AFewFunctionProgramingPrinciplesEveryEngineerShouldKnow

JiamingZhang

03/01/2017

Agenda

Scope

What'sFunctionalPrograming?

FunctionalProgramingPrinciples

Reference/Reading

ScopeThesewon'tbecoveredinthissection(butalreadyonmyradar!)

FPLanguage

FPDesignPattern

FPDataStructures

FPw/OOP

SuccessStories

What'sFunctionalPrograming?Incomputerscience,functionalprogrammingisaprogramming

paradigmthattreatscomputationastheevaluationofmathematical

functionsandavoidschanging-stateandmutabledata.[1]

[1]Wikipedia-FunctionalProgramining

What'sFunctionalPrograming?What'sprogramingparadigm?

Astyleofbuildingthestructureandelementsofcomputerprograms.Commonparadiagmincludes[1]:

Imperative

Declarative

Object-oriented

Functional

Note:Paradigmcanoverlaporcanbeusedw/eachother.

[1]DefinitionofProgramingParadigms

FPPrinciplesTreatcomputationastheevaluationofmathfunctions

PureFunction

High-orderFunction(orFunctor)

AvoidMutation

...

FPPrinciples

TreatcomputationastheevaluationofmathfunctionsAmathfunctionmapsasetofinputtoasetofoutput.

FPPrinciples

TreatcomputationastheevaluationofmathfunctionsFPusuallyprefersexpressionoverstatement

#Assumevalisint

#Statementdefabs1(val):ifval<0:return-valelse:returnval

#Expressiondefabs2(val):return-valifval<0elseval

FPPrinciples

TreatcomputationastheevaluationofmathfunctionsAnequivalentHaskellexample

FPPrinciples

TreatcomputationastheevaluationofmathfunctionsAnequivalentHaskellexample

abs::Int->Intabsval|val<0=-val|otherwise=val

FPPrinciples

TreatcomputationastheevaluationofmathfunctionsFPusuallyprefersrecursionoveriteration

#Iteration-ProceduralStyledefsum1(nums):total=0fornuminnums:total+=numreturntotal

#Recursion-FunctionalStyledefsum2(nums):#NotveryefficientinPythonbutyougottheideareturn0iflen(nums)==0elsenums[0]+sum2(nums[1:])

FPPrinciples

TreatcomputationastheevaluationofmathfunctionsAnequivalentHaskellexample

sum::[Int]->Intsumnums=casenumsof[]->0x:xs->x+sum2xs

FPPrinciples

PureFunction1. Nosideeffect

2. Sameinput,sameoutput

FPPrinciples

PureFunction#1Nosideeffect

ExampleofFunctionw/SiteEffect

FPPrinciples

PureFunction#1Nosideeffect

ExampleofFunctionw/SiteEffect

defsayhi(name)print("Hithere,mynameis%s"%name)

defwrite_to_file(file,text):withopen(file,'w+')asf:f.write(text)

defsort_nums(nums):nums.sort()#inplacesortreturnnums

FPPrinciples

PureFunction#1Nosideeffect

Butwhycreateanappthatcan'tchangethe"world"?

FPPrinciples

PureFunction#1Nosideeffect

Butwhycreateanappthatcan'tchangethe"world"?

Inpractice,applicationsneedtohavesomesideeffects.Simon

Peyton-Jones,amajorcontributortothefunctionalprogramming

languageHaskell,saidthefollowing:"Intheend,anyprogrammust

manipulatestate.Aprogramthathasnosideeffectswhatsoeverisa

kindofblackbox.Allyoucantellisthattheboxgetshotter."Thekey

istolimitsideeffects,clearlyidentifythem,andavoidscattering

themthroughoutthecode.[1]

[1]Whyareside-effectsconsideredevilinfunctionalprogramming?

