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Comment by Animats | Hacker News Reader

This may be too much advanced type theory for a useful language.

You can go all the way to formal verification. This is not enough for that. Or you can stop at the point all memory error holes have been plugged. That's more useful.

You can go way overboard with templates/macros/traits/generics. Remember C++ and Boost. I understand that Boost is now deprecated.

I should work some more on my solution to the back-reference problem in Rust. The general idea is that Rc/Weak/upgrade/downgrade provide enough expressive power for back references, but the ergonomics are awful. That could be fixed, and some of the checking moved to compile time for the single owner/multiple users case.

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> You can go way overboard with templates/macros/traits/generics.

You can go overboard on any language concept imaginable, but conflating all these mechanisms makes it sound like you haven't interacted much with non-C++ languages—particularly since rust doesn't have templates or anything like templates, traits are an entirely unrelated composition mechanism, and macros are entirely unrelated to the type discussion in the article.

This isn't really "advanced type theory" so much as picking up programming language developments from the 90s. I suppose it's "advanced" in the sense that it's a proper type system and not a glorified macro ala templating, but how is that a bad thing?

Agreed. Generics are in most modern typed languages, and traits are essentially interfaces. Maybe templates means C++ templates, which are essentially generics?

In C++, concepts are essentially generics where templates are more like weird macros.

My guess is they meant metaprogramming in general (templates/generics, macros), but traits are not quite like the others.

Thanks for posting this! As a long-time Rust user (and contributor, in the good old days), the thing that has always fascinated me about Rust is the healthy balance it strikes between academic brilliance and industry pragmatism. Radical changes like the ones suggested by the OP risk damaging that balance IMO. I'd rather put up with some language quirks and see Rust achieve "boring technology" status...

But who knows, maybe the "academic brilliance" from the article is more pragmatic than I give it credit for. I sure hope for it if these changes ever go through.

The lack of use cases in that document is a concern. They're all "nice to have" features, but is the payoff there for real work? The "effects" section mentions properties useful for a proof system. But it's not part of a proof system. If it were, most of those could be found automatically by static analysis, without bothering the programmer. (I was doing that decades ago in a very early proof of correctness system.) Getting programmers to annotate items is tough. Just getting C++ programmers to use "const" enough is hard.

"View types" are interesting. But how much of that generality is really needed? We already have it for arrays, with "split_at_mut" and its friends. That's a library function which uses "unsafe" but exports a safe interface. The compiler will already let you pass two different fields of a struct as "&mut". That covers the two most pressing cases.

I would also have liked to see some motivational examples, but I think the most interesting upside of an effect system is composability.

Rust is actually really unique among imperative languages in its general composability - things just compose really well across most language features.

The big missing pieces for composability are higher-kinded types (where you could be generic over Option, Result, etc.), and effects (where you could be generic over async-fn, const-fn, hypothetical nopanic-fn, etc.)

The former becomes obvious with the amount of interface duplication between types like Option and Result. The latter becomes obvious with the number of variants of certain functions that essentially do the same thing but with a different color.

Some uses of higher-kinded types (though not all of them) can be addressed by leveraging Generic Associated Types (GAT).

Part of the problems is that the "things just compose really well" point becomes gradually less and less applicable as you involve the lower-level features Rust must be concerned with. Abstractions start to become very leaky and it's not clear how best to patch things up without a large increase in complexity. A lot of foundational PL research is needed to address this sensibly.

Not so much composability on async Rust.

> Rust is actually really unique among imperative languages in its general composability

Can you compare it to some other imperative language? Because I really don't see anything particularly notable in Rust that would give it this property.

I’m mainly comparing to the progeny of C, where the biggest difference is the fact that almost everything is an expression in Rust.

No need for ternary operators. C# unsafe blocks can only appear as statements (so you cannot delegate from a safe to an unsafe constructor, e.g.). C++ cannot return from the middle of an expression.

A related aspect is the type system, which composes with expressions in really interesting ways, so things like constant array sizes can be inferred.

"View types" addresses one of the top pain points of the borrow checker, which is disjoint borrows on a single value. The compiler can understand values where only some fields are borrowed, so e.g. you can mutably borrow the currently-unborrowed fields, but there's no way to write these partial borrows and so you cannot write functions that either take or return these borrows. This often means having to restructure code to split up one struct into several child structs just to be able to borrow those substructs independently.

