An Interview Question That Prints Out Its Own Source Code (In Ruby)

Another "How would you move Mount Fuji?" interview question. If only people would realize they don't need someone to move Mount Fuji.

Hint in Perl, Python, or Ruby: blank programs are valid.

Check out this "quine relay!" http://d.hatena.ne.jp/ku-ma-me/20090916/p1

The Ruby program outputs Python that outputs Perl that outputs Lua that outputs OCaml that outputs Haskell that outputs C that outputs Java that outputs Brainfuck that outputs Whitespace that outputs Unlambda that outputs the original Ruby program.

(I was unable to get it to work when I tested it, but I still find it quite impressive!)

So here is a much more interesting problem. Can you code a program that runs a function on its own source code?

Hint: It is a generalization of a quine.

I've heard of a quine but only confuse myself trying to think of a way to write one in Ruby that doesn't involve something like eval-ing strings. Reg Braithwaite -> FAIL.

That being said, something tickles the back of my head... Are quines combinatorial? Something about a program that prints its own source code makes me think of some of Raymond Smullyan's various problems in the field...

Dang, I would've failed that interview question. Fortunately, my day-to-day work doesn't involve writing quines as much as trying to configure the right versions of Ruby, Postgres, and Rails 3. Wait--I'm failing at that one, too... ;)

Handy Bash function for testing your quine:

  function quine_test { diff <(ruby $1) <(cat $1); }

Usage:

  $ quine_test quine.rb

Cool because I had never heard of a quine before (so I guess I would probably fail the interview question).