witcher

Track and put down bugs using simple concise error handling

Avoid terminating execution randomly in your code with panics via unwrap and expect varients, or laboriously writing custom enum wrapers, or having to work with Box types which is messy; instead use Result<T> from witcher as the return type and wrap errors easily for additional contextual messaging automatically propogated up the stack. witcher implements std::error::Error retaining downcasting and chaining. Best of all witcher provides the holy grail: automatic simplified backtraces.

What you get

1. Error handling simplified

   by providing type matching on errors
   by automatically handling conversions
   by providing concise and terse user interaction
   by providing conditional colored output

2. Error handling that tells the full story

   by implementing std::error::Error
   by chaining errors together
   by providing contextual messaging
   by providing tracing from point of origin

3. Safety

   100% safe code without any use of unsafe
   Zero low level TraitObject manipulation
   Well tested with over 90% code coverage

Quick links

• Usage
  • Rustc requirments
  • Color
  • Downcasting
    • downcast_ref
    • match_err
    • pass
  • Chaining
    • source
  • Retries
    • err_is
    • retry_on
  • Display
    • Normal
    • Alternate
    • Debug
    • Alternate Debug
• Contribute
  • Git-Hook
• License
  • Contribution
• Backlog
• Changelog

Manifesto

Coming from a Golang background, most recently, I fully expected to just import the defacto standard error package in Rust similar to something like Golang's pkg/errors and I'd be off to the races. Instead, as I dug, I found a rich anthropological layered history of a myriad of projects and authors all professing nobal ideals and principles all trying to solve the same issue. Rust's error handling story isn't full featured enough by itself yet. It feels a lot like Golang's before the existance of pkg/errors. I found a few projects clearly more used than others and saw the tide turn on once popular packages. Literally weeks of research and testing of numerous different patterns and packages later though I have still yet to find anything as simple and usable as the venerable pkg/errors. Thus witcher was born.

As a side note I moved all my research on low level TraitObject manipulation, when I was going down the rabbit hole, over to phR0ze/rust-examples and am happy to say witcher is 100% safe code.

Usage

Use the wrap extension method on Result types to wrap the error with additional contextual messaging and automatically chain errors together. wrap returns a Result<T> so there are fewer symbols and less typing needed.

Requires rustc >= 1.30

This minimum rustc requirement is driven by the enhancements made to Rust's std::error::Error handling improvements

1. Import witcher in your Cargo.toml and keep debug symbols

   [dependencies]
   witcher = "0.1"
   
   [profile.release]
   debug = true
   

2. Use the witcher prelude

   use witcher::prelude::*;
   

3. Use the Result<T> alias as your return type

   fn do_something() -> Result<()>;
   

4. Use the wrap extension method on Result to provide context

   fn do_something() -> Result<()> {
       do_external_thing().wrap("Failed to slay beast")
   }
   fn do_external_thing() -> std::io::Result<()> {
       Err(std::io::Error::new(std::io::ErrorKind::Other, "Oh no, we missed!"))?
   }
   

Color

Color is automatically controlled by the gory based on tty detection. You can disable color manually by setting the TERM_COLOR environment variable to something falsy see gory docs on controlling use.

$ TERM_COLOR=0 cargo run -q --example simple

Downcasting

We can match on error types using downcasting or with the match_err! macro.

downcast_ref - access std::error::Error's downcast_ref

use witcher::prelude::*;

// Wrap our internal error with additional context as we move up the stack
fn do_something() -> Result<()> {
    do_external_thing().wrap("Failed to slay beast")
}

// Function that returns an external error type outside our codebase
fn do_external_thing() -> std::io::Result<()> {
    Err(std::io::Error::new(std::io::ErrorKind::Other, "Oh no, we missed!"))?
}

fn main() {
    let err = do_something().unwrap_err();

    // Get the last error in the error chain which will be the root cause
    let root_cause = err.last();

