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I found it kind of odd that the author says Rust doesn't support concurrent programming out of the box. He links to another comment which points out you don't need Tokio for async (true enough), but even that aside async isn't the only way to achieve concurrency. Threads are built right into the language, and are easier to use than async. The only time they wouldn't be a good choice is if you anticipate needing to spawn so many threads that it causes resource issues, which very few programs will.

(Honest question from non Rustacean.)

How does the cancellation story differ between threads and async in Rust? Or vs async in other languages?

There's no inherent reason they should be different, but in my experience (in C++, Python, C#) cancellation is much better in async then simple threads and blocking calls. It's near impossible to have organised socket shutdown in many languages with blocking calls, assuming a standard read thread + write thread per socket. Often the only reliable way to interrupt a socket thread it's to close the socket, which may not be what you want, and in principle can leave you vulnerable to file handle reuse bugs.

Async cancellation is, depending on the language, somewhere between hard but achievable (already an improvement) and fabulous. With Trio [1] you even get the guarantee that non-compared socket operations are either completed or have no effect.

Did this work any better in Rust threads / blocking calls? My uneducated understanding is that things are actually worse in async than other languages because there's no way to catch and handle cancellations (unlike e.g. Python which uses exceptions for that).

I'm also guessing things are no better in Ada but very happy to hear about that too.

Cancellation in rust async is almost too easy, all you need to do is drop the future.

If you need cleanup, that still needs to be handled manually. Hopefully the async Drop trait lands soon.

> There's no inherent reason they should be different

There is... They're totally different things.

And yeah Rust thread cancellation is pretty much the same as in any other language - awkward to impossible. That's a fundamental feature of threads though; nothing to do with Rust.

To be clear about what I meant: I was saying that, in principle, it would be possible design a language or even library where all interruptable operations (at least timers and networking) can be cancelled from other threads. This can be done using a cancellation token mechanism which avoids even starting the operation of already cancelled token, in a way that avoids races (as you might imagine from a naive check of a token before starting the operation) if another thread cancels this one just as the operation is starting.

Now I've set (and possibly moved) the goalposts, I can prove my point: C# already does this! You can use async across multiple threads and cancellation happens with cancellation tokens that are thread safe. Having a version where interruptable calls are blocking rather than async (in the language sense) would actually be easier to implement (using the same async-capable APIs under the hood e.g., IOCP on Windows).

I wonder where the cut-off is where a work stealing scheduler like Tokio's is noticeably faster than just making a bunch of threads to do work, and then where the hard cut-off is that making threads will cause serious problems rather than just being slower because we don't steal.

It might be quite small, as I found for Maps (if we're putting 5 things in the map then we can just do the very dumbest thing which I call `VecMap` and that's fine, but if it's 25 things the VecMap is a little worse than any actual hash table, and if it's 100 things the VecMap is laughably terrible) but it might be quite large, even say 10x number of cores might be just fine without stealing.

The general rule is that if you need to wait faster use async, and if you need to process faster use threads.

Threads as they are conventionally considered are inadequate. Operating systems should offer something along the lines of scheduler activations[0]: a low-level mechanism that represents individual cores being scheduled/allocated to programs. Async is responsive simply because it conforms to the (asynchronous) nature of hardware events. Similarly, threads are most performant if leveraged according to the usage of hardware cores. A program that spawns 100 threads on a system with 10 physical cores is just going to have threads interrupting each other for no reason; each core can only do so much work in a time frame, whether it's running 1 thread or 10. The most performant/efficient abstraction is a state machine[1] per core. However, for some loss of performance and (arguable) ease of development, threads can be used on top of scheduler activations[2]. Async on top of threads is just the worst of both worlds. Think in terms of the hardware resources and events (memory accesses too), and the abstractions write themselves.

[0] https://en.wikipedia.org/wiki/Scheduler_activations, https://dl.acm.org/doi/10.1145/121132.121151 | Akin to thread-per-core

[1] Stackless coroutines and event-driven programming

[2] User-level virtual/green threads today, plus responsiveness to blocking I/O events

Another way of thinking about this is whether you want to optimize your workload for throughput or latency. It's almost never a binary choice, though.

Can you say more about what you mean by wait faster? Is it as in, enqueue many things faster?

Or in cases where the platform doesn't support threads easily - WASM and embedded (Embassy). Tbh I think that's a better motivation for using async than the usual "but what if 100k people visit my anime blog all at once?"

Ada has some really good ideas which its a shame never took off or got used outside of the safety critical community that mostly used it. The ability to make number types that were limited in their range is really useful for certain classes of bugs. Spark Ada was a relatively easy substandard to learn and apply to start to develop software that was SIL 4 compliant.

