A guide to error handling in Rust

https://nrc.github.io/error-docs/

by nrc • 4 years ago

173 74 4 years ago

nrc.github.io

anacrolix2• 4 years ago

try blocks? is this a bit out of date?

nrc2• 4 years ago

Nope, try blocks are a new (unstable) feature

Spiritus• 4 years ago

I think you're mixing up try blocks with the old try! macro.

tialaramex4• 4 years ago

How so? As it says this is not yet a stable feature, but if you run nightly Rust it's available.

Try blocks let you do what ? (the Try operator) does within a block, rather than needing to split out a separate function for it, which makes sense because why should functions be special in this way?

anacrolix2• 4 years ago

I have indeed confused it with try! macro

tialaramex4• 4 years ago

I feel like this document makes the Try operator (?) and its associated trait more mysterious than necessary. Most people probably won't need to implement Try, especially before it is stabilised, but it's not that much more complicated than say, AddAssign the trait which you implement to make the Add Assignment (+=) operator work on your type.

The key trick of Try is that it converts something (by default an Option or a Result or async Polls of those types) into a ControlFlow†. This is the one nice trick about Exceptions in languages which have them - they influence control flow, but Rust reified it as a vocabulary type which I think is much better. We can pass this thing back to somebody who cares about the resulting control flow, not just suddenly wrench the control flow out from under the rest of the software.

† unlike Try, ControlFlow is actually a stable type you can use today in your Rust and, like std::cmp::Ordering it's useful even just as a vocabulary type, disregarding its semantics. Library A and Library B, written by different people, in different circumstances, both agree that ControlFlow::Continue is continue and ControlFlow::Break is break whereas who knows what the boolean false from Library A means to Library B, let alone what if anything Library B's custom type BPartialResult means to Library A's code.

mmastrac• 4 years ago

TIL what ControlFlow is. This is super interesting and solves some problems I thought were impossible.

Error handling has gone from uber painful in 2018 to pretty decent in the latest editions.

petertodd• 4 years ago

BTW if you have read about the `Try` trait before and are wondering what `ControlFlow` is, read it again: https://doc.rust-lang.org/nightly/std/ops/trait.Try.html

`Try` was recently changed significantly with the introduction of `ControlFlow`. IMO it's a big improvement.

gwbas1c• 4 years ago

I do some hobby projects in Rust.

One gotcha that I hit was using ? in sample code in documentation. It didn't work, so I had to replace all of my ? with .unwrap().

I generally consider .unwrap() a poor example, because it encourages writing code that could crash a program unnecessarily.

erk__• 4 years ago

You can use it in sample code in the documentation, but you will need to add a bit of boilerplate around it: https://doc.rust-lang.org/rustdoc/write-documentation/docume...

cercatrova• 4 years ago

You can set lints for cargo, for example to warn or even disallow compiling with any `unwraps` or `expect`s. I use cargo-cranky which makes using lints super easy, cargo doesn't yet have native functionality to set which lints should be enabled or disabled.

valenterry16• 4 years ago

Too bad Rust doesn't have union types (aka adhoc / anonymous unions) yet.

Without them, using typed errors is very clumsy. Optimally, you would write the following code:

    fn foo(r1: Result<i32, Error1>, r: Result<i32, Error2>) {
        let i1 = r1?;
        let i2 = r2?;
        // ...
    }

and Rust would infer the return type to be Result<String, Error1 | Error2> without having to do any extra definitions or conversions.

insanitybit3• 4 years ago

Anonymous sum types are something I want for error handling as well. In practice though I'm not sure it would really make my life that much better.

masklinn• 4 years ago

It would make life a bit terser, but I’d rather have polymorphic variants. And maybe only anonymous enums over polymorphic variants.

That would make precise error handling on libraries quite a bit better.

valenterry16• 4 years ago

Not sum types. Those are union types. The difference is important, since if you work with two results (or two functions that return results) that use the same error-type you most often don't want to end up with a tuple of two times the same error but simply A<String, Error>.

Of course, if you care about which error is from which function, you can always easily do that by wrapping them into a sumtype, but in practice this is a rather rare use-case in application code at least.

jamincan• 4 years ago

The library ecosystem does fill that gap somewhat with crates like anyhow [1] which also allow you to add context to the errors you return.

