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.

Sticking to the first part: an effect system is not what the user I was responded to was talking about. They were talking about building do notation on top of type classes with higher kinded polymorphism, which cannot effectively abstract over the monadic operations in Rust.

I interpreted OP's comment as complaining about the lack of more general abstractions in Rust that would allow you to implement async/await. Your comment specifically mentioned Haskell-style monads (eg. a `Monad` trait), but that's not the only way to implement something like this.

> the last part is offensive & wrong

Quoting steveklabnik:

> it’s an open research problem if do notation can work in Rust. Until that’s solved at all, we’re just not sure it’s possible. ... "Open question" doesn't mean "impossible", mind you. But nobody has ever come up with a design. In the meantime, we have users to support...

Isn't this what I was saying? "We don't know how to do it, so we're going with the easier option."

Edit: To be clear, I don't think async/await we've ended up with is necessarily in the wrong direction. But I also don't think that "we thoroughly explored the design space of do/monads/effects and concluded that they were impossible to implement ergonomically/efficiently" is really true.

When folks say something is "impossible" in such a context, they mean "given the constraints", which include goals the lang team has for the language. An effects system is pretty heavyweight and may violate these goals.

This is separate from effect systems, which I never said was not possible. rpjohnst's parallel response sums up the key differences between monads and an effect system.

You might want to do a Google Scholar search on Niko Matsakis and Aaron Turon before making silly claims like this.