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The problem of effects in Rust (2020) (without.boats)
92 points by agluszak 3 months ago | hide | past | favorite | 45 comments

There are several languages that are working on solving the composable effects problem that I am aware of:

* Pony - capabilities based language inspired by `E`. Effects are like Rust's (provided by the language, not extensible by end users): https://www.ponylang.io/

* Koka - effects are first class and higher order. `filter` is a higher order function that is "effectfully parameterized" as well as "parameterized over its types". Effects can be discharged at any level of the program and the effects "stack" is manipulable (in what appears to be a principled way) https://koka-lang.github.io/koka/doc/index.html

* Multi-core OCaml is also building parallelism on a version of effects that looks quite a lot like Koka (though I'm not familiar enough with it to say how close they are in practice).

What other ones am I missing?

Flix: https://flix.dev/

(edit) c'n'p from their site:

Why Flix?

Flix aims to offer a unique combination of features that no other programming language offers, including: algebraic data types and pattern matching (like Haskell, OCaml), extensible records (like Elm), type classes (like Haskell, Rust), higher-kinded types (like Haskell), local type inference (like Haskell, OCaml), channel and process-based concurrency (like Go), a polymorphic effect system (unique feature), first-class Datalog constraints (unique feature), and compilation to JVM bytecode (like Scala).

Thanks for the link. Indeed a nice programming language proposal. I love the opinionated set of design principles https://flix.dev/principles/.

There is also a former thread on HN: https://news.ycombinator.com/item?id=25513397

Very nice, good catch. It has UFCS [0] like D!

[0] https://en.wikipedia.org/wiki/Uniform_Function_Call_Syntax

Honestly, I find the announcements of 99% of new languages here at hackernews pretty boring, because they all feel like they didn't learn anything from the last 20 years or so.

But this list actually makes me excited!

Yeah, me too. They published quite a lot of papers using it [0]. The datalog integration is pretty nice too, haven't gotten to learning Prolog yet, but seeing how concise a sudoku solver can be implemented in it was an experience.

(edit) you can import Java, Kotlin and Scala in it too, those are automatically Impure. Would enable purely functional architecture [1] and falling into the "Pit of success" [2].

[0] https://flix.dev/research/ [1] https://blog.ploeh.dk/2016/03/18/functional-architecture-is-... [2] https://www.youtube.com/watch?v=US8QG9I1XW0

Let me tell you what I'm worried about with effects systems: One of the smartest people I know of has a library called Eff https://github.com/hasura/eff that efficiently implements effects through some modifications to GHC. The thing is, she's pretty much stopped working on because she found some really nasty semantic edge-cases that she couldn't resolve to her satisfaction. (I don't understand the problems well enough to describe them, and I think they're specific to lazy languages, but it has left me cautious about the effect model in general.)

Does she have any writings online that describe those nasty semantic edge-cases?

Unfortunately, mostly Twitter, and that’s profoundly unsearchable. Here’s the original announcement before it ran aground: https://mobile.twitter.com/lexi_lambda/status/12262098236640...

What makes you say it ran aground? I thought it was just waiting for [0].

[0]: https://github.com/nicuveo/ghc-proposals/blob/master/proposa...

Freer Monads, More Extensible Effects


Frank does it right, yes.

They use "abilities" as a dual to effects. That's imho the crucial part. Most of the other effect systems only implement half the thing.

But I would change two parts: First of all the co-effects should be proper "capabilities" in the sense of "capability security". The other thing would be modelling the co-effects as co-data. They already use some (ad hoc?) notion of "Interfaces". Those look like co-data definitions (resembling OOP Interfaces). I also thing co-data with co-pattern matching could solve some of the problems mentioned at the end of the paper. Inspiration could be taken form: https://www.cs.uoregon.edu/Reports/DRP-201905-Sullivan.pdf

Seams stale. The paper is a joy to read though...

"Do, be, do, be, do" (those who read it will understand)

This paper really opened my eyes for what's possible.

Unison has an algebraic effect system, under the name "abilities"


How about the languages that have already "solved" the problem (in terms of ergonomically usage), such as Haskell, Purescript, Scala and a lot of other niche languages?

That being said, Rust has a bit of a harder time due to the focus on performance and optimally zero-cost abstraction, whereas other languages are more lenient and focus rather on expressiveness and composability / code reuse.

I mean, monads famously don't compose - you have to write a bunch of monad transformers.

True, but not a huge real world issue in nearly all the relevant cases.

I can very easily have a fallible, asynchronous, multi-value, stateful, database connected computation that returns a Bool and use it in let’s say ‘filterM’.

Most transformers are already written for me.

Edit: switching between no effect and some effect is the tricky part though.

I find in practice that even though I don't have to write the transformers myself, `lift`ing and `unlift`ing things is a pain in the ass. Once you try a proper algebraic effect system, it's hard to feel like transformers are an adequate solution.

Most of the relevant plumbing is generic though. Check out ZIO or cats-effect in Scala which I think both provide highly composable effect systems.

Well, more attempts at this problem come along pretty often (speaking for Haskell at least, and I expect those attempts to be more mature than the other languages you mentioned), so I don't think you can say those languages have solved it. Like the other comment mentioned, it's an active area of research, and some of the research happens in languages designed to solve that problem, and some happens in languages that support solutions to the problem.