1. https://docs.rs/anyhow/latest/anyhow/

valenterry16• 4 years ago

Unfortunately this does not solve the problem. If you use this, you lose typesafety and the documentation you get through the (inferred) types. I.e. with union types you can see at a glance in your IDE which types of errors a function can return - this is too valuable to give it up.

martijnarts• 4 years ago

For anyone interested in what this would look like in Rust now, there's two ways. For libraries, people tend to recommend the thiserror crate. Code sample[0]:

    #[derive(thiserror::Error, Debug)]
    enum Error {
        #[error("One")]
        One(#[from] Error1),
        #[error("Two")]
        Two(#[from] Error2),
    }

    fn foo(r1: Result<i32, Error1>, r2: Result<i32, Error2>) -> Result<..., Error> {
        let i1 = r1?;
        let i2 = r2?;
        // ...
    }

Whereas for binaries, people usually recommend anyhow. Code sample[1]:

    fn foo(r1: Result<i32, Error1>, r2: Result<i32, Error2>) -> anyhow::Result<...> {
        let i1 = r1?;
        let i2 = r2?;
        // ...
    }

[0]: https://play.rust-lang.org/?version=stable&mode=debug&editio... [1]: https://play.rust-lang.org/?version=stable&mode=debug&editio...

exDM692• 4 years ago

For simple tasks you can get away without any external crates by using `Result<T, Box<dyn Error>>`. But it's much more comfortable to use thiserror or anyhow in the long run.

    fn read_string() -> std::io::Result<String> {
        Ok("123".to_owned())
    }

    fn main() -> Result<(), Box<dyn std::error::Error>> {
        let s = read_string()?; // io::Error
        let n = i32::from_str_radix(&s, 10)?; // num::ParseIntError
        println!("read number: {n}");
        Ok(())
    }

icambron• 4 years ago

The binary vs library thing seems like an oversimplification to me. I think it's more like: do you need callers to handle this error specifically? With a library the answer is "I don't know, better let them do it", so you don't want anyhow. But in a binary, you may or may not, and it depends on the error.

The pattern I use in my app is to use thiserror, and then just have an anyhow catch-all. That lets me do specific stuff where I know I'm going to need specific handling, and an easy-to-use fallback for just saying "this bad thing happened" with the anyhow! macro.

    #[derive(Debug, thiserror::Error)]
    pub enum Error {

        #[error("Not logged in")]
        NotLoggedIn,

        #[error(transparent)]
        Api(#[from] ApiError),

        // etc

        #[error(transparent)]
        Other(#[from] anyhow::Error),
    }

I don't know if this is the best pattern but it's worked really well for me.

ithkuil• 4 years ago

what is the TL;DR difference between anyhow and Box<dyn Error> ?

tialaramex4• 4 years ago

Even if the "Ad hoc union" becomes a thing in Rust, you are not likely to get inference of return types.

The return type is part of the function signature and Rust deliberately doesn't infer signatures, in languages with "too much" inference it's impractical for the human programmer to keep track of types because it's all inferred, this has started to be a problem in C++ as more and more things are auto. Rust has some very sophisticated inference inside a function (including partial inference and inferring types from how they're later used), but none for the signature.

valenterry16• 4 years ago

I think it's the job of the IDE to make that work, but I agree, without IDE this can quickly become a problem.

dllthomas5• 4 years ago

While I don't strongly object to Rust's choice here, and I agree that production code should have a type signature on every function, I think this is more a place for lint/clippy/whatever. There's no need to gate the programmer trying something on them having produced a type signature that could be inferred.

yakubin• 4 years ago

„Yet”? Are there any plans to add them?

kibwen2• 4 years ago

There are not. I don't believe they've ever been thoroughly proposed.

xavxav• 4 years ago

I know that this is a common wish, but anonymous sum types have pretty catastrophic impacts on type checking and lead to all sorts of bizarre corner cases like the following:

  let a = if cond {
    1 
  } else {
    1.0
  };
  
  a + 3

Now, the error would be pushed to the `+` operator because there isn't an `Add` for `f32 | u32`. Granted, this is a trivial example, and a programmer can easily see through it, but in general this can get very overwhelming and cause errors to leave their 'root cause'.

valenterry16• 4 years ago

Now, the error would be pushed to the `+` operator because there isn't an `Add` for `f32 | u32`.

Why not? It makes total sense for one to exist. This is something the language needs to deal with, not the programmer. But the programmer always sprinkle annotations so that errors can only reach so far.

dllthomas5• 4 years ago

One option might be to differentiate syntactically between branches of an if/match that are allowed to expand their type (to a union or to a bigger union) and those that are not. I am not sure how far that generalizes, though.

karmakaze• 4 years ago

Typescript seems to have no problem with it (nor do I suspect F#/OCaml):

  class A {
    x = 10;
    scale(n: number): void {
      this.x *= n;
    }
  }
  class B {
    y = 10;
    scale(n: number): void {
      this.y *= n;
    }
  }  
  var z: A | B;
  z = new A
  z.scale(2);
  z = new B
  z.scale(2);

I was also looking for Sum Types/anonymous enums[0].

[0] https://news.ycombinator.com/threads?id=karmakaze&next=33505...

sinistersnare• 4 years ago

I think that taking options and results usually is an anti pattern. You want to handle the errors at creation-site and not propagate it to another function.

