Lazy evaluation in Ruby

1.使用||延迟初始化

def fib(n)
     @result ||= []
     return n if (0..1).include? n
     @result[n] ||= fib(n-1) + fib(n-1)
end

2.使用无穷大Inifinity
Ruby可以使用无穷大：

irb(main):001:0> Inf = 1 / 0.0
=> Infinity
irb(main):002:0> (-Inf..Inf).include?(10)
=> true

使用Inifinity可以构造无穷大小的Lazy List：

lazyList = (0..1/0.0)
lazyList.take(100).select{|i| i % 2 == 0}
 lazyList.take(100).select{rand > 0.9}

3.使用Enumerator:

fib = Enumerator.new { |y|
    a = b = 1
    loop {
      y << a
      a, b = b, a + b
    }
 }
fib.take(10) #=> [1, 1, 2, 3, 5, 8, 13, 21, 34, 55]

natural_numbers = Enumerator.new do |yielder|
  number = 1
  loop do
    yielder.yield number
    number += 1
  end
end

#lazy select
class Enumerator
  def lazy_select(&block)
    Enumerator.new do |yielder|
      self.each do |val|
      yielder.yield(val) if block.call(val)
      end
    end
  end
end
p natural_numbers
    .lazy_select {|n| n % 47 == 0}
    .lazy_select {|n| palindrome_number?(n)}
    .first(5)

4.使用Fiber:

fib = Fiber.new do
   f1 = f2 = 1
   loop do
       Fiber.yield f1
       f1, f2 = f2, f1 + f2
    end
end

10.times { puts fib.resume }

参考：
Lazy Enumerators in Ruby