Theseus: Worry free algebraic and higher order effects

Thank you for citing Heftia. I am its author, and it is encouraging to see more people working in the space of effect libraries for Haskell. I have not been able to spend any time on the library for a while, but I hope to resume work again soon.

I have not had the chance to read Theseus carefully yet, but I think it is a good approach to start from a freer-based implementation to refine the interface and semantics, and then later rewrite the internals using GHC primops while preserving those guarantees. In fact, that is broadly the direction I have been taking as well, and after running some experiments I have become more optimistic than I initially was about how difficult that path is. I hope to be able to share results in the near future.

The possibility of a sound semantics for combining higher-order effects, especially scoping for resource management like bracket or withFile, with coroutines (non-scoped resumption) has been a long-standing mystery. I find it extremely interesting, but because it is so difficult I have paused work on it for now and am focusing on the performance work mentioned above, namely the migration from freer to primops. So I am very happy to see you exploring this area.