[withoutboats]

The first part of your comment is unresponsive to mine; the last part is pretty rude & factually wrong (we are not motivated to implement an effect system right now; we understand the theory).

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.

[canndrew2016]

> The first part of your comment is unresponsive to mine;

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.

[Manishearth]

"impossible" is a highly contextual term here. Adding this to Rust isn't "impossible", of course it isn't. We can "just" slowly turn Rust into Haskell using the edition mechanism. Done.

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.

[withoutboats]

I think that there is not a definition of the Monad trait - not just undesirable, not possible - that can abstract over all Futures and Iterators as implemented in Rust. You would have to use some kind of trait object & lose the incredible inlining benefits that Rust gets from how these interfaces are designed today.

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.