My talk about Functional Programming with Groovy at Greach Greach http://greach.es/ the Groovy spanish conf Date: 04-11-2011

1. Functional Programming
with Groovλ

2. @ArturoHerrero
http://arturoherrero.com

3. Working at OSOCO
Small but outstanding software development shop
Groovy and Grails hackers
on EC2 cloud nine
TDD mantra singers
Quality preachers

4. LISt Processing
(define (factorial n)
(if (= n 1)
1
(* n (factorial (- n 1)))))

5. Lots of Insipid Stupid Parentheses
( (factorial )
( ( )
( (factorial ( )))))

6. Code Complete
The Pragmatic Programmer
Structure and Interpretation of Computer Programs

8. A language that doesn't affect the way you think
about programming, is not worth knowing
Alan Perlis

9. Functional Programming

10. Function

11. Functional Programming
Avoiding Mutable State
λ
Side-Effect-Free Functions
λ
Referential Transparency
λ
First-Class Citizens
λ
Higher-Order Functions
λ
Lambdas and Closures
λ
Lazy Evaluation
λ
Recursion
λ

12. Why Functional Programming?
Referential transparency
λ
Unit testing
λ
Debbuging
λ
Parallelization
λ
Modularity and composition
λ
Increases the quality of code
λ
Abstractions
λ

13. OOP vs. FP

14. Imperative vs. Declarative
Imperative: how to achieve our goal
Take the next customer from a list.
If the customer lives in Spain, show their details.
If there are more customers in the list, go to the beginning
Declarative: what we want to achieve
Show customer details of every customer living in Spain

15. Imperative vs. Declarative
Functional programming is like describing
your problem to a mathematician.
Imperative programming is like giving
instructions to an idiot.
arcus, #scheme on Freenode

16. Functional Programming
with Groovy?
is an imperative language,
but we still can apply functional principles
It's basically a programmer's choice

17. Immutability
Simple
Immutable objects can only be in exactly
one state, the state in which it was created
Always consistent
Less prone to errors and more secure
Immutable objects can be shared freely
Freedom to cache
Inherently thread-safe

18. Immutability
NO: (even being a name rebind and not a real update)
book = 'Fooled by Randomness'
book = "$book - Nassim Taleb"
book = "$book (2001)"
assert 'Fooled by Randomness - Nassim Taleb (2001)' == book
YES:
book = 'Fooled by Randomness'
bookWithAuthor = "$book - Nassim Taleb"
completeBook = "$bookWithAuthor (2001)"
assert 'Fooled by Randomness - Nassim Taleb (2001)' == completeBook

19. Immutability
NO:
years = [2001, 2002]
years << 2003
years += [2004, 2005]
assert [2001, 2002, 2003, 2004, 2005] == years
YES:
years = [2001, 2002]
allYears = years + 2003 + [2004, 2005]
assert [2001, 2002, 2003, 2004, 2005] == allYears

20. Immutability
def list = ['Gr', 'vy']
NO:
list.addAll 1, 'oo'
assert list == ['Gr', 'oo', 'vy']
YES:
assert list.plus(1, 'oo') == ['Gr', 'oo', 'vy']
assert list == ['Gr', 'vy']

21. Immutability
def list = [1, 2, 2, 3]
NO:
list.removeAll 2
assert list == [1, 3]
YES:
assert list.minus(2) == [1, 3]
assert list == [1, 2, 2, 3]

22. Immutability
def list = ['Scala', 'Groovy', 'Java']
NO:
sortedList = list.sort()
assert sortedList == ['Groovy', 'Java', 'Scala']
assert list == ['Groovy', 'Java', 'Scala']
YES:
sortedList = list.sort(false)
assert sortedList == ['Groovy', 'Java', 'Scala']
assert list == ['Scala', 'Groovy', 'Java']

23. Immutability
def list = ['Java', 'Groovy', 'Java']
NO:
uniqueList = list.unique()
assert uniqueList == ['Java', 'Groovy']
assert list == ['Java', 'Groovy']
YES:
uniqueList = list.unique(false)
assert uniqueList == ['Java', 'Groovy']
assert list == ['Java', 'Groovy', 'Java']

