Question for the more knowledgeable Rust folks: Does this code (the echo server example) not handle the situation where the port is already in use? What's the return type of the socket bind in that situation?
EDIT: Wow six responses in as many minutes. I think I've created a new objective measure of how popular a programming language will be!
My view is that sample code such as this should be as idiomatic as possible and that means providing a sample demonstrating a typical real use case. So seeing "unwrap" in this context doesn't sit well with me.
In fairness, this isn't specific to Rust. I find sample code like this in many projects regardless of language. However Rust is billing itself as a safe systems programming alternative to C and C++. It would help its marketing efforts, in my opinion, by having more robust samples than the competition. And let's be honest: given the competition is C and C++, that's a low, low bar--and this comes from a guy who's about as big a fanboy of those two languages as possible.
Edit for more disclaimer: documenting Rust code is a real pleasure. The team has done a stellar job at making documentation an easy thing to do while writing the code. It's already better in most cases than many other projects I can think of.
That is, in terms of safety, this is the same thing as explicitly handling the error and then terminating the process. Which is the only real way you're going to handle this error anyway, unless you wanted some sort of retry logic. Which you might!
But for most example programs there's no easy way to handle errors further than just using expect() unless you want there to be more error handling code than actual useful example code.
The idea is that I expect this to be Ok/Some, and if it's not, please use this error message.
Here, the programmatic object is the grammatical object. That's what I meant by 'backwards'.
could_fail().or_panic_with('message') may be even better.
unwrap should be confined to test and temporary code. It somewhat makes sense in example code, but expect is better for that.
There are sometimes cases when you very easily know that the unwrap will never fail and if it does something has gone very horribly wrong, in which case you might use it.
Libraries should keep usage of unwrap/expect to a minimum. Applications can be more liberal with it, but they should try to use expect or better error handling.
Not everyone perfectly follows this, sadly. But most do.
This will print an error out already.
$ ./target/debug/tokiotest
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Error { repr: Os { code: 98, message: "Address already in use" } }', ../src/libcore/result.rs:837
This really shows a fundamental tension though, in documentation. Is this example supposed to be demonstrating error handling? Or just get you going? Does adding more complex error handling distract from the point it's trying to teach? These are sort of open-ended questions.
If rust had that examples would be even shorter using ? Instead of try! or unwrap
quick_main!(|| -> Result<()> {
// insert Result-returning code here
});
fn main() {
match Args::parse().map(Arc::new).and_then(run) {
Ok(0) => process::exit(1),
Ok(_) => process::exit(0),
Err(err) => {
eprintln!("{}", err);
process::exit(1);
}
}
}
`quick_error` uses a trait like that to determine the exit code of `main()`, allowing it to return either () or i32.
Obviously not if it's used for input errors (network failure). Crashing assertions are made for bugs, not input errors.
We can agree in the cynical interpretation of the laziness of programmers, but the mitigations in this case are so trivial, and the stakes so low, that focusing on unwrap as a point of contention is a poor use of energy.
As I said in the post you're replying to, a nicer error message would be a good thing.
Whoever maintains the server and runs the service is also my user, though, in the general case.
> As I said in the post you're replying to, a nicer error message would be a good thing.
Yeah - I don't mind if unwrap panics with a dev-oriented message as it's basically an assertion, but I guess I expected expect() (no pun intended) to give a more user-friendly error. Maybe the format of the panic! output could be changed to bring the message to the front and the technical details after that.
Such error handling code is usually untested, which is another way of saying 'buggy'. It almost always swallows useful information, like the backtrace. It sometimes lets program execution continue in a messed up state, causing very strange and hard to debug errors later on.
Certainly Rust makes it a lot harder to mess up error handling code than the languages I'm used to but in general I'm definitely in the 'all exceptions fatal' camp.
http://rust-lang.github.io/book/ch12-03-improving-error-hand... shows how to refactor a project to improve its error handling as well, for something a bit less abstract.
* Rust prevents deadlocks
* You can't leak memory in Rust
* Rust prevents race conditions
* Panic is not safe
etc. In my mind, bringing up memory safety here isn't a red herring; the parent said this:
> However Rust is billing itself as a safe systems programming alternative to C and C++.
The way in which we are safer is memory safety, nothing more. And knowing that is crucial.
Rust code will have bugs. Rust code will have security vulnerabilities. Rust is not a panacea.
I know you want to not overstate rust's claims given recent articles, but I think you're actually underselling a little here. For example, a rust `enum` make it much easier for the compiler to enforce code correctness. It's hard to go back to similar code in C or Go once you've gotten used to `match`.
> prevents segfaults
Only outside unsafe blocks.
