Allow multiple includes of Dry::Configurable

Instead of raising AlreadyIncludedError (now removed), update the method undef code in `.included` to handle already-undefined methods gracefully.

Allowing Dry::Configurable to be included multiple times across subclasses now allows for uses cases like a subclass adjusting various extension settings, e.g. `config_class`.

lib/dry/configurable/errors.rb

@@ -7,7 +7,6 @@ module Dry
   module Configurable
     Error = Class.new(::StandardError)
 
-    AlreadyIncludedError = Class.new(Error)
     FrozenConfigError = Class.new(Error)
   end
 end

lib/dry/configurable/extension.rb

@@ -26,8 +26,6 @@ def extended(klass)
 
       # @api private
       def included(klass)
-        raise AlreadyIncludedError if klass.include?(InstanceMethods)
-
         super
 
         klass.class_eval do
@@ -36,8 +34,8 @@ def included(klass)
           prepend(Initializer)
 
           class << self
-            undef :config
-            undef :configure
+            undef :config if method_defined?(:config)
+            undef :configure  if method_defined?(:configure)
           end
         end
 

spec/integration/dry/configurable/included_spec.rb

@@ -12,12 +12,6 @@
         .to include(Dry::Configurable::InstanceMethods)
     end
 
-    it "raises when Dry::Configurable has already been included" do
-      expect {
-        configurable_klass.include(Dry::Configurable)
-      }.to raise_error(Dry::Configurable::AlreadyIncludedError)
-    end
-
     it "ensures `.config` is not defined" do
       expect(configurable_klass).not_to respond_to(:config)
     end
@@ -77,4 +71,42 @@ def finalize!
       expect(instance.finalized).to be(true)
     end
   end
+
+  context "with deep class hierarchy" do
+    let(:configurable_class) do
+      Class.new do
+        include Dry::Configurable
+      end
+    end
+
+    it "allows a subclass also to include Dry::Configurable" do
+      subclass = Class.new(configurable_class) do
+        include Dry::Configurable
+      end
+
+      expect(subclass.new.config).to be_a(Dry::Configurable::Config)
+    end
+
+    it "allows a subclass to reconfigure the behavior" do
+      custom_config_class = Class.new(Dry::Configurable::Config) do
+        def db
+          super + "!!"
+        end
+      end
+
+      subclass = Class.new(configurable_class) do
+        setting :db
+      end
+
+      subsubclass = Class.new(subclass) do
+        include Dry::Configurable(config_class: custom_config_class)
+      end
+
+      obj = subsubclass.new
+      expect(obj.config).to be_a(custom_config_class)
+
+      obj.config.db = "sqlite"
+      expect(obj.config.db).to eq "sqlite!!"
+    end
+  end
 end