FunctionalProgramming

KKishore

FunctionalParadigm>

ImmutablevsMutable>

FunctionsandHigherOrderFunctions>

PatternMatching>

InThisTalk

WhatisFunctionalProgramming?>FunctionalProgramminglanguageisonewhichdoesnothavemutablevariables,assignments,controlstructures.Itenablestheconstructionofelegantprogramsthatfocusonfunctions.(FPhasbeenaroundfor50years).Languages:

Lisp,XQuery,FP,Haskell,Scala,Clojure,etcFunctionalProgrammingisbecomingincreasinglypopularbecauseitoffersanattractivemethodforexploitingparallelismformulticoreandcloudcomputing.

FunctionalParadigm

Needacatalyzer,somethingthatsparksinitialadoptionuntiltheotheradvantagesbecomecleartoeveryone.

>

Multicore(=parallelprogramming)-Cloudcomputing(=distributedprogramming)-

FunctionalParadigm(Why?)

Non‑‑determinism=Parallelprocessing+Mutablestate

>

MutableState>varage=20-age=age+1-

Togetdeterministicprocessing,avoidthemutablestate!Avoidingmutablestatemeansprogrammingfunctionally.

>

TherootoftheProblem

ButwhataboutObjects?>So,shouldweforgetOOandallprograminfunctionalprogramminglanguages?

-

WeneedtocombineOOandFP-

NewObject>Previously:“Objectsarecharacterizedbystate,identity,andbehavior.”(Booch)

-

Now:Eliminateorreducemutablestate.

-

Objects

Scala(ScalableLanguage)

Prof.MartinOdersky

WhatisImmutable?>Examples:-

valage=18//InScala-finalintage=18//Java-constintage=18//C-

WhatisMutable>Examples:-

varage=20//InScala-age=18//Java,C,C++-

ImmutableandMutable

vallist=List(1,2,3)(or)(1::2::3::Nil)>

ImmutableList

1

2

3

Nil

ToDoublethevaluesinthelist.i.e.foralist(1,2,3,4,5)theresultshouldbe(2,4,6,8,10).

>

Let'sdoinJava-

int[]numbers=newint[]{1,2,3,4,5};List<Integer>doubled=newArrayList<>();for(intelem:numbers){doubled.add(elem*2);}System.out.println(doubled);

ImmutableandMutable

PrimitiveObsession

ToDoublethevaluesinthelist.i.e.foralist(1,2,3,4,5)theresultshouldbe(2,4,6,8,10).

>

‑‑ Let'sdoinScalavalnumbers=List(1,2,3,4,5,6)valresult=numbers.map(elem=>elem*2)valresult=numbers.map((elem:Int)=>elem*2)valresult=numbers.map(_*2)

ImmutableandMutable

AnotherSolution:>valnumbers=List(1,2,3,4,5,6)for(elemnumbers){println(elem*2)}

-

Problem:>Ifwelistallthenaturalnumbersbelow10thataremultiplesof3or5,weget3,5,6and9.Thesumofthesemultiplesis23.

Findthesumofallthemultiplesof3or5below1000.

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(1until1000).filter(elem=>elem%3==0||elem%5==0).sum-

ImmutableandMutable

FunctionsarefirstclassCitizens.AfunctionConsistsof4things.

>

Name-Parameters-Body-ReturnType-

Example:defadd(x:Int,y:Int):Int=x+y

Functions

FunctionKeywordNameParametersBodyReturnType

defadd(x:Int,y:Int):Int=x+y>

Functions

Name

Parameters

ReturnType

Body

AFunctionWithoutaName>Example:-valsum=(x:Int,y:Int)=>x+y-println(sum(5,1))-

WriteaSquarefunctioninScala.i.e.ForInput4theresultis16(4*4)Solutions:

‑‑ defsquare(x:Int)=x*x‑‑ defsquares(x:Int):Int=x*x‑‑ valresult=(x:Int)=>x*x

AnonymousFunction

FuntionsthattakesotherfunctionshasparametersorreturningafunctioniscalledHigherOrderFunction.

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Example:>

valpriceList=List(5,10,15,17,20,25,28,30,35,40)

Goal:TosumallthepricesdefsumPrices(priceList:List[Int]):Int={varsum=0for(pricepriceList){sum+=price}sum}

HigherOrderFunctions

Goal:TosumallthepricesUnder25>defsumPricesUnder25(prices:List[Int]):Int={varsum=0for(priceprices){if(price<25)sum+=price}sum}

Goal:TosumallthepricesOver25,Under30,etc>

HowwillYouSolvethisProblem?

Key:Weneedgenericfunctioni.e.SinglefunctiontocomputesumofallpricesOver25,Under30,etc

HigherOrderFunctions

Solution:>defsumPrices(prices:List[Int],cond:Int=>Boolean):Int={varsum=0for(priceprices){if(cond(price))sum+=price}sum}

AlterantiveSolution:>defsumPrices(prices:List[Int],cond:Int=>Boolean):Int=prices.filter(cond).sum

HigherOrderFunctions

Problem:ToTestYes/No> deftest(number:Int):String=numbermatch{case0=>"No"case1=>"Yes"case_=>"Error"

}

PatternMatching

Tosumallthevaluesinthelist.i.eList(1,2,3,4,5)=>15> defsum(list:List[Int]):Int=listmatch{

caseNil=>0casex::xs=>x+sum(xs)

}

PatternMatching

1 2 3 4 5 Nil

x xs

ToPrintHelloWorldNtimes>

defntimes(n:Int)=for(in)println("HelloWorld")

SumofOddelementsinaList>

defoddSum(list:List[Int]):Int=list.filter(elem=>elem%2!=0).sum

Problems

Findlengthofthelistwithoutusinglengthfunction>deflength(list:List[Int]):Int=list.map(elem=>1).sumdeflength(list:List[Int]):Int=listmatch{

caseNil=>0casex::xs=>1+length(xs)

}

Problems

Definehead,tail,initandlastmethodsforlist>

Problems

1 2 3 4 5 12

21

56

45

head last

tail

init

twitter.github.com/scala_school>

twitter.github.io/effectivescala/>

CourseraScalacourse(50Kstudentslastyear)>

Scalamadeeasy>

ProgrammingScala>

HowtoLearn

https://www.coursera.org/>http://www.udacity.com/>https://www.edx.org/>http://www.learnstreet.com/>http://teamtreehouse.com>http://nptel.iitm.ac.in/>

Resources