feat(undefined): introduce undefined

lib/convenient_service/support.rb

@@ -1,6 +1,8 @@
 # frozen_string_literal: true
 
 require_relative "support/not_passed"
+require_relative "support/undefined"
+
 require_relative "support/concern"
 
 require_relative "support/abstract_method"

lib/convenient_service/support/undefined.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+module ConvenientService
+  module Support
+    UNDEFINED = ::Object.new.tap do |object|
+      object.define_singleton_method(:inspect) { "undefined" }
+    end
+  end
+end

spec/lib/convenient_service/support/undefined_spec.rb

@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+require "spec_helper"
+
+require "convenient_service"
+
+# rubocop:disable RSpec/NestedGroups, RSpec/DescribeClass
+RSpec.describe "convenient_service/support/undefined" do
+  example_group "constants" do
+    describe "::UNDEFINED" do
+      it "returns `Object` instance" do
+        expect(ConvenientService::Support::UNDEFINED).to be_instance_of(Object)
+      end
+
+      example_group "instance_methods" do
+        describe "#inspect" do
+          it "returns inspect representation" do
+            expect(ConvenientService::Support::UNDEFINED.inspect).to eq("undefined")
+          end
+        end
+      end
+    end
+  end
+end
+# rubocop:enable RSpec/NestedGroups, RSpec/DescribeClass