[Dessesaf]

Is that sans-IO pattern doing anything particularly novel, or is that basically just a subset of what any functional programmer does anyway?

Don't get me wrong, I'm fully on board with isolating IO. But why not go the slight extra step and just make it completely pure? You've already done the hard part of purity. The rest is easy.

Then you get all those nice benefits of being generic over async and sync, but can also memoize and parallelize freely, and all the other benefits of purity.

[K0nserv]

It's been a while since my Haskell days, but I think the key difference is whether you are abstracting over the IO or if the IO sits outside the pure code entirely When you abstract over IO you have blue(pure code) that contains generic red, green, purple, or orange code. With sans-IO you inverted this so the non-blue code is driving things forward by calling into blue code.

Rust, in particular, does not support abstracting over syncness at the moment, although there's work happening there. Even if you add support for that you also then need to abstract over the executor in use. My fear is that this will be too leaky to be useful, but we'll see. For now sans-IO is the best option in Rust.