#diff method for Ruby Data classes

data_diff.rb

Measure = Data.define(:amount, :unit) do
  def diff(other)
    raise ArgumentError.new("other must be same class") unless other.is_a?(self.class)

    members.inject({}) do |memo, key|
      thisVal, otherVal = send(key), other.send(key)
      memo[key] = [thisVal, otherVal] unless thisVal == otherVal
      
      memo
    end
  end
end

m1 = Measure.new(1, "km")
m2 = m1.with(unit: "g")

m1.diff(m2) # {:unit=>["km", "g"]}
m1.diff(m1) # {}