liskov_example.rb

class Rectangle
  attr_accessor :width, :height, :top_left_corner
  def new(width,height,top_left) ... ; end
  def area ... ; end
  def perimeter ... ; end
end

# A square is just a special case of rectangle...right?

class Square < Rectangle
  # ooops...a square has to have width and height equal
  attr_reader :width, :height, :side
  def width=(w)  ; @width = @height = w ; end
  def height=(w) ; @width = @height = w ; end
  def side=(w)   ; @width = @height = w ; end
end

# But is a Square really a kind of Rectangle?

def make_twice_as_wide_as_high(r, dim)
  r.width = 2*dim
  r.height = dim
end
# violates LSP, if this method is part of "contract"!

# 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
  def initialize(side,top_left_corner)
    @rect = Rectangle.new(side,side,top_left_corner)
  end
  def area      ; @rect.area      ; end
  def perimeter ; @rect.perimeter ; end
  def side=(s) ; @rect.width = @rect.height = s ; end
end