Functional Programming 101 in Groovy

Evgeny GoldinDevCon Tel-Aviv, February 14, 2013

Evgeny Goldin

Java developer since 2000

FP enthusiast: Scheme, Perl, Groovy, Scala

GroovyMag, Gr8Conf

SCM at Trademob

github.com/evgeny-goldin/

@evgeny_goldin

Are you FP-er?

http://thecoolnessfactor.com/2013/01/winds-change/changes-ahead-exit-sign/

In this session

Why FP?

FP building blocks

FP in Groovy

FP candidates

In this session

Mindset switch

Why FP?

Because shit happens

.. or do we make it happen?

Average code is complex

Side-effects go wild

Logical steps are interleaved

Changes are error-prone

http://www.guardian.co.uk/sustainable-business/research-confirms-health-impacts-tire

Average state is fragile and shared

Exposed for updates in any way

Can easily mutate under your feet

Changes are error-prone

http://www.bbcgoodfood.com/content/knowhow/glossary/egg/

Parallelism is hard

“The Multicore Revolution”

Threads + shared mutable state :(

Changes are error-prone

http://www.russel.org.uk/Presentations/euroPython2010_theMulticoreRevolution.pdfhttp://jalopnik.com/5965581/this-is-how-not-to-parallel-park-on-a-hill

Changes are error-prone

http://lenta.ru/sitemap.xml

https://bitbucket.org/evgenyg/devcon-2013/src/master/sitemap-imperative.groovy

https://bitbucket.org/evgenyg/devcon-2013/src/master/sitemap-functional.groovy

ModularityBreak the flow into distinct steps

Do one thing and do it well

FlexibilityShuffle your steps

TestabilityEnsure correctness of your steps

ParallelismScale your steps!

FP building blocks(only a subset this time)

RecursionList = head() + tail()

State is hidden, built up on stack

Recursion eliminationhttps://bitbucket.org/evgenyg/devcon-2013/src/master/recursion.groovy

Immutable StateMoving parts are minimized

No defensive copying

Caching, Parallelism

Immutable StateNew state = f ( old state )

Changes are isolated, constructor only

g ( i + 1 ), g ( list + ‘element’ )

Immutable StatePersistent data structures

Updated in place

Yields a new immutable state

Git

http://git-scm.com/book/en/Git-Branching-What-a-Branch-Is

First-class functionsAnonymous functions

Higher-order functions

Command/Template/Visitor => “Free Willy”

No side-effectssin, cos, max, min, sum, sort

Opens doors for optimizations

Caching, Parallelism

Mindset switch Imperative => Functional

Variables => Values

Classes => Functions

Explicit instructions => Declarative definitions

How (Steps) => What (Result)

Mindset switchPointers, memory allocations, GC - check

State management

Iterations

Filtering, conversion, folding

Parallelism

FP in Groovy

Gradual conversion

http://www.buysgmath.com/p/200/eph-gradually-difficulty-mathematics-1/

final j = 5@Immutable

Anonymous Functionsa.k.a. Closures

Filter - findAll() Map - collect() Fold - inject()

http://www.barlifeuk.com/index.php/2012/08/enter-the-janneau-armagnac-three-musketeers-inspired-competition/

https://bitbucket.org/evgenyg/devcon-2013/src/master/functions.groovy

find()

any()

every()

each()

eachWithIndex()

takeWhile()

dropWhile()

Collection

Range

Map

Array

String

...

Object.iterator()

Categories, Meta, Extensionspublic  static  <T>  Collection<T>  findAll(Collection<T>  self,                                                                                  Closure  filter)  

public  static  <T>  List<T>  collect(Collection<?>  self,                                                                    Closure<T>  transform)

public  static  T  inject(Collection<T>  self,                                                Closure<V>  closure)

public  static  <T>  T              head(List<T>  self)

public  static  <T>  List<T>  tail(List<T>  self)

...

org.codehaus.groovy.runtime.DefaultGroovyMethods

Memoizationa.k.a. Caching

https://bitbucket.org/evgenyg/devcon-2013/src/master/memoization.groovy

TrampolineRecursion => Loop (no stack overflow)https://bitbucket.org/evgenyg/devcon-2013/src/master/trampoline.groovy

GPars!Data Parallelism

https://bitbucket.org/evgenyg/devcon-2013/src/master/gpars.groovy

eachParallel()

collectParallel()

findAllParallel()

everyParallel()

foldParallel()

sumParallel()

gmemoize()

FP Candidates

Loops => any / every

http://en.hdyo.org/tee/questionshttps://bitbucket.org/evgenyg/devcon-2013/src/master/loops.groovy

Split, filter, map, fold

http://www.cityofwaterfalls.ca/hermitage_cascade.html

https://bitbucket.org/evgenyg/devcon-2013/src/master/map-reduce.groovyhttps://bitbucket.org/evgenyg/devcon-2013/src/master/check-versions.groovy

Open your eyes

pinboard.in/u:evgenyg/t:fp/

Arturo Herrero - Functional Programming with Groovy

Neal Ford - Functional Thinking

Martin Odersky - FP Principles in Scala

Erik Meijer - FP Fundamentals

MIT - Structure and Interpretation of Computer Programs

University of Washington - Programming Languages

Stanford - Programming Paradigms

evgenyg@gmail.com

Thank you!

@evgeny_goldin