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Refactoring Design Patterns the Functional Way (in Scala)

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Kfir BlochBackend engineering manager

@kfirondev

What the hell is functional programming?

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• Treats computation as the evaluation of mathematical functions

• Avoid changing state and mutable data• Declarative• Pure functions with no side effects

http://www.clker.com/

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case class Salad(name: String, veggy: Boolean, chopped: Boolean)

Immutability via Scala case class

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sealed trait Salad case class Lentils() extends Salad case class Hummus() extends Salad case class Tahini() extends Salad case class SaladInfo(salad: Salad, spices: Seq[String])

def makeSaladInfo(salad: Salad, f: Salad => Seq[String]): SaladInfo = SaladInfo(salad, f(salad))

// usage makeSaladInfo(Hummus(), spicer)

def spicer(salad: AnyRef): Seq[String] = salad match { case l: Lentils => Seq("Salt", "Olive Oil", "Red paper") case h: Hummus => Seq("Salt", "Garlic", "Onion") case t: Tahini => Seq("Salt", "Garlic", "Lemon") }

Higher order function

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def doSomething(msg: String): Unit= { val formatted = msg.toUpperCase logger.log(Level.WARNING, formatted) }

Function with side effects

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def doSomething(msg: String): String = { val formatted = msg.toUpperCase formatted }

Pure function

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It leads naturally to Hexagonal Architecture(ports & adapters)http://blog.mattwynne.net/2012/05/31/hexagonal-rails-objects-values-and-hexagons/comment-

page-1/

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Hexagonal Architecture with FP gives result which is:

Deterministic Separation of concerns Fast testing lifecycle Concurrent agnostic domain model

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People told you that ->

@kfirondev

People told you that ->

Abstract factory

Builder

Factory Method

Builder

Prototype

Adapter

Bridge

Composite

Decorator

Proxy

Chain of responsibility

Command

Iterator

Mediator

Observer

Object Oriented

http://www.clker.com/

@kfirondev

People told you that ->

Abstract factory

Builder

Factory Method

Builder

Prototype

Adapter

Bridge

Composite

Decorator

Proxy

Chain of responsibility

Command

Iterator

Mediator

Observer

Object Oriented FunctionalProgramming

http://www.clker.com/

@kfirondev

People told you that ->

Abstract factory

Builder

Factory Method

Builder

Prototype

Adapter

Bridge

Composite

Decorator

Proxy

Chain of responsibility

Command

Iterator

Mediator

Observer

Object Oriented FunctionalProgramming

http://www.clker.com/

Incorrect!

@kfirondev

Builder pattern builds a complex object using simple objects and using a step by step approach. This type of design pattern comes under creational pattern as this pattern provides one of the best ways to create an object.

Builder

@kfirondev

Builder pattern builds a complex object using simple objects and using a step by step approach. This type of design pattern comes under creational pattern as this pattern provides one of the best ways to create an object.

Builder

LET’S CODE

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Behavior pattern that helps create objects which represent various strategies and a context object whose behavior varies as per its strategy object. The strategy object changes the executing algorithm of the context object

Strategy

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LET’S CODE

Behavior pattern that helps create objects which represent various strategies and a context object whose behavior varies as per its strategy object. The strategy object changes the executing algorithm of the context object

Strategy

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The chain-of-responsibility pattern is a design pattern consisting of a source of command objects and a series of processing objects.Each processing object contains logic that defines the types of command objects that it can handle.Only qualifying predicate for the request will process it, otherwise it will pass to the next handler in chain

Chain ofResponsibility

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LET’S CODE

The chain-of-responsibility pattern is a design pattern consisting of a source of command objects and a series of processing objects.Each processing object contains logic that defines the types of command objects that it can handle.Only qualifying predicate for the request will process it, otherwise it will pass to the next handler in chain

Chain ofResponsibility

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In software engineering, dependency injection is a software design pattern that implements inversion of control for resolving dependencies. A dependency is an object that can be used (a service)

DependencyInjection

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In software engineering, dependency injection is a software design pattern that implements inversion of control for resolving dependencies. A dependency is an object that can be used (a service)

DependencyInjection

Dependency Injection framework!=

Dependency Injection

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Dependency injection

SpringEJB Guice

Dependency Injection Frameworks

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Dependency injection

SpringEJB Guice

Dependency Injection Frameworks

Constructor Injection

Vanilla

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Dependency injection

SpringEJB Guice

Dependency Injection Frameworks

CakePattern

Constructor Injection

Vanilla

@kfirondev

Dependency injection

SpringEJB Guice

Dependency Injection Frameworks

CakePattern

Constructor Injection

Vanilla

LET’S CODE

@kfirondev

• https://github.com/kfiron/design-pattern-fp-talk

My code is available for you

@kfirondev

• Went over functional programming definition

• Understood that FP as “side effect” help you go towards hexagonal architecture

• Refactor various patterns from Java to Scala Idioms

• Dependency injection using Scala with cake pattern

Summary of what we did

@kfirondev@kfirondev

@kfirondev

linkedin/in/blochkfir github.com/kfiron@kfirondevkfirb@wix.com

Kfir BlochHead of Backend Engineering

Q&A

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