Maybe date monad

maybe_date.rb

# frozen_string_literal: true

require 'minitest/autorun'
require 'date'

class MaybeDate < SimpleDelegator
  include Comparable

  def initialize(date = nil, default: 9999)
    date = begin
             Date.parse(date.to_s)
           rescue ArgumentError
             Date.new(default)
           end
    super(date)
  end

  def <=>(other)
    other = self.class.new(other) unless other.is_a? self.class
    self.__getobj__ <=> other.__getobj__
  end
end

class MaybeFutureDate < MaybeDate
  def initialize(*args)
    super(*args, default: 9999)
  end
end

class MaybePastDate < MaybeDate
  def initialize(*args)
    super(*args, default: 0)
  end
end

describe MaybeDate do
  describe "its constructor" do
    it "returns a Date-like object when passed a real date" do
      subject = MaybeDate.new(Date.today)
      subject.must_be_kind_of Date
    end

    it "parses a compatible string and returns a Date-like object" do
      subject = MaybeDate.new("2018-12-31")
      subject.must_equal Date.new(2018, 12, 31)
    end

    it "parses an incompatible string and returns a Date-like object" do
      subject = MaybeDate.new("No, definitely not")
      subject.must_be_kind_of Date
    end

    it "accepts nil and returns a Date-like object" do
      subject = MaybeDate.new(nil)
      subject.must_be_kind_of Date
    end

    it "does not require an argument to return a Date-like object" do
      subject = MaybeDate.new
      subject.must_be_kind_of Date
    end
  end

  describe "comparision" do
    it "can be sorted by underlying date" do
      d1 = MaybeDate.new("2017-12-23")
      d2 = MaybeDate.new(Date.new(2010, 7, 4))
      d3 = MaybeDate.new("2016-10-03")

      [d1, d2, d3].sort.must_equal [d2, d3, d1]
    end

    it "can be compared to a date" do
      d1 = MaybeDate.new("2017-12-23")
      d2 = Date.new(2018, 7, 4)

      (d1 < d2).must_equal true
    end

    it "returns a 'future' date by default" do
      d1 = MaybeDate.new
      d2 = Date.new(2010, 7, 4)

      (d1 > d2).must_equal true
    end

    it "returns a 'past' date optionally" do
      d1 = MaybeDate.new(default: 0)
      d2 = Date.new(2010, 7, 4)

      (d1 > d2).must_equal false
    end
  end
end

describe MaybeFutureDate do
  it "is in the future" do
    (MaybeFutureDate.new > Date.today).must_equal true
  end
end

describe MaybePastDate do
  it "is in the past" do
    (MaybePastDate.new > Date.today).must_equal false
  end
end