I wasn’t aware of effects until I read the article, but I like the idea. The example you have is just an interface. Effects work a bit like boundary hooks. A pure function only works on things internal to its scope, and so it has no effects. An impure function might affect things outside of its scope, and may want to allow the caller to hook when those effects occur. Think a function that uses the gpu. That’s a side effect, and the function should be decorated to reflect that.

I’m guessing some languages would allow for checking to ensure that any induced effects are correctly noted and advertised so that the system as a whole can be more easily reasoned about.