lazy_enumerables_from_scratch.rb

require 'fiber'

class Step
  def initialize(source, yielder=nil)
    @context = Fiber.new do
      source.each do |element|
        if yielder
          yielder.call(element)
        else
          Fiber.yield(element)
        end
      end
    end
  end

  def take(num)
    to_enum.take(num)
  end

  def element
    to_enum.next
  end

  def first
    to_enum.next
  end

  def all
    to_enum.to_a
  end

  def each
    return to_enum unless block_given?

    loop do
      element = @context.resume
      if @context.alive?
        yield element
      else
        return
      end
    end
  end

  def map(&block)
    yielder = -> (element) do
      Fiber.yield(block.call(element))
    end

    Step.new(self, yielder)
  end

  def filter(&block)
    yielder = -> (element) do
      Fiber.yield(element) if block.call(element)
    end

    Step.new(self, yielder)
  end
end

source = Step.new(["a", "b", "c", "d", "e", "f"])
result = source.map { |v| v.to_sym }.filter { |v| v != :a }

p result.take(2)