24. Immutability
def list = ['Java', 'Groovy']
NO:
reverseList = list.reverse(true)
assert reverseList == ['Groovy', 'Java']
assert list == ['Groovy', 'Java']
YES:
reverseList = list.reverse()
assert reverseList == ['Groovy', 'Java']
assert list == ['Java', 'Groovy']

25. Immutability Collection
def list = ['Groovy', 'Java'].asImmutable()
assert 'Groovy' == list.first()
try {
list.add 'Scala' // Cannot add item
} catch (e) {
assert e instanceof UnsupportedOperationException
}
try {
list.remove 'Java' // Cannot remove item
} catch (e) {
assert e instanceof UnsupportedOperationException
}

26. Immutability Class
@Immutable class Coordinates {
Double latitude, longitude
}
def c1 = new Coordinates(latitude: 48.824068,
longitude: 2.531733)
def c2 = new Coordinates(48.824068, 2.531733)
assert c1 == c2

27. Higher-Order Functions
First-Class Citizen
Can be stored in variables
Can be passed as function parameter
Can be returned from functions
Higher-Order Functions (First-Class Functions)
Functions that take other functions
as arguments or return them as results

28. Closures
def closure = { 'Hello world!' }
assert closure() == 'Hello world!'
def sum = { a, b -> a + b }
assert sum(2,3) == 5
def square = { it * it }
assert square(9) == 81
final BASE = 1000
def salary = { variable -> BASE + variable }
assert salary(500) == 1500

29. Turn Methods into Closures
def salary(variable) {
final BASE = 1000
BASE + variable
}
assert salary(500) == 1500
def salaryClosure = this.&salary
assert salaryClosure(500) == 1500

30. Closures Composition
def minutesToSeconds = { it * 60 }
def hoursToMinutes = { it * 60 }
def daysToHours = { it * 24 }
def hoursToSeconds = minutesToSeconds << hoursToMinutes
def daysToSeconds = hoursToSeconds << daysToHours
assert daysToSeconds(1) == 86400

31. Closures Composition
def upper = { it.toUpperCase() }
def firstLetter = { it.charAt(0) }
def words = ["Don't", "Repeat", "Yourself"]
def acronym = words.collect(firstLetter >> upper).join()
assert acronym == 'DRY'

32. Currying
given: ƒ: (X x Y) -> Z
then: curry(ƒ): X -> (Y -> Z)
Takes a function with a particular
number of parameters and returns a
function with some of the parameter
values fixed, creating a new function

33. Currying
def modulus = { mod, num ->
num % mod
}
assert modulus(2, 5) == 1
assert modulus(3, 5) == 2
def mod2 = modulus.curry(2)
assert mod2(5) == 1
def mod3 = modulus.curry(3)
assert mod3(5) == 2

34. Currying
def bill = { amount, currency ->
"$amount $currency"
}
assert bill(1000, '$') == '1000 $'
assert bill(1000, '€') == '1000 €'
def billInDollars = bill.rcurry('$')
assert billInDollars(1000) == '1000 $'
def billInEuros = bill.rcurry('€')
assert billInEuros(1000) == '1000 €'

35. Currying
def joinWithSeparator = { one, sep, two ->
one + sep + two
}
def joinWithAmpersand =
joinWithSeparator.ncurry(1, '&')
assert joinWithAmpersand('a', 'b') == 'a&b'

36. Classic Operations on
Functional Data Types
list filter
map
fold

37. Classic Operations on
Functional Data Types
list findAll
collect
inject

38. Classic Operations on
Functional Data Types
list any
every
sort
min
sum

39. findAll()
NO:
def result = []
[1, 2, 3, 4].each {
if (it > 2) {
result << it
}
}
assert result == [3, 4]
YES:
assert [1, 2, 3, 4].findAll{ it > 2 } == [3, 4]

40. collect()
NO:
def result = []
[1, 2, 3].each {
result << it * 2
}
assert result == [2, 4, 6]
YES:
assert [1, 2, 3].collect{ it * 2 } == [2, 4, 6]

