lsp_square_rect_fix.rb

# LSP-compliant solution: replace inheritance with delegation
# Ruby's duck typing still lets you use a square in most places where
#  rectangle would be used - but no longer a subclass in LSP sense.
class Square
  attr_accessor :rect
  def initialize(side, top_left)
    @rect = Rectangle.new(side, side, top_left)
  end
  def area      ; rect.area      ; end
  def perimeter ; rect.perimeter ; end
  # A more concise way to delegate, if using ActiveSupport (see text):
  #  delegate :area, :perimeter, :to => :rectangle
  def side=(s) ; rect.width = rect.height = s ; end
end