The trick seems to be how to implement this kind of idea in a composable way, so we can still have a clear, modular design and write generic, reusable code even in the presence of several different types of effect. IMHO, the research into type and effect systems in recent years has been promising, but also shows that this is not an easy problem to solve. I’m hoping that in a few more years we might see a new generation of programming languages that incorporate this kind of “effect safety” as easily and naturally as many developers use const and qualifiers and resource management idioms today.