Implement Recipe#filepath method:

Returns the filepath of the recipe when it is built from a file.

lib/producer/core/recipe.rb

@@ -1,14 +1,15 @@
 module Producer
   module Core
     class Recipe
-      attr_reader :code
+      attr_reader :code, :filepath
 
       def self.from_file(filepath)
-        new(File.read(filepath))
+        new(File.read(filepath), filepath)
       end
 
-      def initialize(code)
+      def initialize(code, filepath = nil)
         @code     = code
+        @filepath = filepath
       end
 
       def evaluate

spec/producer/core/recipe_spec.rb

@@ -8,11 +8,16 @@ module Producer::Core
     subject(:recipe)  { Recipe.new(code) }
 
     describe '.from_file' do
+      let (:filepath)   { fixture_path_for 'recipes/empty.rb' }
+      subject(:recipe)  { Recipe.from_file(filepath) }
+
       it 'builds a recipe whose code is read from given file path' do
-        filepath = fixture_path_for 'recipes/empty.rb'
-        recipe = Recipe.from_file(filepath)
         expect(recipe.code).to eq File.read(filepath)
       end
+
+      it 'builds a recipe whose file path is set from given file path' do
+        expect(recipe.filepath).to eq filepath
+      end
     end
 
     describe '#initialize' do