gistfile1.rb

module MetricSystem
  
  def self.convert(from, to, value)
    raise ArgumentError if from.blank? || to.blank? || value.blank?

    from = from.to_sym
    to   = to.to_sym
    
    if from == :kg && to == :g
      value.to_f * 1000
    elsif from == :g && to == :kg
      value.to_f / 1000
    elsif from == :pound && to == :g 
      value.to_f * 453.59237
    elsif from == :g && to == :pound 
      value.to_f / 453.59237
    elsif from == :mi && to == :km
      value.to_f * 1.60934
    elsif from == :km && to == :mi
      value.to_f / 1.60934
    elsif from == :mi && to == :m
      value.to_f * 1609.34
    elsif from == :m && to == :mi
      value.to_f / 1609.34
    else
      nil
    end
  end
  
end

puts MetricSystem::convert(:km, :mi, MetricSystem::convert(:mi, :km, 1.23456))