takietam.rb

class ArrayMap
  def initialize(array)
    @array = array
  end

  def map(method)
    [method.call(head)] + tail.map(method)
  end

  def head
    @array[0]
  end

  def tail
    ArrayMapFactory.build(@array[1..-1])
  end
end

class EmptyArrayMap < ArrayMap
  def map(method)
    []
  end
end

class ArrayMapFactory
  CONFIG = { 0 => EmptyArrayMap }

  def self.build(array)
    klass = CONFIG[array.count] || ArrayMap
    klass.new(array)
  end
end

def map(array, method)
  ArrayMapFactory.build(array).map(method)
end