chess-pawn.rb

#!/usr/bin/env ruby


class Pawn
  def move_down?(x,y)
    can_move_distance?(x,y)
  end

  def move_up?(x,y)
    can_move_distance?(x,y)
  end

private

  def can_move_distance?(x,y)
    # actually implemented in reality,
    # but `true` here to contrast `false` below
    true
  end
end

class WhitePawn < Pawn
  def move_down?(x,y)
    false
  end
end

class BlackPawn < Pawn
  def move_up?(x,y)
    false
  end
end


white = WhitePawn.new
black = BlackPawn.new


puts "WHITE DOWN -- #{ white.move_down?(5,5) }"
puts "WHITE UP   -- #{ white.move_up?(5,5)   }"

puts "BLACK DOWN -- #{ black.move_down?(5,5) }"
puts "BLACK UP   -- #{ black.move_up?(5,5)   }"


# $ ruby chess-pawn.rb
# WHITE DOWN -- false
# WHITE UP   -- true
# BLACK DOWN -- true
# BLACK UP   -- false