module Funkr
  module Categories
    # Monoids are types that can be added and have a zero element.
    module Monoid

      def mplus
        raise "Monoid#mplus not implemented"
      end
      
      module ClassMethods
        def mzero
          raise "Monoid#mzero not implemented"
        end
        
        def mconcat(list)
          list.inject(mzero){|a,e| a.mplus(e)}
        end
      end
    end
  end
end