Implement MemoizedHelpers#let?

lib/rspec/sleeping_king_studios/deferred/dsl/memoized_helpers.rb

@@ -28,6 +28,7 @@ def self.extended(other)
       )
     end
 
+    # @api private
     def call(example_group)
       super
 
@@ -66,6 +67,25 @@ def let!(helper_name, &)
       before(:example) { send(helper_name) }
     end
 
+    # Defines an optional memoized helper.
+    #
+    # The helper will use the parent value if defined; otherwise, will use the
+    # given value.
+    #
+    # @param helper_name [String, Symbol] the name of the helper method.
+    # @param block [Block] the implementation of the helper method.
+    #
+    # @return [void]
+    def let?(helper_name, &block)
+      wrapped = lambda do
+        next super() if defined?(super())
+
+        instance_exec(&block)
+      end
+
+      let(helper_name, &wrapped)
+    end
+
     # Defines a memoized subject helper.
     #
     # @param helper_name [String, Symbol] the name of the helper method.

spec/integration/deferred/optional_helpers_spec.fixture.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+require 'support/integration/deferred/orbit_examples'
+
+RSpec.describe Spec::Models::Rocket do
+  subject(:rocket) { described_class.new('Imp IV') }
+
+  include Spec::Integration::Deferred::OrbitExamples
+end

spec/integration/deferred/optional_helpers_spec.rb

@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+require 'rspec/sleeping_king_studios/deferred/examples'
+
+require 'support/sandbox'
+
+RSpec.describe RSpec::SleepingKingStudios::Deferred::Examples do
+  let(:fixture_file) do
+    %w[spec/integration/deferred/optional_helpers_spec.fixture.rb]
+  end
+  let(:result) do
+    Spec::Support::Sandbox.run(fixture_file)
+  end
+  let(:expected_examples) do
+    <<~EXAMPLES.lines.map(&:strip)
+      Spec::Models::Rocket#launch when the rocket is launched should set the orbit
+      Spec::Models::Rocket#launch with orbit: value when the rocket is launched should set the orbit
+      Spec::Models::Rocket#orbit is expected to equal nil
+    EXAMPLES
+  end
+
+  it 'should apply the deferred examples', :aggregate_failures do
+    expect(result.summary).to be == '3 examples, 0 failures'
+
+    expect(result.example_descriptions).to be == expected_examples
+  end
+end

spec/support/integration/deferred/orbit_examples.rb

@@ -0,0 +1,47 @@
+# frozen_string_literal: true
+
+require 'support/integration/deferred'
+
+module Spec::Integration::Deferred
+  module ShouldSetTheOrbit
+    include RSpec::SleepingKingStudios::Deferred::Examples
+
+    let?(:expected_orbit) { nil }
+
+    it 'should set the orbit' do
+      launch_rocket
+
+      expect(subject.orbit).to eq expected_orbit
+    end
+  end
+
+  module OrbitExamples
+    include RSpec::SleepingKingStudios::Deferred::Examples
+
+    describe '#launch' do
+      let(:launch_site) { 'KSC' }
+      let(:options)     { {} }
+
+      def launch_rocket
+        subject.launch(launch_site:, **options)
+      end
+
+      context 'when the rocket is launched' do # rubocop:disable RSpec/EmptyExampleGroup
+        include Spec::Integration::Deferred::ShouldSetTheOrbit
+      end
+
+      describe 'with orbit: value' do
+        let(:expected_orbit) { options[:orbit] }
+        let(:options) { { orbit: { periapsis: '100 km', apoapsis: '300 km' } } }
+
+        context 'when the rocket is launched' do # rubocop:disable RSpec/EmptyExampleGroup
+          include Spec::Integration::Deferred::ShouldSetTheOrbit
+        end
+      end
+    end
+
+    describe '#orbit' do
+      it { expect(subject.orbit).to be nil }
+    end
+  end
+end

spec/support/models/rocket.rb

@@ -19,16 +19,19 @@ def initialize(name)
 
         @launched    = false
         @launch_site = nil
+        @orbit       = nil
         @payload     = {}
       end
 
       attr_reader \
         :launch_site,
+        :orbit,
         :payload
 
-      def launch(launch_site:, payload: {})
+      def launch(launch_site:, orbit: nil, payload: {})
         @launched    = true
         @launch_site = launch_site
+        @orbit       = orbit
         @payload     = payload
       end
 

spec/support/shared_examples/deferred_examples.rb

@@ -445,7 +445,7 @@ module DeferredExamples
 
     shared_examples 'should define memoized helpers' do
       shared_examples 'should define a memoized helper macro' \
-      do |method_name, before: false, subject: false|
+      do |method_name, before: false, optional: false, subject: false|
         shared_context 'when the helper is defined' do
           before(:example) { define_helper }
         end
@@ -545,19 +545,27 @@ def call_helper
             parent_group.let(helper_name, &existing_block)
           end
 
-          include_examples 'should call and memoize the block value'
+          if optional
+            include_examples 'should call the existing implementation'
+          else
+            include_examples 'should call and memoize the block value'
 
-          include_examples 'should wrap the existing implementation'
+            include_examples 'should wrap the existing implementation'
+          end
         end
 
         context 'when a method is declared in a parent example group' do
           before(:example) do
             parent_group.define_method(helper_name, &existing_block)
           end
 
-          include_examples 'should call and memoize the block value'
+          if optional
+            include_examples 'should call the existing implementation'
+          else
+            include_examples 'should call and memoize the block value'
 
-          include_examples 'should wrap the existing implementation'
+            include_examples 'should wrap the existing implementation'
+          end
         end
 
         context 'when a helper is defined in the example scope' do
@@ -597,15 +605,23 @@ def call_helper
             ancestor_class.let(helper_name) { -1 }
           end
 
-          include_examples 'should call and memoize the block value'
+          if optional
+            include_examples 'should call the existing implementation'
+          else
+            include_examples 'should call and memoize the block value'
+          end
         end
 
         context 'when a method is defined in inherited examples' do
           before(:example) do
             ancestor_class.define_method(helper_name) { -1 }
           end
 
-          include_examples 'should call and memoize the block value'
+          if optional
+            include_examples 'should call the existing implementation'
+          else
+            include_examples 'should call and memoize the block value'
+          end
         end
 
         if before
@@ -717,9 +733,11 @@ def call_helper
           before: true
       end
 
-      # describe '.let?' do
-      #   pending
-      # end
+      describe '.let?' do
+        include_examples 'should define a memoized helper macro',
+          :let?,
+          optional: true
+      end
 
       describe '.subject' do
         include_examples 'should define a memoized helper macro',