Of course this is just a minimal example... Others have talked about the real point of this comment, and it's just a minor nit. I wanted to point this out for new rust programmers reading this.

oxff2• 4 years ago

It is perhaps too verbose by default, as indicated by popularity of thiserror and anyhow crates.

nrc2• 4 years ago

Many of the features of such crates are making their way to the standard library, so things will definitely improve. Figuring out what is best has taken some time and Rust has not wanted to prematurely commit.

oxff2• 4 years ago

I like what the error handling achieves, it is actually readable way to understand the divergent control flow paths (and probably majority of code is read more than written), but I do not enjoy writing the initial boilerplate, so that's good to hear.

kibwen2• 4 years ago

If anyone's interested in helping to shape the future of Rust's built-in error-handling story, there's an error handling project group that's been doing great work recently, e.g. the major effort to move the Error trait into libcore ( https://github.com/rust-lang/project-error-handling/issues/3 ) and stabilizing std::backtrace. You can follow along or get involved via the #project-error-handling channel on the Rust zulip: https://rust-lang.zulipchat.com/

exDM692• 4 years ago

Perhaps, but it's also much better than it was two years ago and there is work going on to make it better two years in the future.

A myriad of experimental prototypes (like the failure crate and its descendants) have been made, experimented with and then retired and looks like the progress is converging to these two complementary error handling crates (anyhow, thiserror, and a few mostly-compatible variants like eyre), and work going on to standardize some aspects of it so (parts of) these crates can be retired. There's also core::error that's bringing this to no-std environments.

So yeah, it definitely was not great on day 1 and there's been a lot of churn on error handling but it is going in the right direction.

Ciantic• 4 years ago

It can be as verbose as you want. This guide as well as others suggests using concrete error types for libraries but anyhow "catch all" method for applications.

There is disadvantages for going "catch all" libraries as then you can't be sure you are catching all errors.

svnpenn2• 4 years ago

This glaring omission from this is the "enum idiom":

https://doc.rust-lang.org/std/convert/trait.From.html#exampl...

they talk about it here:

https://nrc.github.io/error-docs/error-design/error-type-des...

but including more than a snippet would go a long way to that "aha" moment I think. This was frustrating for me browsing this site. The author wrote 10 pages of docs, but nearly all the examples are like 5 line snippets of code. I think examples are equally important as the discussion itself. Rust itself suffers from the same problem:

https://github.com/rust-lang/book/issues/3348

epage• 4 years ago

Whats the "aha" moment for it, the Froms?

The author might not have included that as they call out you likely shouldn't directly wrap another error.

I go a step further and think that public errors shouldn't have From's for concrete types, exposing your implementation details, and that enum errors are more generally too tied to implementation details to be used in libraries.

svnpenn2• 4 years ago

OK, but what do you do then? Its not really helpful to say "this bad", if you don't offer a "this good". Of the maybe 10 approaches I have seen to Rust error handling (including using external crates, gross), the "enum idiom" is the most elegant and flexible to me, and coming from another language feels the most natural.

shepmaster• 4 years ago

public errors shouldn't have From's for concrete types
enum errors are more generally too tied to implementation details to be used in libraries

I generally agree. SNAFU addresses these problems in two ways:

1. The `From` implementation is not created for the underlying error but for an intermediate type (by default). That type is private to the crate (by default) and cannot expose implementation details.

2. There's an opaque error facility to completely hide the enum details.

Put together, that looks something like...

    use snafu::prelude::*;
    use std::{
        fs,
        path::{Path, PathBuf},
    };
    
    #[derive(Debug, Snafu)]
    enum ErrorImpl {
        #[snafu(display("Could not read the config file {}", path.display()))]
        UnableToReadConfig {
            source: std::io::Error,
            path: PathBuf,
        },
    
        #[snafu(display("Could not write the config file {}", path.display()))]
        UnableToWriteConfig {
            source: std::io::Error,
            path: PathBuf,
        },
    }
    
    #[derive(Debug, Snafu)]
    pub struct Error(ErrorImpl);
    
    pub type Result<T, E = Error> = std::result::Result<T, E>;
    
    pub fn do_stuff_with_config(path: &Path) -> Result<()> {
        let config = fs::read_to_string(path).context(UnableToReadConfigSnafu { path })?;
        fs::write(path, config).context(UnableToWriteConfigSnafu { path })?;
        Ok(())
    }

Other things about SNAFU:

- It's very easy to add valuable context to the errors. See how the `&Path` context is transformed to a `PathBuf` with low ceremony in the example.

- You can create struct- or enum-based errors.

- You can use "stringly-typed" errors (akin to anyhow) but in combination with strongly-typed errors. This allows you to start out with a loose error handling regimen and make it stronger as you go along.

- There's support for capturing backtraces or lightweight file/line/column information.

- There's a pretty error reporter for usage with `main` functions or tests.

- There's support for the nightly-only Provider API.

MontagFTB2• 4 years ago

An aside: what’s the template that gets articles formatted this way in GitHub pages? I found it very appealing.

mrec• 4 years ago

This is `mdbook` - https://rust-lang.github.io/mdBook/

MontagFTB2• 4 years ago

Thanks!

A guide to error handling in Rust

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