I can't help but feel that we just went through a huge period of growth at all costs and now there is a desire to return, after 30-years of anything goes, to trying to make software that is safer again. Would be nice to start to build languages based on all the safety learnings over the decades to build some better languages, the good ideas keep getting lost in obscure languages and forgotten about.

Nim was inspired by Ada & Modula, and has subranges [1]:

  type
    Age = range[0..200]

  let ageWorks = 200.Age
  let ageFails = 201.Age

  $ nim c main.nim
  Error: 201 can't be converted to Age

I know quite some people in the safety/aviation domain that kind of dislike the subranges, as it inserts run-time checks that are not easily traceable to source code, thus escaping the trifecta of requirements/tests/source-code (which all must be traceable/covered by each other).

Weirdly, when going through the higher assurance levels in aviation, defensive programming becomes more costly, because it complicates the satisfaction of assurance objectives. SQLite (whiches test suite reaches MC/DC coverage which is the most rigorous coverage criterion asked in aviation) has a nice paragraph on the friction between MC/DC and defensive programming:

https://www.sqlite.org/testing.html#tension_between_fuzz_tes...

What happens when you add 200+1 in a situation where the compiler cannot statically prove that this is 201?

Your example also gets evaluated at comptime. For more complex cases I wouldn't be able to tell you, I'm not the compiler :) For example, this get's checked:

  let ageFails = (200 + 2).Age
  Error: 202 can't be converted to Age

  import std/strutils

  stdout.write("What's your age: ")
  let age = stdin.readLine().parseInt().Age

  $ nim r main.nim
  What's your age: 999
  Error: unhandled exception: value out of range: 999 notin 0 .. 200 [RangeDefect]

Exactly this. Fails at runtime. Consider rather a different example: say the programmer thought the age were constrained to 110 years. Now, as soon as a person is aged 111, the program crashes. Stupid mistake by a programmer assumption turns into a program crash.

Why would you want this?

I mean, we've recently discussed on HN how most sorting algorithms have a bug for using ints to index into arrays when they should be using (at least) size_t. Yet, for most cases, it's ok, because you only hit the limit rarely. Why would you want to further constrain the field, would it not just be the source of additional bugs?

Ada, or at least GNAT, also supports compile-time dimensional analysis (unit checking). I may be biased, because I mostly work with engineering applications, but I still do not understand why in other languages it is delegated to 3rd party libraries.

https://docs.adacore.com/gnat_ugn-docs/html/gnat_ugn/gnat_ug...

> The ability to make number types that were limited in their range is really useful for certain classes of bugs.

This is a feature I use a lot in C++. It is not part of the standard library but it is trivial to programmatically generate range-restricted numeric types in modern C++. Some safety checks can even be done at compile-time instead of runtime.

It should be a standard feature in programming languages.

> The ability to make number types that were limited in their range is really useful for certain classes of bugs.

Yes! I would kill to get Ada's number range feature in Rust!

It is worked on under the term "pattern types" mainly by Oli oli-obk Scherer I think, who has an Ada background.

Can't tell you what the current state is but this should give you the keywords to find out.

Also, here is a talk Oli gave in the Ada track at FOSDEM this year: https://hachyderm.io/@oli/113970047617836816

That feature is actually from Pascal, and Modula-2, before making its way into Ada.

For some strange reason people always relate to Ada for it.

I would guess that Ada is simply more known. Keep in mind that tech exploded in the past ~3.5 decades whereas those languages are much older and lost the popularity contest. If you ask most people about older languages, the replies other than the obvious C and (kind of wrong but well) C++ are getting thin really quickly. COBOL, Ada, Fortran, and Lisp are probably what people are aware of the most, but other than that?

You've forgotten about BASIC, SNOBOL, APL, Forth, and PL/1. There were many interesting programming languages back then. Good times!

The first five languages I learned back in the 70s: FORTRAN, Pascal, PL/I, SNOBOL, APL. Then I was an Ada and Icon programmer in the 80s. In the 90s, it was C/C++ and I just never had the enthusiasm for it.

For me it's because I learned Ada in college.

18 year old me couldn't appreciate how beautiful a language it is but in my 40s I finally do.

Turbo Pascal could check ranges on assignment with the {$R+} directive, and Delphi could check arithmetic overflow with {$Q+}. Of course, nobody wanted to waste the cycles to turn those on :)

Most Pascal compilers could do that actually.

Yeah not wanting to waste cycles is how we ended up with the current system languages, while Electron gets used all over the place.

There is RFC but I guess the work stopped.

As a sibling comment[0] mentioned, pattern types are actively being worked on.