(Just to add- there's a really wide range of ergonomics that you can support and it's hard to get everything right. And performance is super tricky!)

Does Idris qualify?


Idris is great.

F* (f star) too! https://www.fstar-lang.org/

Maybe also ATS, but I'm unsure if it qualifies

Last I looked, Ocaml's upcoming effect system is untyped, so doesn't really belong in the discussion.

Upcoming OCaml multicore won't expose effects at all. I believe the intention is to finish work on typed effects[1] and only then add it to the language.

[1]: https://www.janestreet.com/tech-talks/effective-programming/

They have an effects branch but admitted it won't be ready in time for OCaml 5.0 (supposedly arriving around end of 2021 / early 2022) so they have a separate compiler variant with it where the progress can be tracked.

Last I read, they really want to nail down the entire multi-threaded environment to the last detail first (which mostly means make sure no parallel bugs are introduced + single-threaded code won't become slower; both pretty huge goals).

It's not quite built into the language but Scala has a really well-developed ecosystem of effect libraries (ZIO, Monix, cats-effect) that solve this problem really nicely.

There's also the Eff programming language: https://www.eff-lang.org/

Koka looks very promising to me. I wonder why it’s not getting more attention. Probably because of being research at this point.

Did not MSFT pull funding?

Oh didn’t know that. Sad to hear that :(

Compared with Rust’s Result monad, which allows developers to clearly see the effects of error handling, there are two other hidden fallible effects in Rust that are much harder to tackle:

* Panic rewinding. I am not sure how to ensure your Rust function being panic safe. It is quite easy to cause soundness issue if some invariants no longer hold due to panic. I see `PanicGuard` sometimes used in Rust std library.

* Future cancellation. `tokio::select` is one of the infamous examples, where it is quite easy to introduce bug if the future cannot handle cancellation gracefully.

When trying to handle them properly, it feels more like writing traditional C code than Rust.

>I am not sure how to ensure your Rust function being panic safe.

You could use the linking trick in which your panic handler uses non-existent extern fn. For example, this approach is used in the no-panic crate. Of course, this approach is nothing more than a clever hack with several significant limitations.

>Future cancellation

I would say it's a more general problem of Rust lacking linear types.

IMHO both panics and async are a mistake. The latter should be a standard library with some macros and the former should be replaced by proper error handling everywhere.

Yes out of memory errors should be able to be handled. You might not care when writing web backend slop designed to run under orchestrators but for systems code it is necessary sometimes and Rust is a systems language.

It’s possible to work around both shortcomings but they contradict the languages mission and are warts. Async is a big “excessive cleverness” mistake.

It is not possible to implement async/await efficiently and safely with macros, and so doing so would be “contradicting the language’s mission.”

They were considered and even originally built that way, but empirically it was worse.

I’d go further and say it’s not possible to fully implement async/await without compiler help.

I got really far with stateful, back in 2016 [1]. Stateful was an attempt to write a coroutine library in a proc macro, which generated state machines, as opposed to using os primitives like green threading. This was back before the rust community really started working in this space. I ended up extracting the type system from rustc to do much of the analysis, but it ultimately failed due to how difficult it was to output rust code that respected the borrow checker rules. I also didn’t have anything like the pinning system, so I couldn’t catch move issues either.

It was a much better idea to just implement this in the compiler.

[1]: http://erickt.github.io/blog/2016/01/27/stateful-in-progress...

Hmm… didn’t know they tried.

Personally I just loathe async in general. Go has it right, but I understand why you can’t do that in Rust. Async is an ugly workaround for the inefficiency of OS threads, and I wish they would just fix that so we can stop all this madness.

Rust as a language is in a funny place — somewhere between C and Haskell.

Whenever Rust adds new features, C users panic "it's going to end up like C++!", but at the same time from Haskellers' perspective Rust's type system is still not advanced enough.

As the article explains, it's incredibly hard to add these high-level features while still preserving "portable assembly" aspect of the language.

Rust occupies the C++ space, and is a massive improvement over C++. It’s type system is relatively complex but in practice is simpler to use than C++ once you learn it and far less bug prone.

The dogmatic C partisans who insist on building houses out of sand with tweezers will never like it.

It’s basically C++ rebooted with ideas from Haskell, Go, and other modern languages. If Rust completely displaced C++ that would be a huge win for productivity and security / reliability.

There's nothing modern about Go.

Yes there is: simplicity and lack of needless cleverness.

Overengineering is the great plague of modern software.

Rust contains a little bit of needless cleverness but most of its complexity is essential to the problem domain of low level systems programming and intrinsic safety. Go operates at a different level with a fat library (almost a VM) and garbage collection, allowing for a very simple type system. Both languages are modern but fill different niches.

Rust is new C++ while Go is closer to Python or early pre-complexity-orgy Java.

On the other hand, the type system of vanilla Haskell is not especially powerful compared to current stable Rust’s. It’s pretty much all GHC extensions where the real magic is.

Things that take closures should support both paradigms, since pure function closures that cannot abort can enable certain kinds of compiler optimizations like vectorizing more easily.

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