    // Match single concrete error type
    if let Some(e) = root_cause.downcast_ref::<std::io::Error>() {
        println!("Root cause is a std::io::Error: {}", e)
    } else {
        println!("{}", err)
    }
}

Results:

$ cargo run -q --example downcast_ref
Root cause is std::io::Error: Oh no, we missed!

match_err! - matches on concrete error typese

use witcher::prelude::*;

fn do_something() -> Result<()> {
    do_external_thing().wrap("Failed to slay beast")
}

fn do_external_thing() -> std::io::Result<()> {
    Err(std::io::Error::new(std::io::ErrorKind::Other, "Oh no, we missed!"))?
}

fn main() {
    let err = do_something().unwrap_err();

    // Match multiple downcasted cases to handle errors differently
    match_err!(err.last(), {
        x: Error => println!("Root cause is witcher::Error: {}", x),
        x: std::io::Error => println!("Root cause is std::io::Error: {}", x),
        _ => println!("{}", err)
    });
}

Results:

$ cargo run -q --example downcast_match
Root cause is std::io::Error: Oh no, we missed!

pass - pass error through transparently

In some cases it might be nice to be able to use the Witcher common error pattern but treat particular top level errors as a pass through.

WARNING: this will not work with the match_err! macro which uses the type directly for matching rather than indirectly when working with an Error type.

use witcher::prelude::*;

fn do_something() -> Result<()> {
    do_external_thing().pass()
}

fn do_external_thing() -> std::io::Result<()> {
    Err(std::io::Error::new(std::io::ErrorKind::Other, "Oh no, we missed!"))?
}

fn main() {
    let err = do_something().unwrap_err();

    // Since we used `pass` we can match on the error directly
    match err.downcast_ref::<std::io::Error>() {
        Some(err) => println!("Root cause is std::io::Error: {}", err),
        None => println!("Root cause is witcher::Error: {}", err),
    }
}

Results:

$ cargo run -q --example pass
Root cause is std::io::Error: Oh no, we missed!

Chaining

We can continue to leverage std::error::Error's source method for chaining of errors. The first error wrapped will retain its concrete type but errors there after in the chain have lost that information.

source - std::error::Error's source method is exposed

use witcher::prelude::*;
#[derive(Debug)]
struct SuperError {
    side: SuperErrorSideKick,
}
impl std::fmt::Display for SuperError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "SuperError is here!")
    }
}
impl std::error::Error for SuperError {
    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
        Some(&self.side)
    }
}

#[derive(Debug)]
struct SuperErrorSideKick;
impl std::fmt::Display for SuperErrorSideKick {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "SuperErrorSideKick is here!")
    }
}
impl std::error::Error for SuperErrorSideKick {}

fn do_something() -> Result<()> {
    do_external_thing().wrap("Failed doing super hero work")
}

fn do_external_thing() -> std::result::Result<(), SuperError> {
    Err(SuperError {side: SuperErrorSideKick})
}

fn main() {
    if let Err(err) = do_something() {

        // Traverse the error chain
        let mut source = Some(err.std());
        while let Some(err) = source {
            match_err!(err, {
                // Using alternate form of display for `Error` to get just the message
                x: Error => println!("Found witcher::Error: {:#}", x),
                x: SuperError => println!("Found SuperError: {}", x),
                x: SuperErrorSideKick => println!("Found SuperErrorSideKick: {}", x),
                _ => println!("unknown")
            });
            source = err.source();
        }
    }
}

Results:

$ cargo run -q --example chain
Found witcher::Error: Failed doing super hero work
Found SuperError: SuperError is here!
Found SuperErrorSideKick: SuperErrorSideKick is here!

