I’ve been reading the book in anticipation of giving it to my son to get him started with programming and Ruby. It seems to me a couple of things have been left out, such as the facts that 1.) in order to be executable from the shell, a “chmod +x calc.rb” must be performed, and 2.) in order to invoke it, one will have to give a full pathname, like for instance “./calc.rb”. Certainly an explanation of PATH would be overkill, but this much is necessary for success.
As a footnote, I really like the book. Well written. Congratulations on a good job!