Update Struct#[] to return nil for unset optional attributes (fix #171

, close #177)

lib/dry/struct.rb

@@ -146,7 +146,11 @@ def initialize(attributes)
     #   rom_n_roda[:subtitle] #=> nil
     def [](name)
       @attributes.fetch(name) do
-        raise MissingAttributeError.new(attribute: name, klass: self.class)
+        if self.class.attribute_names.include?(name)
+          nil
+        else
+          raise MissingAttributeError.new(attribute: name, klass: self.class)
+        end
       end
     end
 

spec/integration/struct_spec.rb

@@ -356,6 +356,19 @@ class Task < Dry::Struct
         .with_message("Missing attribute: :name on Test::Task")
     end
 
+    describe "optional attributes" do
+      before do
+        class Test::Task
+          attribute :name?, "string"
+        end
+      end
+
+      it "returns nil if the attribute is not set" do
+        value = Test::Task[user: "Jane"]
+        expect(value[:name]).to be_nil
+      end
+    end
+
     describe "protected methods" do
       before do
         class Test::Task