> guarantees thread safety.
But you just said you can have deadlocks and race condition, so... perhaps change the frontpage?
This is a relatively uninteresting quibble: the guarantees of any language only apply outside their equivalent of unsafe blocks (e.g. Python is memory safe... until you use ctypes). Pretty much everything has a way to FFI down to interact with the machine; in Rust, it's just called `unsafe`.
(One way to look at `unsafe` is that it's a tightly integrated FFI to another language called "unsafe Rust", and it benefits from having zero performance or semantic overhead.)
If a language frontpage is inexact I'll assume the rest of the website is inexact, simple as that.
Terminology matters.
As it cost me 20+ downvotes to make a rational conversation with you all, I will stop interacting with the Rust community.
As I mentioned in https://news.ycombinator.com/item?id=13377957, in the world of programming languages, the term "memory safety" means something specific.
D calls itself safe on its website, but it too has the ability to escape-hatch into C/C++.
Language websites putting forth subjective claims or claims based on definitions which may be subjective is totally normal:
Go says that it "makes it easy to build simple, reliable, and efficient software", which is totally subjective.
Ruby says "It has an elegant syntax that is natural to read and easy to write.", also subjective.
Python says "lets you work quickly and integrate systems more effectively", also subjective.
By the same token, you should be requiring that every subjective statement on the other langs' pages have the caveat "in the opinion of the $LANG developers." Obviously no one would wear that.
The page clarifies later on that the definition of thread safety in question is "threads without data races." The 16 words on the front page of the website are not the appropriate place to go into caveats and nuances.
> Only outside unsafe blocks.
This is just true of all of our guarantees. Given that the vast, vast, vast, vast majority of Rust code is safe code, I don't feel this is misleading. Do you say that Ruby doesn't prevent segfaults due to its C FFI?
edit: I'm just saying "don't lie". I shouldn't have to defend this view point and get downvotes for it.
1/ It says on the tin "prevents segfaults". It does not prevent segfaults in all cases, but OK, let's debate the other claim.
2/ It also says "guarantees thread safety". What is the Wikipedia definition of thread safety? It varies, so let's take the most common "freedom from race conditions."
Steve come and say Rust can have race conditions https://news.ycombinator.com/item?id=13376485 and that it's a common misunderstanding to think it prevents race conditions. Surely it would be great if the frontpage would not promote it!
Ergo the frontpage claim is false.
The wikipedia definition is pretty vague, it relies on a concept of "safe" that isn't defined there. It's acceptable to say that "data race safety" is "thread safety", though confusing. Rust's homepage does clarify what it means in the bullet points below that statement, so I wouldn't call this a lie. It may be misleading though, and this is a common misunderstanding as steve mentioned, so I submitted a PR to fix it https://github.com/rust-lang/rust-www/pull/685
In the punchline, "prevent segfaults" is framed in negative terms. If you put "provides memory safety" it will be a bit less evocative of specific pain, but will give a warm feeling.
Not too sure of the segfaults bit, feel free to put in your thoughts on that PR
Rust is a systems programming language that runs blazingly fast, guarantees memory safety and provides correct concurrency.
Rust is a systems programming language that provides uncompromised performance, convenience and security.
Rust is a systems programming language that enables unprecedented levels of performance, productivity and safety.
edit: I like (2) best
While teaching Rust it's a recurring theme among students to believe that all crashes are created equal and that an exit from a panic must be equivalent to a segfault, despite this being significantly false. So it's common when discussing panics to novice audiences to sprinkle in "by the way, this isn't a crash, it's a controlled exit".
Is it? It's out of bounds of any object, and therefore undefined behavior.
It's one of the more common ways of using unwrap.
(The examples unwrap elsewhere in less known-good scenarios, I agree that we should improve there)
I've starting trying to use `if let Some/Ok(...) = ...` in examples for my projects rather than `unwrap` for pretty much this reason. I feel like it strikes a pretty good balance between providing a relatively terse example without the risk of implying that `unwrap/expect` is considered normal usage for the library.
My intermediate impression is: Basics and the whole idea behind it looks very good and suits Rusts general concepts. However from a productivity point of view it yet can't match the ergonomics of Go or C# with TPL and async/await for IO tasks. But I think the rust contributors are well aware of this and working on some things (async/await like sugar and that impl trait thing which might fix my composition problems).
More likely, you'll be using something like this: https://tokio.rs/docs/getting-started/simple-server/.
Perhaps something like that should also be on the homepage to show that, while you can do low-level, it has already scaled up to a higher abstraction for you.
