# File lib/rspec/core/configuration.rb, line 178
      def formatter=(formatter_to_use)
        if string_const?(formatter_to_use) && (class_name = eval(formatter_to_use)).is_a?(Class)
          formatter_class = class_name
        elsif formatter_to_use.is_a?(Class)
          formatter_class = formatter_to_use
        else
          formatter_class = case formatter_to_use.to_s
          when 'd', 'doc', 'documentation', 's', 'n', 'spec', 'nested'
            RSpec::Core::Formatters::DocumentationFormatter
          when 'h', 'html'
            RSpec::Core::Formatters::HtmlFormatter
          when 't', 'textmate'
            RSpec::Core::Formatters::TextMateFormatter
          when 'p', 'progress'
            RSpec::Core::Formatters::ProgressFormatter
          else
            raise ArgumentError, "Formatter '#{formatter_to_use}' unknown - maybe you meant 'documentation' or 'progress'?."
          end
        end
        self.formatter_class = formatter_class
      end