One of the things I find most exciting about effect systems is that they solve some of the issues with typeclasses and traits. The issue with these is you can provide exactly one implementation for a type, and this implementation then applies to your entire program. (The underlying theoretical issue, I guess, is coherence.) This means that code cannot request injection of behaviour _except_ through the type of its data.

In practice, what that means is you get a strong temptation to hang behaviour on data even when it doesn't fit perfectly. Because of the natural desire to reduce the number of entity definitions, you end up defining a typeclass on a data type that doesn't fit exactly just to get the behaviour to the right place without having to introduce a new policy or something.

Effects change this by essentially letting you provide multiple names implementations for the same data type, and you don't need to pass around a policy type because the polymorphism lets you tie handlers to scope.

So, if the fancy type-safe library-based control flow doesn't really do much for you, I think that their potential for code design is a good reason to still be excited!