Add InstanceDSL to isolate #initialize method from mixin

.rspec

@@ -1,3 +1,4 @@
 --format documentation
 --color
 --require spec_helper
+--warnings

lib/dry/initializer.rb

@@ -8,7 +8,8 @@ module Initializer
     require_relative "initializer/param"
     require_relative "initializer/option"
     require_relative "initializer/builder"
-    require_relative "initializer/dsl"
+    require_relative "initializer/instance_dsl"
+    require_relative "initializer/class_dsl"
 
     # rubocop: disable Style/ConstantName
     Mixin = self # for compatibility to versions below 0.12
@@ -18,7 +19,7 @@ module Initializer
       obj.define_singleton_method(:inspect) { "Dry::Initializer::UNDEFINED" }
     end.freeze
 
-    extend Dry::Initializer::DSL
+    extend Dry::Initializer::ClassDSL
 
     def param(*args)
       __initializer_builder__.param(*args).call(__initializer_mixin__)
@@ -31,11 +32,7 @@ def option(*args)
     private
 
     def __initializer_mixin__
-      @__initializer_mixin__ ||= Module.new do
-        def initialize(*args)
-          __initialize__(*args)
-        end
-      end
+      @__initializer_mixin__ ||= Module.new
     end
 
     def __initializer_builder__(**settings)

lib/dry/initializer/dsl.rb → lib/dry/initializer/class_dsl.rb

@@ -1,22 +1,17 @@
 module Dry::Initializer
-  module DSL
+  module ClassDSL
     attr_reader :config
 
     def [](**settings)
       Module.new do
-        extend  Dry::Initializer::DSL
+        extend  Dry::Initializer::ClassDSL
         include Dry::Initializer
         @config = settings
       end
     end
 
     def define(fn = nil, &block)
-      mixin = Module.new do
-        def initialize(*args)
-          __initialize__(*args)
-        end
-      end
-
+      mixin   = Module.new { include InstanceDSL }
       builder = Builder.new Hash(config)
       builder.instance_exec(&(fn || block))
       builder.call(mixin)
@@ -29,12 +24,10 @@ def extended(klass)
       super
       mixin   = klass.send(:__initializer_mixin__)
       builder = klass.send(:__initializer_builder__, Hash(config))
-
       builder.call(mixin)
-      klass.include(mixin)
-      klass.send(:define_method, :initialize) do |*args|
-        __initialize__(*args)
-      end
+
+      klass.include(InstanceDSL) # defines #initialize
+      klass.include(mixin)       # defines #__initialize__ (to be redefined)
     end
 
     def mixin(fn = nil, &block)

lib/dry/initializer/instance_dsl.rb

@@ -0,0 +1,15 @@
+module Dry::Initializer
+  module InstanceDSL
+    private
+
+    # The method is reloaded explicitly
+    # in a class that extend [Dry::Initializer], or in its subclasses.
+    def initialize(*args)
+      __initialize__(*args)
+    end
+
+    # The method is redefined implicitly every time
+    # a `param` or `option` is invoked.
+    def __initialize__(*); end
+  end
+end

spec/definition_spec.rb

@@ -2,13 +2,13 @@
   shared_examples :initializer do |in_context|
     subject { Test::Foo.new(1, bar: 2) }
 
-    it "sets variables when defined by #{in_context}" do
+    it "sets variables when defined via `#{in_context}`" do
       expect(subject.instance_variable_get(:@foo)).to eql 1
       expect(subject.instance_variable_get(:@bar)).to eql 2
     end
   end
 
-  it_behaves_like :initializer, "extending Dry::Initializer" do
+  it_behaves_like :initializer, "extend Dry::Initializer" do
     before do
       class Test::Foo
         extend Dry::Initializer
@@ -18,7 +18,7 @@ class Test::Foo
     end
   end
 
-  it_behaves_like :initializer, "extending Dry::Initializer::Mixin" do
+  it_behaves_like :initializer, "extend Dry::Initializer::Mixin" do
     before do
       class Test::Foo
         extend Dry::Initializer::Mixin
@@ -28,7 +28,7 @@ class Test::Foo
     end
   end
 
-  it_behaves_like :initializer, "extending Dry::Initializer[undefined: false]" do
+  it_behaves_like :initializer, "extend Dry::Initializer[undefined: false]" do
     before do
       class Test::Foo
         extend Dry::Initializer[undefined: false]
@@ -38,7 +38,7 @@ class Test::Foo
     end
   end
 
-  it_behaves_like :initializer, "including Dry::Initializer with block" do
+  it_behaves_like :initializer, "include Dry::Initializer with block" do
     before do
       class Test::Foo
         include Dry::Initializer.define {
@@ -49,7 +49,7 @@ class Test::Foo
     end
   end
 
-  it_behaves_like :initializer, "including Dry::Initializer with lambda" do
+  it_behaves_like :initializer, "include Dry::Initializer with lambda" do
     before do
       class Test::Foo
         include Dry::Initializer.define -> do
@@ -60,7 +60,7 @@ class Test::Foo
     end
   end
 
-  it_behaves_like :initializer, "including Dry::Initializer[undefined: false]" do
+  it_behaves_like :initializer, "include Dry::Initializer[undefined: false]" do
     before do
       class Test::Foo
         include Dry::Initializer[undefined: false].define {
@@ -71,7 +71,7 @@ class Test::Foo
     end
   end
 
-  it_behaves_like :initializer, "including Dry::Initializer::Mixin" do
+  it_behaves_like :initializer, "include Dry::Initializer::Mixin" do
     before do
       class Test::Foo
         include Dry::Initializer::Mixin.define {