FPPrinciples

PureFunction#2Sameinput,sameoutput

ExampleofFunctionthatViolatesthis

FPPrinciples

PureFunction#2Sameinput,sameoutput

ExampleofFunctionthatViolatesthis

gloabl_counter=0

functionfoo(x){gloabl_counter+=1;returngloabl_counter+x;}

foo(1)//=>2foo(1)//=>3

FPPrinciples

PureFunction#2Sameinput,sameoutput

ExampleofFunctionthatViolatesthis

gloabl_counter=0

functionfoo(x){gloabl_counter+=1;returngloabl_counter+x;}

foo(1)//=>2foo(1)//=>3

Otherexamples?

FPPrinciples

High-orderFunction(orFunctor)1. Takeatleastonefunctionasinputor

2. Returnafunction

FPPrinciples

High-orderFunction(orFunctor)Example:Afunctionthattakesanotherfunctionasinput

nums=[1,2,3]

map(lambdax:x*2,nums)#=>[2,4,6]

filter(lambdax:x%2==0,nums)#=>[2]

reduce(lambdatotal,num:total*num,nums)#=>6

FPPrinciples

High-orderFunction(orFunctor)Example:Afunctionthatreturnsafunctionasoutput

dirs=['lib','bin','log']

#`functools`isabuilt-inPythonmodule#`functools.partial`returnapartial-appliedfuncfunc=functools.partial(os.path.join,'/User/jason')

map(func,dirs)#=>['/User/jason/lib','/User/jason/bin','/User/jason/log']

FPPrinciples

High-orderFunction(orFunctor)Q:Canyoudothisw/justlambda?

FPPrinciples

High-orderFunction(orFunctor)Q:Canyoudothisw/justlambda?

map(lambdadirname:os.path.join('/User/jason',dirname),dirs)#=>['/User/jason/lib','/User/jason/bin','/User/jason/log']

FPPrinciples

High-orderFunction(orFunctor)AnotherExample--Compose

defcompose(f,g):defcomposed_func(val):returng(f(val))returncomposed_func

FPPrinciples

High-orderFunction(orFunctor)AnotherExample--Compose

defcompose(f,g):defcomposed_func(val):returng(f(val))returncomposed_func

dirname='~/lib/../log'

os.path.abspath(os.path.expanduser(dirname))#=>/Users/jiamingz/log

expandpath=compose(os.path.expanduser,os.path.abspath)expandpath(dirname)expandpath('~/lib/python/..')

FPPrinciples

High-orderFunction(orFunctor)Composeisacommonhigh-orderfunctioninFPlang

FPPrinciples

High-orderFunction(orFunctor)Composeisacommonhigh-orderfunctioninFPlang

add5::Int->Intadd5x=5+x

mul5::Int->Intmul5x=5*x

mul5_and_add5::Int->Intmul5_and_add5=add5.mul5

FPPrinciples

AvoidMutation[1,2,3]->[1,2,42]

FPPrinciples

AvoidMutation[1,2,3]->[1,2,42]

Example:Mutation

defchange(arr,idx,new_val):arr[idx]=new_valreturnarr

FPPrinciples

AvoidMutation[1,2,3]->[1,2,42]

Example:Mutation

defchange(arr,idx,new_val):arr[idx]=new_valreturnarr

Example:NoMutation

defchange(arr,idx,new_val):returnarr[:idx]+[new_val]+arr[idx+1:]

change([1,2,3],2,42)#=>[1,2,42]

FPPrinciples

AvoidMutationWait...whatabouttheefficiency?

FPPrinciples

AvoidMutationWait...whatabouttheefficiency?

Persistentdatastructure

Notgonnacovertothistime.Ifyou'reinterested,checkthisIdealHashTree

Reference/ReadingWikipedia-FunctionalPrograming

Wikipedia-PureFunction

Wikipedia-HigherOrderFunction

AnjanaVakil:LearningFunctionalProgrammingwithJavaScript-

JSUnconf2016

TheOriginsofScala

Eassy-WhyFunctionalProgramingMattersbyJohnHughes

Youtube-WhyFunctionalProgrammingMattersbyJohnHughesat

FunctionalConf2016