The problem is that the caller may write yield instead of block.call. The code I have given is possible caller's code. Extended method definition in my library can be simplified to my code above. Client provides block passed to define_method (body of a method), so he/she can write there anything. Especially yield. I can write in documentation that yield simply does not work, but I am trying to avoid that, and make my library 100% compatible with Ruby (alow to use any language syntax, not only a subset).