This is probably only because you didn't develop the answer fully but i still struggle to see how monads (or other structures in that family) couldn't apply here: they aren't implemented in any way and just provide interface (eg not caring about rust at all, the semantic of that feature here is very monadic, it should mostly behave like the CPS monad). Anyway, i'm gonna stop arguing, look at how it is/will be implemented and hopefully wait for that essay of yours, to have a broader picture.
Higher kinded polymorphism results in trivially undecidable type inferences without something like currying; the restrictions needed to support it would be arbitrary and weird given that Rust does not have currying (essentially some rules to reconstruct the restrictions of currying in type operator context).
Instances of both Future and Iterator do not implement the Monad type class as defined in Haskell, because the "fmap" operator does not return the "self" type parameterized by a new type, it returns it own new type. This is because the state machine they represent is leaked through their function signature.
Do notation doesn't work with the imperative control flow that Rust has, which other people have already discussed.
Sorry, but I don't buy it. I had a half-baked, unfinished proposal for an effects system that would have allowed Rust to implement async/await just as efficiently (no stackful coroutines) along with any number of other effects [0]. Maybe it wouldn't have been a good idea due to stretching Rust's complexity budget too far, but that's very different from saying it's impossible. Having watched the development of Rust closely I really think that the design team just didn't understand the theory side well enough to be able explore the design space here. (I'm not being as critical as I might sound, PL theory is hard and the Rust devs have wielded it much more competently than the designers of any other non-research language).
[0] https://internals.rust-lang.org/t/start-of-an-effects-system...
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.
...I saw this thread too late, hopefully you will still see this comment. I'm genuinely curious.
- I wrote the generic associated types RFC (how Rust will implement higher kinded polymorphism).
- I wrote the const generics RFC (the closest Rust will get to dependent types).
- I wrote the async/await RFC, as well as the linked blog post.
That is to say that I am intimately familiar with how Rust's type system can be extended to support more "powerful" abstractions.
Monads as implemented in pure functional programming languages like Haskell cannot usefully abstract over asynchronous and synchronous IO in Rust for a variety of reasons having to do with the way the type system exposes low level details by virtue of Rust being a systems programming language. I do not believe that `do` notation could be a useful mechanism for achieving either the ergonomics or the performance that async/await syntax will have in Rust.
I'm responding to you because you're the top comment, but I could write a similar response to a lot of comments here. Monads, stackful coroutines, green threads, CSP, etc - we've heard of them! :) We have well-motivated reasons to choose async/await: its the only solution that meets our requirements.