bp.blogspot.com/_RpRScqAI8e4/SbNPgumsqPI/AAAAAAAAAK4/ZK8ZsfKJCmQ/s400/wolverine+hugh+jackman.jpg

Referential Transparency Higher Order functions Lazy Evaluation Pattern Matching

Higher Order functions “Functions as first class values”

http://www.dreamstime.com/looking-up-to-the-sky-thumb4655038.jpg

Higher Order functions “Functions that can be passed as arguments”

def square(x): return x * x def add(function, x): return function(x) + function(x) print add(square, 5)

Higher Order functions “Functions that can be returned as results”

function addNumber(x) { return function(y) { return x + y } } var add4With = addNumber(4) var add5With = addNumber(5) add4with(8) add5with(8) 12 13

Higher Order functions “Functions that can be returned as results”

function addNumber(x) { return function(y) { return x + y } } var add4With = addNumber(4) var add5With = addNumber(5) add4with(8) add5with(8) 12 13

Lambda Functions

Higher Order functions “Functions that can be returned as results”

function addNumber(x) { return function(y) { return x + y } } var add4With = addNumber(4) var add5With = addNumber(5) add4with(8) add5with(8) 12 13

Lambda Functions

Functions stored in data structures

Higher Order functions “Currying”

let add x y = x + y val add : int -> int -> int > let add5 = add 5;; val add5 : (int -> int) > add5 4;; val it : int = 9 >

Referential Transparency Higher Order functions Lazy Evaluation Pattern Matching

Lazy Evaluation “Arguments in a function call are evaluated at most once.”

http://www.arngren.dk/image/Wallpaper/Comic/Calvin%20Hobbes/Sleeping1024.jpg

Lazy Evaluation “Ability to invoke expressions only when its value is needed.”

static void Main(string[] args) { var lazy = new Lazy<int>(() => { Console.WriteLine("calculating..."); return 6 * 7; }); Console.WriteLine(lazy.Value); Console.WriteLine(lazy.Value); }

Lazy Evaluation “Ability to invoke expressions only when its value is needed.”

static void Main(string[] args) { var lazy = new Lazy<int>(() => { Console.WriteLine("calculating..."); return 6 * 7; }); Console.WriteLine(lazy.Value); Console.WriteLine(lazy.Value); }

calculating... 42 42

Lazy Evaluation “Memoization”

function Lazy_Memoized(def) { var cache = []; return function(i) { return (i in cache) ? cache[i] : (cache[i] = def.call(arguments.callee, i)); }; } var factorial = new Lazy_Memoized(function(i) { return i <= 1 ? i : i * this(i - 1); }); factorial(6)

Lazy Evaluation “Ability to create infinite sequence in data structures”

http://softwarezku.co.cc/wp-content/uploads/2010/09/Visual-Studio-2010-Logo.png

Referential Transparency Higher Order functions Lazy Evaluation Pattern Matching

Pattern Matching “Writing several equations defining the same function”

http://softwareandfinance.com/images/vcpp-trig1.jpg

f(x) = 1, when x = 0 = 3x + ex, when x > 0

Pattern Matching A typical function definition

Pattern Matching A typical function definition

fac :: Integer -> Integer fac 0 = 1 fac n | n > 0 = n * fac(n-1)

let rec factorial = function | 0 -> 1 | n -> n * factorial(n - 1)

References

• Concepts, Evolution and Application of Functional Programming Languages – Paul Hudak

• http://msdn.microsoft.com/en-us/library/dd547125.aspx - Pattern Matching in F#

• http://appden.com/javascript/lazy-list-comprehensions-in-javascript-a-lazy-evalution/

• http://images.google.com

• http://en.wikipedia.org

• http://www.haskell.org/haskellwiki/Haskell

• http://homepages.inf.ed.ac.uk/wadler/