Failing in Rust - Speaker Deck

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Failing in Rust Armin @mitsuhiko Ronacher

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800°C 36° 2' 0.4662" N 118° 15' 38.7792" W 795°C 789°C 797°C 793°C 805°C 782°C we show you your crashes

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— Robert F. Kennedy “Only those who dare to fail greatly can ever achieve greatly.”

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Why do we care?

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Errors are Important • Errors are part of your API • Exceptions let you forget about this easily • A lot more relevant when you can catch them and there are multiple versions of libraries involved

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ways to fail greatly

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Mechanisms •Result •Option •panic!

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Result Propagation vs Panic • Results/Options are for handling • panics are for recovering at best

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Examples of Panics • out of bound access • runtime

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Examples of Option • safe signalling absence of data • "the one obvious error"

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— Douglas Adams

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But when you do •panic!("…"); •unreachable!();

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let's talk results

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But if you don't panic … how do you result?

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fn square_a_number() -> Result { let num = get_a_random_float()?; Ok(num * num) }

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let val = expr?;

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let val = match Try::into_result(expr) { Ok(v) => v, Err(e) => return Try::from_error(From::from(e)); };

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error propagation can be hooked!

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The Err in Result can be anything :-/

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So let's use some traits for Err

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pub trait Error: Debug + Display { fn description(&self) -> &str; fn cause(&self) -> Option<&Error>; }

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impl Error + 'static { pub fn downcast_ref(&self) -> Option<&T> where T: Error + 'static; }

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— Charles Darwin “To kill std::error is as good a service as, and sometimes even better than, the establishing of a new trait”

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Problems • Generic errors give no guarantees • no Send / Sync / Debug • causes() returns non static errors • description() is useless • no backtraces

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Enter Failure

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— Winston Churchill “Success consists of going from std::error to failure without loss of enthusiasm”

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some std errors are fails nice! impl Fail for E where E: StdError + Send + Sync + 'static

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failure 0.1 ➠ failure 1.0

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pub trait Fail: Display + Debug + Send + Sync + 'static { fn cause(&self) -> Option<&Fail>; fn backtrace(&self) -> Option<&Backtrace>; fn context(self, context: D) -> Context where D: Display + Send + Sync + 'static, Self: Sized; }

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Fail can be derived

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#[derive(Fail, Debug)] #[fail(display = "my failure happened")] pub struct MyFailure;

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#[derive(Fail, Debug)] #[fail(display = "my failure happened")] pub struct MyFailure { backtrace: failure::Backtrace,  }

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#[derive(Fail, Debug)] #[fail(display = "my failure happened")] pub struct MyFailure { backtrace: failure::Backtrace, #[fail(cause)] io_cause: ::std::io::Error,  }

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Fail & Error

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Fail ⟷ Error

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Fail for libraries Error for applications

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What's in the Package

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Main Functionality • Fail trait • Error type • Context

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Bonus Points • Fail works with no_std • Fail works with many std::errors • error-chain is deprecating itself for failure • actix and others are already using it!

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— rustc an Error is not a Fail

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failure 0.1: error.cause() failure 1.0: error.as_fail() Error to &Fail

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Examples

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#[derive(Debug, Fail, PartialEq, Eq, PartialOrd, Ord)] #[fail(display = "invalid value for project id")] pub struct ProjectIdParseError; Parse Errors

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#[derive(Debug, Fail)] pub enum DsnParseError { #[fail(display = "no valid url provided")] InvalidUrl, #[fail(display = "no valid scheme")] InvalidScheme, #[fail(display = "username is empty")] NoUsername, #[fail(display = "no project id")] NoProjectId, #[fail(display = "invalid project id")] InvalidProjectId(#[fail(cause)] ProjectIdParseError), } Complex Parse Errors

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fn parse(url: Url) -> Result { let project_id: i64 = url.path() .trim_matches('/') .parse() .map_err(DsnParseError::InvalidProjectId)?; Ok(Dsn { project_id }) } Mapping Errors

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#[derive(Debug, Fail, Copy, Clone, PartialEq, Eq, Hash)] pub enum ErrorKind { #[fail(display = "governor spawn failed")] TroveGovernSpawnFailed, #[fail(display = "governor shutdown failed")] TroveGovernShutdownFailed, } Error Kinds

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#[derive(Debug)] pub struct Error { inner: Context, } Custom Errors

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impl Fail for Error { fn cause(&self) -> Option<&Fail> { self.inner.cause() } fn backtrace(&self) -> Option<&Backtrace> { self.inner.backtrace() } } impl fmt::Display for Error { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { fmt::Display::fmt(&self.inner, f) } } Error Pass Through

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pub fn run(config: Config) -> Result<(), Error> { let trove = Arc::new(Trove::new(config)); trove.govern().context(ErrorKind::TroveGovernSpawnFailed)?; // … } Example Usage

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use failure::{Error, ResultExt}; pub fn attach_logfile(&mut self, logfile: &str) -> Result<(), Error> { let f = fs::File::open(logfile) .context("Could not open logfile")?; let reader = BufReader::new(f); for line in reader.lines() { let line = line?; User Facing with Error