Add methods for introspection Struct attributes

closes #45

Author: flash-gordon

lib/dry/struct/class_interface.rb

@@ -187,6 +187,21 @@ def primitive
       def optional?
         false
       end
+
+      # Checks if this {Struct} has the given attribute
+      #
+      # @param [Symbol] key Attribute name
+      # @return [Boolean]
+      def attribute?(key)
+        schema.key?(key)
+      end
+
+      # Gets the list of attribute names
+      #
+      # @return [Array<Symbol>]
+      def attribute_names
+        schema.keys
+      end
     end
   end
 end

spec/shared/struct.rb

@@ -133,6 +133,19 @@
         expect { optional_type[foo: :bar] }.to raise_error(Dry::Types::ConstraintError)
       end
     end
+
+    describe '.attribute?' do
+      it 'checks if a struct has an attribute' do
+        expect(type.attribute?(:name)).to be true
+        expect(type.attribute?(:last_name)).to be false
+      end
+    end
+
+    describe '.attribute_names' do
+      it 'returns the list of schema keys' do
+        expect(type.attribute_names).to eql(%i(name age address root))
+      end
+    end
   end
 
   it 'registered without wrapping' do