Enumerator::Lazy

Presented at SF.rb on August 2, 2016.

Transcript

1. Enumerator::Lazy
   Erik Michaels-Ober
   @sferik
   

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2. Imperative languages

   do iteration like this:
   int sum = 0;
   for(i = 1; i < 10; i = i + 1) {
   sum = sum + i;
   }
   

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3. Functional languages

   do iteration like this:
   rec_sum [] = 0
   rec_sum (x:xs) = x + rec_sum xs
   rec_sum [1..9]
   

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4. Object oriented languages

   (should) do iteration like this:
   sum = 0
   (1..9).each do |i|
   sum += i
   end
   

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5. Object oriented languages

   (should) do iteration like this:
   sum = (1..9).inject(&:+)
   

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6. Iterators
   Introduced in CLU by

   Barbara Liskov
   (1975)
   Copied in Ruby by
   Yukihiro Matsumoto
   (1995)
   

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7. Ruby’s iterator is called
   Enumerator
   

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8. enum = Enumerator.new do |yielder|
   yielder.yield("sf")
   yielder.yield("dot")
   yielder.yield("rb")
   end
   

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9. ["sf", "dot", "rb"].each
   ["sf", "dot", "rb"].to_enum
   Enumerator.new(["sf", "dot", "rb"])
   

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10. enum = Enumerator.new do |yielder|
    n = 0
    loop do
    yielder.yield(n)
    n += 1
    end
    end
    

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11. fib = Enumerator.new do |yielder|
    a = b = 1
    loop do
    yielder.yield(a)
    a, b = b, a + b
    end
    end
    

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12. module Enumerable
    def lazy_map(&block)
    Enumerator.new do |yielder|
    return to_enum(__method__) unless block_given?
    each do |n|
    yielder.yield(block.call(n))
    end
    end
    end
    end
    

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13. module Enumerable
    def lazy_select(&block)
    Enumerator.new do |yielder|
    return to_enum(__method__) unless block_given?
    each do |n|
    yielder.yield(n) if block.call(n)
    end
    end
    end
    end
    

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14. Ruby 2.0 introduced
    Enumerator::Lazy
    

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15. What are the first five even
    perfect squares over a thousand?
    

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16. lazy_integers = (1..Float::INFINITY).lazy
    lazy_integers.collect { |x| x ** 2 }.
    select { |x| x.even? }.
    reject { |x| x < 1000 }.
    first(5)
    #=> [1024, 1156, 1296, 1444, 1600]
    

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17. What are the first five

    twin primes?
    

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18. require "prime"
    lazy_primes = Prime.lazy
    lazy_primes.select { |x| (x - 2).prime? }.
    collect { |x| [x - 2, x] }.
    first(5)
    #=> [[3, 5], [5, 7], [11, 13], [17, 19], [29, 31]]
    

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19. module Enumerable
    def repeat_after_first
    return to_enum(__method__) unless block_given?
    each.with_index do |*val, index|
    index.zero? ? yield(*val) : 2.times { yield(*val) }
    end
    end
    end
    

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20. require "prime"
    lazy_primes = Prime.lazy
    lazy_primes.repeat_after_first.
    each_slice(2).
    select { |x, y| x + 2 == y }.
    first(5)
    #=> [[3, 5], [5, 7], [11, 13], [17, 19], [29, 31]]
    

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21. When are the next five

    Friday the 13ths?
    

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22. require "date"
    lazy_dates = (Date.today..Date.new(9999)).lazy
    lazy_dates.select { |d| d.day == 13 }.
    select { |d| d.friday? }.
    first(10)
    

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23. Detect whether a text file
    contains a string?
    (without reading the entire file into memory)
    

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24. lazy_file = File.readlines("/path/to/file").lazy
    lazy_file.detect { |x| x =~ /regexp/ }
    

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25. Being lazy is efficient.
    

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26. Being lazy is elegant.
    

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27. View Slide

28. Thank you
    

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