[0] https://news.ycombinator.com/item?id=45474777

>Ada has some really good ideas which its a shame never took off or got used outside of the safety critical community that mostly used it. The ability to make number types that were limited in their range is really useful for certain classes of bugs.

As pjmlp says in a sibling comment, Pascal had this feature, from the beginning, IIRC, or from an early version - even before the first Turbo Pascal version.

30+ years ago I was programming in Ada, and I feel the same way and have been repeatedly disappointed. Maybe this time around things will be different.

The author indicates some obvious differences, including the fact that Ada has a formal spec and rust doesn't -- rustc seems to be both in flux as well as the reference implementation. This might matter if you're writing a new compiler or analyzer.

But the most obvious difference, and maybe most important to a user, was left unstated: the adoption and ecosystem such as tooling, libraries, and community.

Ada may have a storied success history in aerospace and life safety, etc, and it might have an okay standard lib which is fine for AOC problems and maybe embedded bit poking cases in which case it makes sense to compare to Rust. But if you're going to sit down for a real world project, ie distributed system or OS component, interfacing with modern data formats, protocols, IDEs, people, etc is going to influence your choice on day one.

Rust has now a donated spec that was provided by Ferrocene. This spec style was influenced by the Ada spec. It is available publicly now on https://rust-lang.github.io/fls/ .

This is part of the effort of Ferrocene to provide a safety certificate compiler. And they are already available now.

This is only meaningful if Rust compiler devs give any guarantees about never breaking the spec and always being able to compile code that adheres to this spec.

Why so?

Specs for other languages are also for a specific version/snapshot.

It's also a specific version of a compiler that gets certified, not a compiler in perpetuity, no matter what language.

That's not how it works for most language standards, though. Most language standards are prescriptive, while Rust is descriptive.

Usually the standard comes first, compiler vendors implement it, and between releases of the spec the language is fixed. Using Ada as an example, there was Ada 95 and Ada 2003, but between 95 and 2003 there was only Ada 95. There was no in-progress version, the compiler vendors weren't making changes to the language, and an Ada95 compiler today compiles the same language as an Ada95 compiler 30 years ago.

Looking at the changelog for the Rust spec (https://rust-lang.github.io/fls/changelog.html), it's just the changelog of the language as each compiler verion is released, and there doesn't seem to be any intention of supporting previous versions. Would there be any point in an alternative compiler implementing "1.77.0" of the Rust spec?

And the alternative compiler implementation can't start implementing a compiler for version n+1 of the spec until that version of rustc is released because "the spec" is just "whatever rustc does", making the spec kind of pointless.

How is this different from the existing situation that Rust remains compatible since Rust 1.0 over a decade ago?

Neither the Rust nor the Ada spec is formal, in the sense of consumable by a theorem prover. AFAIK for Ada Spark, there is of course assumptions on the language semantics built-in to Spark, but: these are nowhere coherently written down in a truly formal (as in machine-readable) spec.

On strings in Ada vs Rust. Ada's design predates Unicode (early 1980s vs 1991), so Ada String is basically char array whereas Rust string is a Unicode text type. This explains why you can index into Ada Strings, which are arrays of bytes, but not into Rust strings, which are UTF8 encoded buffers that should be treated as text. Likely the Rust implementation could have used a byte array here.

This write-up shows that while Ada may have some cultural and type-related disadvantages compared to Rust, Ada seems to generally win the readability contest.

What is missing from the comparison is compiler speed - Ada was once seen as a complex language, but that may not be the case if compared against Rust.

In any case, thanks for the post, it made me want to try using Ada for a real project.

Interesting that Ada has an open source compiler. For whatever reason when I looked at it years ago I thought it was proprietary compilers only so I never really looked at it again. Maybe I’ll look again now.

GNAT has been around for 30 years. There were some limitations with (one version of?) it due to it being released without the GPL runtime exception, which meant linking against its runtime technically required your programs to also be released under GPL. That hasn't been an issue for a very long time either, though.

GNAT has been around since the 90s, based on GCC. My university did some work on the compiler and used it for some undergrad courses like real-time programming. IIRC there was an attempt to use Ada for intro to programming courses, but I think they chose Pascal and then eventually C++.

In Case Study 2, near the end it says "if the client may need to know SIDE_LENGTH, then you can add a function to return the value"

Which yeah, you can do that but it's a constant so you can also more literally write (in the implementation just like that function):

        pub const SIDE_LENGTH: usize = ROW_LENGTH;

Tangentially related, one of the more interesting projects I've seen in the 3D printing space recently is Prunt. It's a printer control board and firmware, with the latter being developed in Ada.

https://prunt3d.com/

https://github.com/Prunt3D/prunt

It's kind of an esoteric choice, but struck me as "ya know, that's really not a bad fit in concept."

