POC type checking when calculating with unit-based values

poc_meter.rb

class Meter
  attr_reader :value, :unit

  def initialize(value, unit = 'm')
  	@value = value
	  @unit = unit
  end

  def self.[](value)
	  self.new(value)
  end

  def +(other)
    raise TypeError, 'Added incompatiple units, please use "m"(Meter[<Number>])' unless other.is_a? self.class
	  self.class.new(value + other.value)
  end
  
  def *(other)
	  if other.respond_to?(:unit)
      unit = self.unit + other.unit
	    value = value * other.value
	  else
	    value = @value * other
	  end
    self.class.new(value, unit)
  end
end

Meter[4] + Meter[9] * 4