# File lib/rspec/core/configuration.rb, line 271 def add_formatter(formatter_to_use, output=nil) formatter_class = built_in_formatter(formatter_to_use) || custom_formatter(formatter_to_use) || (raise ArgumentError, "Formatter '#{formatter_to_use}' unknown - maybe you meant 'documentation' or 'progress'?.") formatters << formatter_class.new(output ? File.new(output, 'w') : self.output) end