Namespaces 101

Ruby has recently merged namespaces as an experimental feature that is disabled by default, as of this writing. This is a non-trivial development driven by @matz himself, and mainly implemented by @tagomoris, who just became a Ruby committer (🎉). The feature has been cooking for a long time, with a first ticket opened a couple of years ago (#19744) and a revised one opened just last week (#21311). There is already some documentation here, and the tickets have tons of discussions. About details, understanding the proposal, performance considerations, objections, ..., it is a bit hard to follow. In this post I'd like to present a digested summary of namespaces. I would like to give you a mental model for them so you know what is coming.