Retries

We can retry failing code with a few different Result extension functions.

err_is - will return true if an error exists and is the given type

use witcher::prelude::*;

fn retry_on_concreate_error_type_using_err_is() -> Result<()> {
    let mut retries = 0;
    let mut result = do_external_thing();
    while retries < 3 && result.err_is::<std::io::Error>() {
        retries += 1;
        println!("retrying using err_is #{}", retries);
        result = do_external_thing();
    }
    result.wrap("Failed while attacking beast")
}
fn do_external_thing() -> std::io::Result<()> {
    Err(std::io::Error::new(std::io::ErrorKind::Other, "Oh no, we missed!"))
}

fn main() {
    println!("{}", retry_on_concreate_error_type_using_err_is().unwrap_err());
}

Results:

cargo run -q --example retry_err_is
retrying using err_is #1
retrying using err_is #2
retrying using err_is #3
 error: witcher::Error: Failed while attacking beast
 cause: std::io::error::Error: Oh no, we missed!
symbol: retry_err_is::retry_on_concreate_error_type_using_err_is
    at: examples/retry_err_is.rs:11:5
symbol: retry_err_is::main
    at: examples/retry_err_is.rs:18:22

retry_on - is a cleaner simplified way to do a similar thing as our err_is example

use witcher::prelude::*;

fn retry_on_concreate_error_type() -> Result<()> {
    do_external_thing().retry_on(3, TypeId::of::<std::io::Error>(), |i| {
        println!("std::io::Error: retrying! #{}", i);
        do_external_thing()
    }).wrap("Failed while attacking beast")
}
fn do_external_thing() -> std::io::Result<()> {
    Err(std::io::Error::new(std::io::ErrorKind::Other, "Oh no, we missed!"))
}

fn main() {
    println!("{}", retry_on_concreate_error_type().unwrap_err());
}

Results:

cargo run -q --example retry_on
std::io::Error: retrying! #1
std::io::Error: retrying! #2
std::io::Error: retrying! #3
 error: witcher::Error: Failed while attacking beast
 cause: std::io::error::Error: Oh no, we missed!
symbol: retry_on::retry_on_concreate_error_type
    at: examples/retry_on.rs:4:5
symbol: retry_on::main
    at: examples/retry_on.rs:16:22

retry - is similar to retry_on but doesn't take the type of error into account

use witcher::prelude::*;

fn retry() -> Result<()> {
    do_external_thing().retry(3, |i| {
        println!("std::io::Error: retrying! #{}", i);
        do_external_thing()
    }).wrap("Failed while attacking beast")
}
fn do_external_thing() -> std::io::Result<()> {
    Err(std::io::Error::new(std::io::ErrorKind::Other, "Oh no, we missed!"))
}

fn main() {
    println!("{}", retry().unwrap_err());
}

Results:

cargo run -q --example retry
std::io::Error: retrying! #1
std::io::Error: retrying! #2
std::io::Error: retrying! #3
 error: witcher::Error: Failed while attacking beast
 cause: std::io::error::Error: Oh no, we missed!
symbol: retry::retry
    at: examples/retry.rs:4:5
symbol: retry::main
    at: examples/retry.rs:16:22

Display

Witcher's Error type implements different functionality for each of the Display format options. They follow a level of verbosity in witcher from least information to most i.e. {} {:#} {:?} {:#?}

Normal: {} - will write out the first error message only

Alternate: {:#} - will write out all error messages in the chain

Debug: {:?} - will write out all error messaging with simplified backtracing

Alternate Debug: {:#?} - will write out all error messaging with simplified backtracing

Contribute

Pull requests are always welcome. However understand that they will be evaluated purely on whether or not the change fits with my goals/ideals for the project.

Git-Hook

Enable the git hooks to have automatic version increments

cd ~/Projects/witcher
git config core.hooksPath .githooks

License

This project is licensed under either of:

• MIT license LICENSE-MIT or http://opensource.org/licenses/MIT
• Apache License, Version 2.0 LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in this project by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.

Backlog

• Add rust doc comments
• Mechanism for converting to JSON maybe use {:#?}

Changelog

• 12/30/2020
  • Corrected the minimum required rustc badge