6_10.rb

class Bicycle
  attr_reader :style, :size,
              :tape_color,
              :front_shock, :rear_shock

  def initialize(**opts)
    @style       = opts[:style]
    @size        = opts[:size]
    @tape_color  = opts[:tape_color]
    @front_shock = opts[:front_shock]
    @rear_shock  = opts[:rear_shock]
  end

  # checking 'style' starts down a slippery slope
  def spares
    if style == :road
      { chain:       '11-speed',
        tire_size:   '23',       # milimeters
        tape_color:  tape_color }
    else
      { chain:       '11-speed',
        tire_size:   '2.1',      # inches
        front_shock: front_shock }
    end
  end
  # ...
end


bike = Bicycle.new(
        style:       :mountain,
        size:        'S',
        front_shock: 'Manitou',
        rear_shock:  'Fox')

puts bike.spares
# => {:chain=>"11-speed", :tire_size=>"2.1", :front_shock=>"Manitou"}

bike = Bicycle.new(
        style:       :road,
        size:        'M',
        tape_color:  'red')

puts bike.spares
# => {:chain=>"11-speed", :tire_size=>"23", :tape_color=>"red"}