Copyright © SELA Software & Education Labs, Ltd. | 14-18 Baruch Hirsch St., Bnei Brak 51202, Israel | www.selagroup.com

Functional Programming

High Level Introduction

elada@sela.co.il

http://il.linkedin.com/in/eladavneri/

http://blogs.microsoft.co.il/elada

Elad AvneriArchitect, ALM and Software Development

Agenda

The BasicsPrinciplesFP Languages

FeaturesComparison

Demo: Imperative to FP Evolution

3

Software Development Challenges

Modern software becomes larger and more complexWe’d like to reduce the time and cost of software developmentWe’d like to increase the quality of the developed software

FP to the Rescue

Promotes composition and modularityPromotes simpler codeConcise, promotes smaller codeConcurrency is not an issueDramatically reduces number of bugs

The Goal of this Presentation

Is not to convince to abandon OOPIs to open your mind for additional paradigmLearn techniques which you can utilize in your favorite programming language

http://theburningmonk.com/2015/04/dont-learn-a-syntax-learn-to-change-the-way-you-think/

A

AAn appetizer:

sorted :: [Integer] -> Boolsorted [] = Truesorted [_] = Truesorted (x:y:zs) = x < y && sorted (y:zs)

So, What Is It All About?Functional Programming Other programming styles

Classes, procedures Building Blocks

N/A A must State changes

Function calls, Loops, conditionals, and function (method) calls

Main control flow

Don’t Panic!!!

FP Principles

ImmutabilityPure functionsModularity and compositionDeclarative

Immutability

Thread safeData flow is explicitCacheable objects

"How many Haskell programmers does it take to change a lightbulb?“ "Haskell

programmers don't "change" lightbulbs, they "replace" them. And you must also

replace the whole house at the same time.”

Idempotence: Get the same results no matter how many times a function is called

We gain the ability to cache results for function calls

Insanity: doing the same thing over and over again and expecting different results.

Albert Einstein

Pure Functions

A function has no side effect We gain clarity We gain “laziness”:

int foo(){

bar();return 0;

}

Pure Functions

Function calls are independentCan be parallelized

result = func1(x,y) + func2(x,z)

Pure Functions

Pure Functions

Encapsulation is in the function levelEasier to debugEasier to test in isolation

Modularity and Composability

A function’s output is only its return valueWe gain:

Modularity with light couplingComposability

Declarative

The focus is on what the computer should do rather than how it should do itThink of a spreadsheet:

We don’t specify the order of calculationsWe don’t deal with variablesWe deal with expressions and not with stepsWe tell it what we want not how to achieve it

Think of SQL…FP is the same but for general purpose

Imperative

List<int> results = new List<int>();foreach(var num in collection){ if (num % 2 != 0) results.Add(num);}

Declarative vs Imperative

Declarativevar results = collection.Where( num => num % 2 != 0);

List<int> collection = new List<int> { 1, 2, 3, 4, 5 };

http://stackoverflow.com/a/1784702/871579

Myths and Misconceptions

It’s difficultIt’s not difficult it’s “just” unfamiliar

We can do it also in XXX (= your favorite language)

Maybe, but “real” FP language is “pushing” you there

It’s not practical for real-world applicationsSimply wrong

FP has poor performanceActually some of the features gives great optimization opportunitiesThe quality code vs optimized code tradeoff

FP is here from the ‘50sAt the beginning computers were slow

Not enough power to process abstractions

Now it’s back!

The Retro of FP

Functional Programming/Language

Functional programming is style of programmingA functional language is one that supports and encourages the functional style

(Some) Functional Language Features

Separate data from behaviorFunctions as First-Class ValuesHigh-Order FunctionsRecursionPattern matchingAlgebraic data types Lazy evaluation

Separate Data from Behavior

No classes and objectsData passes between functions as parameters and return values

Functions as First-Class Values

You can do with functions whatever you can do with other data-types. E.g.:

Bind identifier to a function

Store a function in a data structure

let squareIt = fun n -> n * nlet doubleIt = fun n -> 2 * n

let funList = [ squareIt; doubleIt ]

Functions as First-Class Values

High order functionslet compose = fun op1 op2 -> // Use a let expression to build the function that

will be returned. let funToReturn = fun n -> op1 (op2 n) // Then just return it. funToReturn

let squareAndDouble = compose doubleIt squareIt// The following expression squares 3, doubles 9, returns

18, and // then displays 18.System.Console.WriteLine(squareAndDouble 3)

Recursion

No loops

factorial n = if n < 2 then 1 else n * factorial (n-1)

Pattern Matchingfactorial :: Integer -> Integerfactorial 0 = 1factorial n = n * factorial (n - 1)

fib :: Integer -> Integerfib 0 = 1fib 1 = 1fib n = fib (n-1) + fib (n-2)

length' :: (Num b) => [a] -> b length' [] = 0 length' (_:xs) = 1 + length' xs

Algebraic Data Typesdata Bool = False | True

data Point = Point Float Float deriving (Show)  data Shape = 

Circle Point Float | Rectangle Point Point deriving (Show)

surface :: Shape -> Float  surface (Circle _ r) = pi * r ^ 2  surface (Rectangle (Point x1 y1) (Point x2 y2)) = (abs $ 

x2 - x1) * (abs $ y2 - y1)

Lazy Evaluation

Evaluation is deferred until their results are neededAllows using infinite listsnats = [0..]

odds = [1,3..]ones = 1 : ones

-- What will happen when executing the following?length oddstake 5 odds

Language ComparisonF# Scala Haskell

CLR (.Net) JVM Native Runtime

Medium1 High (-) High FP Adherence

Default (but may be not)

Default (but may be not)

Yes Immutability

Yes Not by default (but can be)

Yes Lazy Evaluation

Yes Yes Yes Algebraic Data Types

1) E.g. functions are not guaranteed to be pure: let tm = DateTime.Now

Demo

Imperative to FP Evolution

What should you Take?

Don’t learn a syntax, learn to change the way you thinkAdd new paradigms to your toolboxGet out of your comfort zone

Functional Programming is an Opportunity

Questions

Copyright © SELA Software & Education Labs, Ltd. | 14-18 Baruch Hirsch St., Bnei Brak 51202, Israel | www.selagroup.com

Thank You

elada@sela.co.il

http://il.linkedin.com/in/eladavneri/

http://blogs.microsoft.co.il/elada

Elad AvneriArchitect, ALM and Software Development