I found the inclusion of arrays indexed on arbitrary types in the feature table as a benefit of Ada surprising. That sounds like a dictionary type, which is in the standard library of nearly every popular Language. Rust includes two.

I think they're focused very much specifically on the built-in array type. Presumably Ada is allowed to say eggs is an array and the index has type BirdSpecies so eggs[Robin] and eggs[Seagull] work but eggs[5] is nonsense.

Rust is OK with you having a type which implements Index<BirdSpecies> and if eggs is an instance of that type it's OK to ask for eggs[Robin] while eggs[5] won't compile, but Rust won't give you an "array" with this property, you'd have to make your own.

My guess is that this makes more sense in a language where user defined types are allowed to be a subset of say a basic integer type, which I know Ada has and Rust as yet does not. If you can do that, then array[MyCustomType] is very useful.

I call out specifically User Defined types because, Rust's NonZeroI16 - the 16-bit integers except zero - is compiler-only internal magic, if you want a MultipleOfSixU32 or even U8ButNotThreeForSomeReason that's not "allowed" and so you'd need nightly Rust and an explicit "I don't care that this isn't supported" compiler-only feature flag in your source. I want to change this so that anybody can make the IntegersFiveThroughTwelveU8 or whatever, and there is non zero interest in that happening, but I'd have said the exact same thing two years ago so...

I really don't understand this so I hope you won't mind explaining it. If I would have the type U8ButNotThreeForSomeReason wouldn't that need a check at runtime to make sure you are not assigning 3 to it?

Ada also has hash maps and sets.

http://www.ada-auth.org/standards/22rm/html/RM-TOC.html - See section A.18 on Containers.

The feature of being able to use a discrete range as an array index is very helpful when you have a dense map (most keys will be used) or you also want to be able to iterate over a sequential block of memory (better performance than a dictionary will generally give you, since they don't usually play well with caches).

Thanks for the clarification. I can imagine that being a useful optimization on occasion.

It's not a dictionary, that's a totally different data structure.

In ADA you can subtype the index type into an array, i.e. constraining the size of the allowed values.

The Americans with Disabilities Act doesn't cover subtyping the index type into an array. Ada, the language, does though.

EDIT: Seems I'm getting downvoted, do people not know that ADA is not the name of the programming language? It's Ada, as in Ada Lovelace, whose name was also not generally SHOUTED as ADA.

There does seem to be a strain of weird language fanatics who insist all programming language names must be shouted, so they'll write RUST and ADA, and presumably JAVA and PYTHON, maybe it's an education thing, maybe they're stuck in an environment with uppercase only like a 1960s FORTRAN programmer ?

I have found a strong correlation between people who say JAVA and country of origin. And thus have assumed it's an education thing.

It's funny because Fortran's official name now is Fortran, not FORTRAN.

Maybe who cares?

Very nice text. As i am very sceptical to Rust i apreciate the comarison to a language i know better. I was not aware that there is no formal spec for rust. Isn't that a problem if rustc makes changes?

A few things:

1. There is a spec now, Ferrocene donated theirs, and the project is currently integrating it

2. The team takes backwards compatibility seriously, and uses tests to help ensure the lack of breakage. This includes tests like “compile the code being used by Linux” and “compile most open source Rust projects” to show there’s no regressions.

Ferrocene is a specification but it’s not a formal specification. [Minirust](https://github.com/minirust/minirust) is the closest thing we have to a formal spec but it’s very much a work-in-progress.

It depends on who you are.

For implementers of third-party compilers, researchers of the Rust programming language, and programmers who write unsafe code, this is indeed a problem. It's bad.

For the designers of Rust, "no formal specification" allows them to make changes as long as it is not breaking. It's good.

I'd love to use Ada as my primary static language if it had broader support. It's in my opinion the best compiled language with strong static typing. Although it has gained traction with Alire, it unfortunately doesn't have enough 3rd party support for my needs yet.

What 3rd party things would you like to see?

It's just everything and the kitchen sink, I'm afraid, from a NATS server and client library over cross-platform GUIs including mobile, common file format reading and writing like Excel, Word, and PDF to post-quantum cryptography. I'm unfortunately in the business of Lego-brick software development using well-maintained 3rd-party libraries whenever possible. It's the same with CommonLisp, I love it but in the end I'd have to write too many things on my own to be productive in it.

I envy people who can write foundational, self-contained software. It's so elegant.

You have Alire crates for generating Excel and PDF streams/files. Of course you want everything else ;-).

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