I think we need to distinguish two questions:
- Should the sync/async distinction be reflected in types?
- How does async get implemented?
I believe the answer to the first question is “yes, but not in the traditional way”. The traditional way leads to the colored function problem: We need to duplicate large bodies of code in sync and async versions. Or, alternatively, we have to sprinkle a lot of type variables around just to accommodate the
possibility that everything can be async. I believe we can do much better by switching the viewpoint
from effects to capabilities. An async computation would then be a computation that references the
Suspend capability. It turns out that this change in viewpoint gives a much better solution to the effect polymorphism problem.
The second question, how async is implemented, is largely orthogonal to the first. It could be by mapping to a monad, which is the most common solution for Scala today. Or by mapping to state machines, provided we can extend that to higher-order functions. Or by building on a runtime with support for coroutines and continuations, which (hopefully) Loom will provide. Once we buy into capabilities, we’ll have another interesting option: duplicate every function that can take a suspendable argument into sync and async versions. This may look like it reintroduces function coloring but doesn’t really since it is all codegen instead of source. Also, the capability type system tells us what we need to duplicate, so it’s hopefully not pervasive.
We are just starting a large scale (7 persons over 5 years) project to research these things. The aim is to find a unified approach to resources and effects that could become a high-level analogue to what Rust is for lower-level systems programing. The sync/async problem is one of the fundamental problems we study. We will work on a concrete type system for suspendability, and will look into different implementation strategies and compare them.
Here is a slide deck that describes the project. Btw, I am still looking to fill some roles in this project. Here’s a job add for a post doctoral scientist: https://recruiting.epfl.ch/Vacancies/2452/Description/2. Another ad for a research engineer will follow.