A description of Functional Programming basics and how they are applied in Groovy.

1. Functional Programming
101 in Groovy
Evgeny Goldin
DevCon Tel-Aviv, February 14, 2013

2. Evgeny Goldin
Java developer since 2000
FP enthusiast: Scheme, Perl, Groovy, Scala
GroovyMag, Gr8Conf
SCM at Trademob
github.com/evgeny-goldin/
@evgeny_goldin

3. Are you FP-er?

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

5. In this session
Why FP?
FP building blocks
FP in Groovy
FP candidates

6. In this session
Mindset switch

7. Why FP?

8. Because shit happens

9. .. or do we make it happen?

10. 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

11. 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/

12. Parallelism is hard
“The Multicore Revolution”
Threads + shared mutable state :(
Changes are error-prone
http://www.russel.org.uk/Presentations/euroPython2010_theMulticoreRevolution.pdf
http://jalopnik.com/5965581/this-is-how-not-to-parallel-park-on-a-hill

13. Changes are error-prone

14. 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

15. Modularity
Break the flow into distinct steps
Do one thing and do it well

16. Flexibility
Shuffle your steps

17. Testability
Ensure correctness of your steps

18. Parallelism
Scale your steps!

19. FP building blocks
(only a subset this time)

20. Recursion
List = head() + tail()
State is hidden, built up on stack
Recursion elimination
https://bitbucket.org/evgenyg/devcon-2013/src/master/recursion.groovy

21. Immutable State
Moving parts are minimized
No defensive copying
Caching, Parallelism

22. Immutable State
New state = f ( old state )
Changes are isolated, constructor only
g ( i + 1 ), g ( list + ‘element’ )

23. Immutable State
Persistent data structures
Updated in place
Yields a new immutable state

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

25. First-class functions
Anonymous functions
Higher-order functions
Command/Template/Visitor => “Free Willy”

26. No side-effects
sin, cos, max, min, sum, sort
Opens doors for optimizations
Caching, Parallelism

27. Mindset switch
Imperative => Functional
Variables => Values
Classes => Functions
Explicit instructions => Declarative definitions
How (Steps) => What (Result)

28. Mindset switch
Pointers, memory allocations, GC - check
State management
Iterations
Filtering, conversion, folding
Parallelism

29. FP in Groovy

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

31. final j = 5
@Immutable

32. Anonymous Functions
a.k.a. Closures

33. Filter - findAll()
Map - collect()
Fold - inject()
https://bitbucket.org/evgenyg/devcon-2013/src/master/functions.groovy
http://www.barlifeuk.com/index.php/2012/08/enter-the-janneau-armagnac-three-
musketeers-inspired-competition/

34. find()
any()
every()
each()
eachWithIndex()
takeWhile()
dropWhile()

35. Collection
Range
Map
Array
String
...
Object.iterator()

36. Categories, Meta, Extensions
public
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

37. Memoization
a.k.a. Caching
https://bitbucket.org/evgenyg/devcon-2013/src/master/memoization.groovy

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

39. GPars!
Data Parallelism
https://bitbucket.org/evgenyg/devcon-2013/src/master/gpars.groovy

40. eachParallel()
collectParallel()
findAllParallel()
everyParallel()
foldParallel()
sumParallel()
gmemoize()

41. FP Candidates

42. Loops => any / every
https://bitbucket.org/evgenyg/devcon-2013/src/master/loops.groovy
http://en.hdyo.org/tee/questions

43. Split, filter, map, fold
https://bitbucket.org/evgenyg/devcon-2013/src/master/map-reduce.groovy
https://bitbucket.org/evgenyg/devcon-2013/src/master/check-versions.groovy
http://www.cityofwaterfalls.ca/hermitage_cascade.html

44. Open your eyes

45. 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

46. Thank you!
@evgeny_goldin
evgenyg@gmail.com