Enumerator::Lazy - Speaker Deck

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Enumerator::Lazy Erik Michaels-Ober @sferik

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

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

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

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

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Ruby’s iterator is called Enumerator

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

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

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

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

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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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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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Ruby 2.0 introduced Enumerator::Lazy

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

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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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What are the ﬁrst ﬁve  twin primes?

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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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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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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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When are the next ﬁve  Friday the 13ths?

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

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