Clarify block behavior

README.md

@@ -86,7 +86,7 @@ Search.new(query: "ruby", limit: 50).call
 
 ### Post-initialize hook
 
-Pass a block to `initialize_with` to run code after assignments. The block runs in the instance context.
+Pass a block to `initialize_with` to run code after assignments. The block runs in the instance context. You can use either the reader (e.g. `foo`) or the instance variable (`@foo`) to read values; to assign back to an attribute, assign to the instance variable (e.g. `@foo = foo.to_s`).
 
 ```ruby
 class Rectangle

spec/post_initialize_block_spec.rb

@@ -17,4 +17,45 @@
     it { is_expected.to have_attributes(foo: 1, bar: 2) }
     it { expect(subject.instance_variable_get("@baz")).to eq(1) }
   end
+
+  describe "referencing attributes in the block: @attr vs reader" do
+    # When normalizing/setting defaults in the block (e.g. @foo = @foo.to_s),
+    # confirm whether we must use the instance variable or can use the reader.
+
+    let(:with_ivar) do
+      Class.new do
+        include DeclarativeInitialization
+
+        initialize_with :foo do
+          @foo = @foo.to_s
+        end
+      end
+    end
+
+    let(:with_reader) do
+      Class.new do
+        include DeclarativeInitialization
+
+        initialize_with :foo do
+          @foo = foo.to_s
+        end
+      end
+    end
+
+    it "allows normalizing using instance variable on both sides (@foo = @foo.to_s)" do
+      expect(with_ivar.new(foo: 42).foo).to eq("42")
+    end
+
+    it "allows normalizing using reader on RHS (@foo = foo.to_s)" do
+      expect(with_reader.new(foo: 42).foo).to eq("42")
+    end
+
+    it "both forms produce the same result" do
+      [with_ivar, with_reader].each do |k|
+        expect(k.new(foo: 100).foo).to eq("100")
+        expect(k.new(foo: :sym).foo).to eq("sym")
+        expect(k.new(foo: nil).foo).to eq("")
+      end
+    end
+  end
 end