Experimental implementation of special subsets with + operator.

subset.rb

require 'set'

class FullSubset

  def +(other)
    self
  end

  def inspect
    "<FullSubset>"
  end

end

class NonFullSubset

  attr_reader :elements

  def initialize(elements)
    @elements = Set.new(elements)
  end

  def +(other)
    case other
    when NonFullSubset
      NonFullSubset.new(self.elements + other.elements)
    else
      other + self
    end
  end

  def inspect
    "<NonFullSubset(#{@elements.to_a.sort.join(', ')})>"
  end

end

###############################################################################

a = NonFullSubset.new([ 1, 2, 3 ])
b = NonFullSubset.new([ 3, 4, 5 ])
full = FullSubset.new
empty = NonFullSubset.new([])

p a
p b
p full
p empty
puts
puts '- - - - -'
puts

p a+a
p a+b
p a+full
p a+empty
puts

p b+a
p b+b
p b+full
p b+empty
puts

p full+a
p full+b
p full+full
p full+empty
puts

p empty+a
p empty+b
p empty+full
p empty+empty
puts