require 'set'
require 'active_support/core_ext/module/attribute_accessors'

module ActionView
  class Template
    class Types
      class Type
        SET = Struct.new(:symbols).new(Set.new([ :html, :text, :js, :css, :xml, :json ]))

        def self.[](type)
          if type.is_a?(self)
            type
          else
            new(type)
          end
        end

        attr_reader :symbol

        def initialize(symbol)
          @symbol = symbol.to_sym
        end

        def to_s
          @symbol.to_s
        end
        alias to_str to_s

        def ref
          @symbol
        end
        alias to_sym ref

        def ==(type)
          @symbol == type.to_sym unless type.blank?
        end
      end

      cattr_accessor :type_klass

      def self.delegate_to(klass)
        self.type_klass = klass
      end

      delegate_to Type

      def self.[](type)
        type_klass[type]
      end

      def symbols
        type_klass::SET.symbols
      end
    end
  end
end

require 'set'
require 'active_support/core_ext/module/attribute_accessors'

module ActionView
  class Template
    class Types
      class Type
        SET = Struct.new(:symbols).new([ :html, :text, :js, :css, :xml, :json ])

        def self.[](type)
          if type.is_a?(self)
            type
          else
            new(type)
          end
        end

        attr_reader :symbol

        def initialize(symbol)
          @symbol = symbol.to_sym
        end

        def to_s
          @symbol.to_s
        end
        alias to_str to_s

        def ref
          @symbol
        end
        alias to_sym ref

        def ==(type)
          @symbol == type.to_sym unless type.blank?
        end
      end

      cattr_accessor :type_klass

      def self.delegate_to(klass)
        self.type_klass = klass
      end

      delegate_to Type

      def self.[](type)
        type_klass[type]
      end

      def symbols
        type_klass::SET.symbols
      end
    end
  end
end

Store the symbols as an array.
Store the symbols as an array.

A Set can't be implicitly converted into an Array:

```
irb(main):012:0> formats = [ :rss ]
=> [:rss]
irb(main):013:0> formats &= SET.symbols
TypeError: no implicit conversion of Set into Array
    from (irb):13:in `&'
    from (irb):13
    from /Users/kasperhansen/.rbenv/versions/2.2.3/bin/irb:11:in `<main>'
```

Besides `Mime::SET.symbols` returns an Array, so we're closer to that.
