Fix Marshaller interface to copy extension names to a subclass

Fixes #13

CHANGELOG.md

@@ -9,6 +9,9 @@
 ### Changed
 * Change #delete to allow removing any subkey of a deeply nested key
 
+### Fixed
+* Fix Marshaller interface to copy extension names to a subclass
+
 ## [v0.4.0] - 2020-01-25
 
 ### Added

lib/tty/config/marshaller.rb

@@ -16,7 +16,7 @@ def self.included(base)
 
       module ExtensionsStore
         def ext
-          @ext ||= []
+          @_ext ||= []
         end
 
         # Set a list of extensions
@@ -27,11 +27,21 @@ def ext
         # @api public
         def extension(*extensions)
           if extensions[0].is_a?(Array)
-            @ext = extensions[0]
+            @_ext = extensions[0]
           else
-            @ext = extensions
+            @_ext = extensions
           end
         end
+
+        # Copy extensions to a subclass
+        #
+        # @param [Object] subclass
+        #
+        # @api private
+        def inherited(subclass)
+          super
+          subclass.instance_variable_set(:@_ext, @_ext.dup)
+        end
       end
 
       # Marshal object into a given format

spec/unit/register_marshaller_spec.rb

@@ -60,4 +60,27 @@ def unmarshal(file); end
       TTY::Config::Marshallers::JavaPropsMarshaller
     ])
   end
+
+  it "inherits from existing marshaller" do
+    stub_const("CustomYAMLMarshaller",
+               Class.new(TTY::Config::Marshallers::YAMLMarshaller) do
+                 def marshal(data)
+                   YAML.safe_load(data, aliases: true)
+                 end
+               end)
+
+    config = TTY::Config.new
+    config.register_marshaller :yaml, CustomYAMLMarshaller
+
+    expect(config.marshallers).to eq([
+      CustomYAMLMarshaller,
+      TTY::Config::Marshallers::JSONMarshaller,
+      TTY::Config::Marshallers::TOMLMarshaller,
+      TTY::Config::Marshallers::INIMarshaller,
+      TTY::Config::Marshallers::HCLMarshaller,
+      TTY::Config::Marshallers::JavaPropsMarshaller
+    ])
+    expect(CustomYAMLMarshaller.ext).to eq(%w[.yaml .yml])
+    expect(CustomYAMLMarshaller.dep_name).to eq(%w[yaml])
+  end
 end