FP in OOP

Lessons learned from Functional Programming applied

to Object Oriented Programming

Functional Programming: Why?

OOP was the great paradigm shift of the 90s

FP will be the next great paradigm shift

Excels at parallel and distributed computing

Background: Church-Turing Thesis

Anything that could possibly be computed, can

be computed using any of the following:

● A Turing machine

● The Lambda Calculus

● Recursion

See Google for more details!

/* Factorial in PHP */function factorial($n){

$result = 1;

for ($c = 1; $c <= $n; $c++)$result = $result * $c;

return $result;}

The Imperative Way

The Functional Way

-- Factorial in Haskellfactorial 0 = 1factorial n = n * factorial (n - 1)

Value Objects

Value objects

Immutable

Safe to compose

Easy to test

Use to express as much logic as possible

Value Objects

class Date {private $day, $month, $year;public function __construct($day, $month, $year) { ... }public function isAfter(Date $other) { ... }public function isBefore(Date $other) { ... }public function isSame(Date $other) { ... }

}class DateRange {

private $start, $end;public function __construct(Date $start, Date $end) {

if ($start->isAfter($end)) {throw new InvalidArgumentException(); }

}public function contains(Date $day) {

return $day->isAfter($this->start) &&$day->isBefore($this->end);

}}

Command/Query

Separation

Command/query separation

Isolates state-modifying logic

Queries are safe to reorder, easier to

optimize

Easier to test

Provides opportunity to use different

models for read and write operations

(CQRS)

Command/query separation

class ShoppingCart {

private $total = 0;

private $products = array();

public function calculateSubtotal() {

$subtotal = 0;

foreach ($this->products as $product) {

$subtotal += $product->getPrice();

}

return $subtotal;

}

public function calculateTax() {

return $this->total * 0.075;

}

public function addProduct($product) {

$this->products[] = $product;

}

public function updateTotal($amount) { $this->total = $amount;

}

Command/query separation

$product1 = new Product(25.00);

$product2 = new Product(10.00);

$cart = new ShoppingCart;

$cart->addProduct($product1);

$cart->addProduct($product2);

assert($cart->calculateSubtotal() == 35.00);

$cart->updateTotal($cart->calculateSubtotal());

assert($cart->getTotal() + $cart->calculateTax() ==

37.625);

assert($cart->calculateTax() + $cart->getTotal() ==

37.625);

$cart->updateTotal($cart->getTotal() + $cart-

>calculateTax());

assert($cart->getTotal() == 37.625);

Higher Order Functions

Higher-order functions

Let you parameterize behavior or

designate new behavior at runtime

OOP design patterns:

● Strategy pattern

● Visitor pattern

Strategy Example - PHP 1

abstract class PricingStrategy {private $customer;public function __construct($customer) { ... }abstract public function calculatePrice($prodcut);

}class GoldClubPricing extends PricingStrategy {

public function calculatePrice($prodcut) { ... }}class HighRiskPricing extends PricingStrategy {

public function calculatePrice($prodcut) { ... }}class LoyalCustomerPricing extends PricingStrategy {

public function calculatePrice($prodcut) { ... }}

Strategy Example - PHP 2

class ShoppingCart {private $products;private $customer;private function getPricingStrategy($customer) { ... }

public function calculateSubtotal() {$subtotal = 0;$pricingStrategy = $this->getPricingStrategy($this->customer);

foreach ($this->products as $product) {$subtotal += $pricingStrategy->calculatePrice($product);

}return $subtotal;

}}

Strategy Example - Functional

goldClubPricing product = ...highRiskPricing product = ...loyalCustomerPricing product = ...

getPricingStrategy customer = ...

calculateSubtotal customer products = sum (map pricingStrategy products)where pricingStrategy = getPricingStrategy customer

Other Techniques

Other techniques

Lazy evaluation

Map/Reduce (Folds)

Message passing concurrency and the Actor

model

Thank you!

Further Reading

Patterns of Enterprise Application Architecture -

Martin Fowler

Domain Driven Design - Eric Evans

A Functional Pattern System for Object-Oriented

Design - Thomas Kuhne

The Power of Value - Dan Johnssonhttp://www.infoq.com/presentations/Value-Objects-Dan-Bergh-Johnsson

Objects, Anomalies, and Actors: The Next Revolutionhttp://www.infoq.com/presentations/Objects-Anomalies-and-Actors-The-Next-Revolution

Actor Model Concurrency - Jeff Darcyhttp://pl.atyp.us/wordpress/index.php/2009/07/actor-model-concurrency