test, and the code that makes the test pass, to understand whole values and exceptional values, from Avdi Grim's https://avdi.codes/courses/moom/, based on https://josh.works/primative-obsession-and-exceptional-values

exceptional_values.rb

class Object
  def exceptional?
    false
  end
end

Course = Struct.new(:name, :duration) do  
  def duration=(new_duration)
    self[:duration] = Duration(new_duration)
  end
end

class Duration
  def self.[](magnitude)
    new(magnitude)
  end
  
  attr_reader :magnitude
  def initialize(magnitude)
    @magnitude = magnitude
    freeze
  end
  
  def to_s
    "#{self.magnitude} #{self.class.name.downcase}"
  end
  
  def inspect
    "#{self.class}[#{self.magnitude}]"
  end
  
  alias_method :to_i, :magnitude
end

class Days < Duration; end
class Weeks < Duration; end
class Months < Duration; end

class ExceptionalValue
  attr_reader :raw_value, :reason
  def initialize(raw_value, reason: "Unspecified")
    @raw_value = raw_value
    @reason = reason
  end
  def exceptional?
    true
  end
  
  def to_s
    @raw_value.to_s
  end
end

# helper methods
def render_course_info(course)
  "#{course.name} (#{render_value(course.duration)})"
end

def render_value(value)
  case value
  when Months
    "#{value.to_i} gruling months"
  when Weeks
    "#{value.to_i} delightful weeks"
  when Days
    "a paultry #{value.to_i} days"
  end
end

# in the video, this method was capitalized, I don't know why but I imagine
# it was for a good reason
def Duration(raw_value)
  case raw_value
  when Duration
    raw_value
  when /\A(\d+)\s+months\z/i
    Months[$1.to_i]
  when /\A(\d+)\s+weeks\z/i
    Weeks[$1.to_i]
  when /\A(\d+)\s+days\z/i
    Days[$1.to_i]
  else 
    ExceptionalValue.new(raw_value, reason: "unrecognized format")
  end
end

exceptional_values_test.rb

require_relative './exceptional_values'
gem 'minitest'
require 'minitest/autorun'

class DurationTest < MiniTest::Test
  
  def setup
    @math = Course.new("Math", Months[4])
    @grammar = Course.new("Grammar", Days[50])
    @skrying = Course.new("Skrying", Weeks[7])
  end
  
  def test_create_new_duration
    assert @math
    assert_equal "Math", @math.name
  end
  
  def test_duration_has_magnitude
    assert_equal 4, @math.duration.magnitude
  end
  
  def test_duration_is_months_object
    assert_instance_of Months, @math.duration
  end
  
  def test_duration_formatting_is_not_string
    refute_equal String, @math.duration.class
  end
  
  def test_duration_to_i_is_aliased_to_magnitude
    assert_equal 4, @math.duration.to_i
  end
  
  def test_duration_to_string
    assert_equal "4 months", @math.duration.to_s
  end
  
  def test_duration_inspect
    assert_equal "Months[4]", @math.duration.inspect
  end
  
  def test_duration_can_change
    @math.duration = Weeks[3]
    
    assert_instance_of Weeks, @math.duration
  end
  
  def test_duration_can_be_denominated_in_days
    assert_instance_of Days, @grammar.duration
    assert_equal 50, @grammar.duration.magnitude
  end
  
  def test_duration_can_be_denominated_in_weeks
    assert_instance_of Weeks, @skrying.duration
    assert_equal 7, @skrying.duration.magnitude
  end
  
  def test_duration_can_be_denominated_in_months
    potions = Course.new("Potions", Months[3])
    
    assert_instance_of Months, potions.duration
    assert_equal 3, potions.duration.magnitude
  end
  
  # test helper methods
  def test_render_months_class
    results = render_value(@math.duration)
    
    assert_equal "4 gruling months", results
  end
  
  def test_render_weeks_class
    results = render_value(@skrying.duration)
    
    assert_equal "7 delightful weeks", results
  end
  
  def test_render_days_class
    results = render_value(@grammar.duration)
    
    assert_equal "a paultry 50 days", results
  end
  
  def test_render_course_info_returns_nicely_formatted_strings
    assert_equal "Math (4 gruling months)", render_course_info(@math)
    assert_equal "Grammar (a paultry 50 days)", render_course_info(@grammar)
    assert_equal "Skrying (7 delightful weeks)", render_course_info(@skrying)
  end
  
  def test_freeze_magnitude_in_initialize
    skip
    # not sure what I can assert to make sure value is frozen
    # https://blog.honeybadger.io/when-to-use-freeze-and-frozen-in-ruby/
  end
  
  # testing exceptional values
  def test_Duration_converts_months_string_to_duration_object
    math = Course.new("Math")
    math.duration = "4 months"
    assert_equal Months, math.duration.class
    assert_equal 4, math.duration.magnitude
  end
  
  def test_Duration_convert_weeks_string_to_duration_object
    math = Course.new("Math")
    math.duration = "7 days"
    assert_equal Days, math.duration.class
    assert_equal 7, math.duration.magnitude
  end
  
  def test_convert_weeks_string_to_duration
    math = Course.new("Math")
    math.duration = "3 weeks"
    assert_instance_of Weeks, math.duration
    assert_equal 3, math.duration.magnitude
  end
  
  def test_exceptional_values
    math = Course.new("Math")
    math.duration = "a blink of an eye"
    assert_instance_of ExceptionalValue, math.duration
    assert_equal "a blink of an eye", math.duration.to_s
  end
  
  def test_exceptional_values_know_they_are_exceptional
    math = Course.new("Math")
    math.duration = "a blink of an eye"
    assert math.duration.exceptional?
    refute math.name.exceptional?
  end
end