41. inject()
NO:
def total = 0
[1, 2, 3].each {
total += it
}
assert total == 6
YES:
def total = [1, 2, 3].inject(0) { acc, n ->
acc + n
}
assert total == 6

42. find()
NO:
def result
try {
[1, 2, 3].each {
if (it > 1) {
result = it
throw new Exception() // monstrous
}
}
} catch(exception) { }
assert result == 2
YES:
assert [1, 2, 3].find{ it > 1 } == 2

43. max()
@TupleConstructor // import groovy.transform.*
class Person {
String name
Integer age
}
def person1 = new Person('Arturo', 26)
def person2 = new Person('Luis', 61)
def person3 = new Person('Laura', 19)
def family = [] << person1 << person2 << person3
assert family.max{ it.age }.age == 61
assert family.collect{ it.age }.max() == 61
assert family*.age.max() == 61

44. Refactoring
def exists = false
family.each { person ->
if (person.age > 60) {
exists = true
}
}
assert exists == true
def exists = family.inject(false) { found, person ->
if (person.age > 60) {
found = true
}
return found
}
assert exists == true
assert family.any{ it.age > 60 } == true

45. Combinator Functions
@TupleConstructor // import groovy.transform.*
class Person {
String name
String lastname
Integer age
}
def rafa = new Person('Rafael', 'Luque', 36)
def marcin = new Person('Marcin', 'Gryszko', 34)
def arturo = new Person('Arturo', 'Herrero', 26)
def osokers = [] << rafa << marcin << arturo << rafa
assert osokers.unique(false)
.findAll{ it.age > 30}
.sort{ it.lastname } == [marcin, rafa]
assert osokers == [rafa, marcin, arturo, rafa]

46. Combinator Functions
// Procedural style
def count = 0
for (i in (1 .. 1000)) {
if (i % 2) {
count += ("$i".size())
}
}
assert count == 1445
// Functional style
def count = (1 .. 1000).findAll{ it % 2 }
.collect{ "$it" }
.inject(0) { sum, num ->
sum + num.size()
}
assert count == 1445

47. Lazy Evaluation
Only does as much work as necessary
Delays the evaluation of the expression
until it's needed
CPU efficient
The value is not calculated or assigned
until the value is requested
Manage potentially infinite data structures
Only a manageable subset of the data
will actually be used

48. Lazy Evaluation
class Person {
@Lazy String name = 'Arturo'
}
def person = new Person()
assert !(person.dump().contains('Arturo'))
assert person.name.size() == 6
assert person.dump().contains('Arturo')

49. Lazy Evaluation
class Get {
String url
@Lazy URL urlObj = { url?.toURL() }()
@Lazy(soft=true) String text = urlObj?.text
}
def get = new Get(url: 'http://arturoherrero.com')
assert get.url == 'http://arturoherrero.com'
assert get.dump().contains('text=null')
assert get.dump().contains('urlObj=null')
assert get.urlObj.protocol == 'http'
assert get.urlObj.host == 'arturoherrero.com'
assert get.text.contains('Arturo Herrero')

50. Lazy Evaluation
groovy.sql.DataSet.DataSet
groovy.util.XmlSlurper
@Singleton(lazy=true)
class Util {
Integer count(text) {
text.size()
}
}
assert 6 == Util.instance.count('Arturo')

51. Infinite structures
class LazyList {
...
}
def naturalnumbers = integers(1)
assert '1 2 3 4 5 6' == naturalnumbers.take(6).join(' ')
def evennumbers = naturalnumbers.filter{ it % 2 == 0 }
assert '2 4 6 8 10 12' == evennumbers.take(6).join(' ')

52. Infinite structures
@Grab('org.functionaljava:functionaljava:3.0')
import fj.data.Stream
Stream.metaClass.filter = { Closure c ->
delegate.filter(c as fj.F) }
Stream.metaClass.asList = {
delegate.toCollection().asList() }
def evens = Stream.range(1).filter{ it % 2 == 0 }
assert [2, 4, 6, 8, 10, 12] == evens.take(6).asList()