The result of parse().unwrap() is of type F, which means addr is of type 'F'. afterwards when we call `let sock = TcpListener::bind(&addr, &handle).unwrap();` the compiler can infer from the signature of `TcpListener::bind`[2] what F has to be, namely `std::net::SocketAddr`
[1] https://doc.rust-lang.org/std/primitive.str.html [2] https://tokio-rs.github.io/tokio-core/tokio_core/net/struct....
impl FromStr for SocketAddr
fn parse<F>(&self) -> Result<F, F::Err> where F: FromStr
Due to the resulting thread, I feel legitimately excited to sit down and learn a new-to-me language for the first time in ages :)
The TCPListener::bind call returns a Result enum, which can have the values Ok(SomeType) or Err(SomeError). The compiler forces you to decide how you want to handle that, for the sake of brevity the example code calls unwrap() which assumes the result is Ok and will either return SomeType or else panic.
> Does this code
So
let sock = TcpListener::bind(&addr, &handle).unwrap();
1: https://docs.rs/tokio-core/0.1.3/tokio_core/net/struct.TcpLi...
This line tries to bind the port and returns a `Result` type which is then unwraped, which will cause a panic when the port already is in use.
It's generally considered bad form to use unwrap() (and by extension panic) in production code, but for examples where you just want to show something it's fine.
bind would either return a Option type or a Result type, unwrap is defined for both.
edit: Others have looked through the docs and confirmed it's a Result
TcpListener::bind(&addr, &handle).unwrap()
Rather than raising exceptions, the paradigm in Rust is to return a Response item, which has an Ok and an Err subtype. unwrap() here assumes that the response is Ok and just gives that back the data returned, or crashes if its an error.
> It won't crash. It's a "clean" shutdown.
:+1:
let addr = "127.0.0.1:12345".parse().unwrap();
I'm not trying to start a language war, but IMO that's too bad: this feature is, in my experience, the single largest source for difficult-to-detect errors that characterize the Ruby programming experience, e.g. anything that uses Rails. It also contributes to deep stack traces which are essentially useless, because the departure point from your actual code is some 20+ frames above the site of the error.
In this case, both of those methods are in the standard library.
From reading the docs, something provides a method called `FromStr` and the `str.parse` method does some magic to figure out which `FromStr` of all the registered traits is the one which we want? So then, it's not arbitrary monkey-patching, but using a defined protocol for defining and extending string parsing?
> From reading the docs, something provides a method called `FromStr` and the `str.parse` method does some magic to figure out which `FromStr` of all the registered traits is the one which we want?
The method isn't doing any magic: the language's type inference is deducing that the return type has to be a SocketAddr based on how the return value is used (it is passed to a function expecting a SocketAddr), and this forces the compiler to use the SocketAddr implementation of the FromStr trait (which has a from_str method), all at compile time.
(If the compiler deduced that the return type was something that didn't implement FromStr, i.e. no idea how to parse that value from a string, one would get a compile error.)
> So then, it's not arbitrary monkey-patching, but using a defined protocol for defining and extending string parsing?
Yes, parse is entirely driven by the FromStr trait.
It's closer to refinements than monkey-patching, that is, you can, but you must bring the trait into scope for it to work.
> And that merely including a library can alter the definition of those objects everywhere in your project's code?
No, based on the above rule. We don't want that.
So, specifically. Rust has "traits", which, for these purposes, work like Ruby modules. But. Rust has "coherence rules." That is,
You can only implement a trait for a type if you defined either the trait, or the type, or both.
In tokio-core, they bring SocketAddr into scope: https://docs.rs/tokio-core/0.1.3/src/tokio_core/net/tcp.rs.h...
However, I was thinking of a slightly different case when I said that, which is methods. That is, you couldn't call methods unless they've had their traits brought into scope. But parse isn't on a trait; it's defined on `str` itself. In that definition, it also has to bring FromStr into scope.
So basically, this is very muddled by the fact that this particular example uses the standard library heavily, which already implicitly brings a lot of things (like &str) into scope for you. External packages don't have that.
It's unclear to me what you're saying here. It's true that you can't make an inherent method `parse` on str like the one being called here, it's also true that one couldn't implement FromStr for SocketAddr in an external package, but one can create a custom generic function that uses FromStr and have that generic type be SocketAddr for some call sites. Of course, if this function wanted to have the syntax of a method call on str, it would have to be defined in a trait that users then import.
> In tokio-core, they bring SocketAddr into scope: https://docs.rs/tokio-core/0.1.3/src/tokio_core/net/tcp.rs.h....
To be clear, this isn't necessary for calling methods---including inherent methods on SocketAddr---unlike the methods in a trait. The only reason to do this is that it lets one abbreviate the name: "SocketAddr" instead of "::std::net::SocketAddr".