How are these suggestions not pragmatic? You don't have to use them, but if you need them they are there. From a security point of view I can see many of these being incredibly useful.

No, that's not how it works. See for example async: I don't need it (and indeed hate working with it, because colored functions are incredibly unpleasant to work with). But a huge swath of libraries in the ecosystem are designed with the assumption that you will use it. If the language adds more features like the article is proposing, it will most likely balkanize the crate ecosystem even further. This stuff does affect people even if they never use the features in question.

If it slows down Rust development it's not pragmatic. And if it creates a cultural schism between full commitment and pragmatic approaches, it's also trouble. Remember Scala?

>If it slows down Rust development it's not pragmatic.

I truly don't understand. If you don't want rust to become complex, you don't want it to "develop" fast anyways. Unless you mean you think it will be slower to write code?

> And if it creates a cultural schism between full commitment and pragmatic approaches, it's also trouble.

Zero clue what this is supposed to mean. WTF is "full commitment" here?

> Remember Scala?

Scala, haskell, and others are high level languages in "academic terms." They have high levels of abstraction. The proposals are the opposite of high level abstractions, they instead formalize very important low level properties of code. If anything they decrease abstraction.

Rust is nowhere near the complexity of Scala wrt. seemingly arbitrary high-level features. There's a low-level, systems programming featureset that involves quite a bit of complexity but that's also less arbitrary when comparing across similarly low-level languages.

> This may be too much advanced type theory for a useful language.

I think a lot of things taken for granted these days were considered "too complicated" some time ago: think of how widespread pattern matching, closures, generics, or functional idioms in imperative languages are, and compare to e.g. Java 1.0.

My feeling is that the "acceptable level of complexity" for programming languages goes up over time, so probably stuff like effect types will be almost everywhere in another 10 years.

> This may be too much advanced type theory for a useful language.

Maybe but:

- Move fixes Pin

- Linear types, prevent memory leaks

- potentially effects simplify so many things

Each of these functionalities unlock capabilities people have complained about Rust. Namely async, gen blocks, memory leaks.

Yes. The appalling level of ignorance in the comments here and failure to understand the article is disturbing and a little scary ... hopefully it won't get in the way of actually simplifying and unifying the language, generalizing special cases, removing limitations and roadblocks, etc. as outlined in the article.

Counterpoint: if any language could thrive in that valley of despair between pragmatic and theoretical excellence you're referring to, it would be Rust. Because so much of the cost is already paid for once you have satisfied the borrow checker. At least that's what I'd imagine, I could certainly be wrong.

I'm not 100% convinced that "plugging memory error holes" was right at the compiler level.

Currently building out clr, which uses a heuristic (not formal verification) method for checking soundness of zig code, using ~"refinement types". In principle one could build a more formal version of what I'm doing.

https://github.com/ityonemo/clr

Boost is as actual as ever.

Also the way nowadays is with constexpr, templates, and around the corner, static reflection.

Boost is stronger than ever.

> I understand that Boost is now deprecated.

Huh?? Boost is used basically everywhere.

Definitely not. Boost is specifically prohibited in many companies. I haven’t run into Boost in a source tree in over a decade.

There are many reasons for this. Boost has uneven quality. Many of the best bits end up in the C++ standard. New versions sometimes introduce breaking changes. Recent versions of C++ added core language features that make many Boost library features trivial and clean to implement yourself. Boost can make builds much less pleasant. Boost comes with a lot of baggage.

Boost was a solution for when template metaprogramming in C++ was an arcane unmaintainable mess. Since then, C++ has intentionally made massive strides toward supporting template metaprogramming as a core feature that is qualitatively cleaner and more maintainable. You don’t really need a library like Boost to abstract those capabilities for the sake of your sanity.

If you are using C++20 or later, there isn’t much of a justification for using Boost these days.

Not at all. Most of the good parts of boost are now part of the standard library, and the rest of boost that is of high quality have stand-alone implementations, like ASIO, pybind11 (which was heavily influenced by Boost.python), etc...

A lot of the new stuff that gets added into boost these days is basically junk that people contribute because they want some kind of resume padding but that very few people actually use. Often times people just dump their library into boost and then never bother to maintain it thereafter.

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