A nonsensical example of what I want to do. Someone please fill in the blanks.

parser_plugin.rb

class Parser
  class << Base
    # what to do here?
    def create(sym)
      eval 'Parser::#{sym.to_s.capitalize}.new'
    end
  end
end

class Parser::Html < Parser::Base
end

class Parser::Pdf < Parser::Base
end

parser = Parser.create(type: :html)

parser.class
#> Parser::Base::Html 

Are you planning on many parsers, or just a small subset? and there is a better way than eval

Object.get_const("Parser::#{sym.to_s.capitalize}").new

yeah, that's helpful. definitely planning on many parsers. I want it to be extendable so others can write plugins as well.