A Rust-flavored effect system is quite a bit different from do-notation. You're probably right that such a thing could achieve the same ergonomics and efficienc

A Rust-flavored effect system is quite a bit different from do-notation. You're probably right that such a thing could achieve the same ergonomics and efficiency as built-in async/await, but you're also probably right that it would be a huge amount of complexity. When people say that monads and do-notation don't work in Rust, they're talking about a user-level Monad trait and a simple CPS transform built on top of it: - Such a monad trait is impossible to write even with HKT because it would have to abstract over both Option/Result (type constructors) and Iterator/Future (traits) - Such a CPS transform would be extremely limited (not composable with built-in loop structures) and/or extremely tricky around TCE, lifetimes, and allocation (the typical type for `>>=` would involve `Fn` trait objects...). Effects bypass this by leaving the CPS transform in the compiler, instead only exposing delimited continuations to userspace. Which is basically what async/await does, just non-generically.