Laziness

Slides from my talk about lazy evaluation at the May RUG::B (Ruby Users Group Berlin) meetup.

Centered in Ruby, contains also some Haskell, Clojure, and pictures of cute sleeping animals.

Transcript

1. Laziness porras http://iamserg.io porras RUG::B · May 2013
2. Thank you.
3. Questions?
4. Hallo RUG::B!
5. Ich heiße Sergio porras http://iamserg.io porras
6. Laziness
7. Lazy Evaluation
8. “evaluation strategy which delays the evaluation of an expression until its value is needed” http://en.wikipedia.org/wiki/Lazy_evaluation
9. @articles = Article.where(status: 'published')
10. @articles = Article.where(status: 'published') @articles.each do |article| ...
11. @articles = Article.where(status: 'published') @articles.limit(5).each do |article| ...
12. take 5 (filter odd [1..])
13. take 5 (filter odd [1..]) [1,3,5,7,9]
14. (defn fibonacci [] (map first (iterate (fn [[a b]] [b (+ a b)]) [0 1])))
15. (defn fibonacci [] (map first (iterate (fn [[a b]] [b (+ a b)]) [0 1]))) (take 20 (fibonacci))
16. def fibonacci all_pairs.map(&:first) end def all_pairs pairs = [[0, 1]] loop do pairs << next_pair(*pairs.last) end pairs end def next_pair(a, b) [b, a + b] end fibonacci.take(20)
19. def fibonacci all_pairs.map(&:first) end def all_pairs pairs = [[0, 1]] loop do pairs << next_pair(*pairs.last) end pairs end def next_pair(a, b) [b, a + b] end fibonacci.take(20)
20. def fibonacci all_pairs.map(&:first) end def all_pairs pairs = [[0, 1]] loop do pairs << next_pair(*pairs.last) end pairs end def next_pair(a, b) [b, a + b] end fibonacci.take(20)
21. def fibonacci all_pairs.map(&:first) end def all_pairs pairs = [[0, 1]] loop do pairs << next_pair(*pairs.last) end pairs end def next_pair(a, b) [b, a + b] end fibonacci.take(20) never
22. (defn fibonacci [] (map first (iterate (fn [[a b]] [b (+ a b)]) [0 1])))
23. (defn fibonacci [] (map first (iterate (fn [[a b]] [b (+ a b)]) [0 1]))) (take 20 (fibonacci))
24. (defn fibonacci [] (map first (iterate (fn [[a b]] [b (+ a b)]) [0 1]))) (take 20 (fibonacci)) ; (0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597 2584 4181)
25. But why
26. Performance no faster operation than no operation
27. Infinite lists
28. Non-evaluated code
29. Expressiveness
31. Poor man’s laziness in Ruby
32. class Whatever attr_reader :data def initialize(param) @data = expensive_calculation(param) end private def expensive_calculation(param) ... end end
33. class Whatever def initialize(param) @param = param end def data expensive_calculation(@param) end private def expensive_calculation(param) ... end end
34. class Whatever def initialize(param) @param = param end def data @data ||= expensive_calculation(@param) end private def expensive_calculation(param) ... end end
36. Can you imagine a language without unless?
37. unless :: Bool -> a -> Maybe a unless condition code = if condition then Nothing else Just code
38. unless :: Bool -> a -> Maybe a unless condition code = if condition then Nothing else Just code > unless (10 > 5) "whatever"
39. unless :: Bool -> a -> Maybe a unless condition code = if condition then Nothing else Just code > unless (10 > 5) "whatever" Nothing
40. unless :: Bool -> a -> Maybe a unless condition code = if condition then Nothing else Just code > unless (10 > 5) "whatever" Nothing > unless (10 > 15) "whatever"
41. unless :: Bool -> a -> Maybe a unless condition code = if condition then Nothing else Just code > unless (10 > 5) "whatever" Nothing > unless (10 > 15) "whatever" Just "whatever"
43. def my_unless(condition, code) if condition nil else code end end
44. def my_unless(condition, code) if condition nil else code end end def whatever puts "whatever method" :whatever end
45. def my_unless(condition, code) if condition nil else code end end def whatever puts "whatever method" :whatever end > my_unless(10 > 15, whatever)
46. def my_unless(condition, code) if condition nil else code end end def whatever puts "whatever method" :whatever end > my_unless(10 > 15, whatever) whatever method => :whatever
47. def my_unless(condition, code) if condition nil else code end end def whatever puts "whatever method" :whatever end > my_unless(10 > 15, whatever) whatever method => :whatever > my_unless(10 > 5, whatever)
48. def my_unless(condition, code) if condition nil else code end end def whatever puts "whatever method" :whatever end > my_unless(10 > 15, whatever) whatever method => :whatever > my_unless(10 > 5, whatever) whatever method => nil
49. def my_unless(condition, &code) if condition nil else code.call end end def whatever puts "whatever method" :whatever end > my_unless(10 > 15) { whatever } whatever method => :whatever > my_unless(10 > 5) { whatever } => nil
50. Procs = lazy code
51. “evaluation strategy which delays the evaluation of an expression until its value is needed” http://en.wikipedia.org/wiki/Lazy_evaluation we
53. https://speakerdeck.com/michaelfairley/extending-ruby-with-ruby http://www.youtube.com/watch?v=ZjYgNnCtpg4
54. https://speakerdeck.com/michaelfairley/extending-ruby-with-ruby http://www.youtube.com/watch?v=ZjYgNnCtpg4
55. https://github.com/porras/trenza
56. class Wadus def calculate # whatever end end wadus1 = Wadus.new.lazy wadus2 = Wadus.new.lazy x = wadus1.calculate y = wadus2.calculate # no calculation is actually done puts x # now wadus1.calculate is called # wadus2.calculate never gets called
58. Ruby 2.0 came along...
59. Enumerator::Lazy Enumerable#lazy http://railsware.com/blog/2012/03/13/ruby-2-0-enumerablelazy/ https://tomafro.net/2012/12/infinite-sequences-in-ruby
60. N = (1..Float::INFINITY).lazy N.select(&:odd?).take(5)
61. N = (1..Float::INFINITY).lazy N.select(&:odd?).take(5) [1, 3, 5, 7, 9]
62. Fibonacci = Enumerator.new do |e| pair = [0, 1] loop do e << pair pair = [pair[1], pair[0] + pair[1]] end end.lazy.map(&:first)
63. Fibonacci = Enumerator.new do |e| pair = [0, 1] loop do e << pair pair = [pair[1], pair[0] + pair[1]] end end.lazy.map(&:first) > Fibonacci.take(10) => [0, 1, 1, 2, 3, 5, 8, 13, 21, 34]
64. Fibonacci. select { |n| n % 5 == 0 }. take_while { |n| n < 10 ** 6 }
65. Fibonacci. select { |n| n % 5 == 0 }. take_while { |n| n < 10 ** 6 } [0, 5, 55, 610, 6765, 75025, 832040]
66. Thank you. porras http://iamserg.io porras
67. Thank you. porras http://iamserg.io porras No animals were harmed woken up in the making of this talk