53. Recursive
factorial(6)
6 * factorial(5)
6 * (5 * factorial(4))
6 * (5 * (4 * factorial(3)))
6 * (5 * (4 * (3 * factorial(2))))
6 * (5 * (4 * (3 * (2 * factorial(1)))))
6 * (5 * (4 * (3 * (2 * 1))))
6 * (5 * (4 * (3 * 2)))
6 * (5 * (4 * 6))
6 * (5 * 24)
6 * 120
720

54. Recursive
factorial(6)
6 * factorial(5)
6 * (5 * factorial(4))
6 * (5 * (4 * factorial(3)))
6 * (5 * (4 * (3 * factorial(2))))
6 * (5 * (4 * (3 * (2 * factorial(1)))))
6 * (5 * (4 * (3 * (2 * 1))))
6 * (5 * (4 * (3 * 2)))
6 * (5 * (4 * 6))
6 * (5 * 24)
6 * 120
720

55. Tail-Recursive
factorial(6, 1)
factorial(5, 6)
factorial(4, 30)
factorial(3, 120)
factorial(2, 360)
factorial(1, 720)

56. Tail-Recursive
factorial(6, 1)
factorial(5, 6)
factorial(4, 30)
factorial(3, 120)
factorial(2, 360)
factorial(1, 720)

57. Tail Call Optimization
3 techniques:
The compiler transform the recursion into a loop
λ
Let the JVM recognize the recursion and eliminate it
λ
Transform the recursion into iterative by hand
λ

58. Tail Call Optimization
3 techniques:
The compiler transform the recursion into a loop
λ
Let the JVM recognize the recursion and eliminate it
λ
Transform the recursion into iterative by hand
λ
really?

59. Tail Call Optimization
def factorial
factorial = { n ->
n == 1 ? 1 :
n * factorial(n - 1)
}
factorial(1000)

60. Tail Call Optimization
rflow
kO ve
def factorial S tac
factorial = { n ->
n == 1 ? 1 :
n * factorial(n - 1)
}
factorial(1000)

61. Tail Call Optimization
def factorial
factorial = { n, BigInteger acc = 1 ->
n == 1 ? acc :
factorial(n - 1, n * acc)
}
factorial(1000)

62. Tail Call Optimization
rflow
kO ve
def factorial S tac
factorial = { n, BigInteger acc = 1 ->
n == 1 ? acc :
factorial(n - 1, n * acc)
}
factorial(1000)

63. Tail Call Optimization
def factorial
factorial = { n, BigInteger acc = 1 ->
n == 1 ? acc :
factorial.trampoline(n - 1, n * acc)
}.trampoline()
factorial(1000)

64. Trampolining
def even, odd
even = { x -> x == 0 ? true :
odd.trampoline(x - 1) }.trampoline()
odd = { x -> x == 0 ? false :
even.trampoline(x - 1) }.trampoline()
assert even(1000) == true

65. Memoization
def fibonacci
fibonacci = { n ->
n <= 1 ? n :
fibonacci(n - 1) + fibonacci(n - 2)
}
fibonacci(35) // 9.935 seconds

66. Memoization
def fibonacci
fibonacci = { n ->
n <= 1 ? n :
fibonacci(n - 1) + fibonacci(n - 2)
}.memoize()
fibonacci(35) // 0.002 seconds

67. Memoization
def plus = { a, b -> sleep 1000; a + b }.memoize()
assert plus(1, 2) == 3 // after 1000ms
assert plus(1, 2) == 3 // return immediately
assert plus(2, 2) == 4 // after 1000ms
assert plus(2, 2) == 4 // return immediately
def plusAtLeast = { ... }.memoizeAtLeast(10)
def plusAtMost = { ... }.memoizeAtMost(10)
def plusBetween = { ... }.memoizeBetween(10, 20)

68. “Functional” is more a way of thinking
than a tool set
Neal Ford

69. Be a craftsman

71. Thank you!
@ArturoHerrero
Join us at OSOCO