Agreed, instance_eval is definitely disgusting, but this trick scores well on readability, it's no more of a trap than some other Ruby idioms, and it serves a good purpose (robustness!).

    def foo(bar, baz=(default = true; 'default')) 
    # it still looks separated from other arguments
      if default
        puts "#{bar}.times { puts #{baz} }" 
      else
        bar.times { puts baz }
      end
    end

To anyone who knows more Ruby than I - is there a good reason against that I'm missing?