I don't think there's anything inherently evil, bad, or even particularly confusing about this. As long as you understand how it's being attached to a running f

I don't think there's anything inherently evil, bad, or even particularly confusing about this. As long as you understand how it's being attached to a running function there's very little confusion to be had. The problem is that most of the resources for learning completely gloss over this incredibly important aspect of the language.