[rmgk]

First of, I do like that style.

In the context of my argument its quite interesting because you essentially argue that one can get the benefits of separate runtime implementations without actually needing pure functions (in a very strict sense) or any of the IO encodings.

What I had in mind when I said application specific handling, I meant things like the application developer writing a custom optimization pass on the returned IO values to make the execution more efficient for the specific application. Or adding custom way of handling/reporting errors. And I meant doing so while not making their own interfaces and instead working with whatever the common IO abstraction is.