Double dispatch in Ruby example (my presentation at a local Ruby meetup)

01-before.rb

module SocialPresenter

  # ...

  class Twitter < Struct.new(:object)

    def message
      case object
      when String

        case object
        when "/chart/artists" then t("Popular artists via %{twitter_tag}")
        when "/chart/albums"  then t("Popular albums via %{twitter_tag}")
        when "/chart/tracks"  then t("Popular tracks via %{twitter_tag}")
        end

      when Playlist

        t "#NowPlaying %{playlist_title} on %{twitter_tag}",
          playlist_title: object.name

      when Post

        t "%{post_title} on %{twitter_tag}",
          post_title: object.title

      when Genre

        t "#NowPlaying %{genre_name} music on %{twitter_tag}",
          genre_name: object.display_name

      when Artist

        t "#NowPlaying %{artist_name} music on %{twitter_tag}",
          artist_name: object.name

      when Album

        t "#NowPlaying %{album_name} by %{artist_name} on %{twitter_tag}",
          album_name: object.name,
          artist_name: object.artist.name

      end
    end

    private

    def t(text, options = {})
      I18n.t text, **options, twitter_tag: "@Rhapsody"
    end
  end

  # ...

end

02-after.rb

module SocialPresenter

  # ...

  class Twitter < Struct.new(:object)

    def message
      send("#{object.class}_message")
    end

    def String_message
      case object
      when "/chart/artists" then t("Popular artists via %{twitter_tag}")
      when "/chart/albums"  then t("Popular albums via %{twitter_tag}")
      when "/chart/tracks"  then t("Popular tracks via %{twitter_tag}")
      end
    end

    def Playlist_message
      t "#NowPlaying %{playlist_title} on %{twitter_tag}",
        playlist_title: object.name
    end

    def Post_message
      t "%{post_title} on %{twitter_tag}",
        post_title: object.title
    end

    def Genre_message
      t "#NowPlaying %{genre_name} music on %{twitter_tag}",
        genre_name: object.display_name
    end

    def Artist_message
      t "#NowPlaying %{artist_name} music on %{twitter_tag}",
        artist_name: object.name
    end

    def Album_message
      t "#NowPlaying %{album_name} by %{artist_name} on %{twitter_tag}",
        album_name: object.name,
        artist_name: object.artist.name
    end

    private

    def t(text, options = {})
      I18n.t text, **options, twitter_tag: "@Rhapsody"
    end
  end

  # ...

end