Make it generic, with some kind of Resource interface that Network is one of the implementations¹, and you get a dynamic effect system.
But that thing you wrote is a static effect system. Having a singleton that you need to carry with the type doesn't make it not part of the type system.
1 - Or actually use Resource as a class. But Java wouldn't do this.
I think you're talking about what power a type system needs to say that this function doesn't have undesirable side effects, while Justin is saying if you can't just "import std.net" and access the network by side effects (and can't make raw syscalls either, or poke around in raw memory), and nothing hands you a Network value, a plain old boring type system is enough.
