enumerable_ext.rb

module Enumerable
  def lazy_map mapper = nil, &block
    return to_enum(:lazy_map, block) unless mapper
    return to_enum(:lazy_map, mapper) unless block

    each do |*args|
      yield mapper.call(*args)
    end
  end
end

examples.rb

# Allows applying multiple transformations in one iteration
# Examples:

[1, 2, 3, 4].lazy_map { |x| x + 1 }
# => #<Enumerator: ...>

[1, 2, 3, 4].lazy_map { |x| x + 1 }.to_a
# => [2, 3, 4, 5]

[1, 2, 3, 4].lazy_map { |x| x + 1 }.lazy_map { |y| y + 1 }.to_a
# => [3, 4, 5, 6]
# Equivalent to:
[1, 2, 3, 4].map { |x| x + 1 + 1 }

# But it gets better!

# More to come...