I was in the audience at Rails World 2026 when Aaron Patterson (tenderlove) kicked off the closing keynote. Aaron opened with a well-worn line: “the purpose of a system is what it does.” He wasn’t satisfied with it. The purpose of a system, he argued, is what it does for whoever is observing it.

Hand an old Nokia 3310 to someone who has never seen a cell phone before, and they might use it as a hammer. In that moment, to that person, it is a hammer. What a system does depends on who’s watching.

That distinction has a name in compiler design: The as-if rule.

In this post, I walk through what the as-if rule means for Ruby’s compiler, where Ractors push it right up to its edges, and how the same idea explains a real RubyGems.org security incident that played out earlier this year.

Read more here: The As-If Rule, Ractors & AI