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failure + error-chain = πŸ’–
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failchain is a tiny companion crate to the failure crate, which aims to recover the ergonomics of error_chain. It does this by bringing back:

  • chain_err
  • non-verbose Error, ErrorKind pairs
  • support for bail! and ensure! with custom ErrorKind-s

The failure library recommends three different patterns for errors. This library implementes the most complex one (and the most useful one) but without all the boilerplate.

What it looks like

use failchain::{BoxedError, ChainErrorKind};
use failure::Fail;
use std::path::PathBuf;
use std::result::Result as StdResult;

pub type Error = BoxedError<ErrorKind>; // Use `UnboxedError` instead for
                                        // non-allocating, but bigger `Error`.
pub type Result<T> = StdResult<T, Error>;

#[derive(Clone, Eq, PartialEq, Debug, Fail)]
pub enum ErrorKind {
    #[fail(display = "Metadata I/O error {:?}.", 0)]

    #[fail(display = "Corrupt metadata file: {}", 0)]

    #[fail(display = "WAD I/O error {:?}.", 0)]

    #[fail(display = "Corrupt WAD file: {}", 0)]

impl ChainErrorKind for ErrorKind {
    type Error = Error;

use super::errors::{ErrorKind, Result};
use failchain::{
    ResultExt, // for `chain_err`,
use std::fs::File;
use std::io::Read;
use std::path::Path;

fn validate_metadata(path: &Path, metadata: &str) -> Result<()> {
    // `ensure` is like `assert` (or failure's ensure), except it allows you to
    // specify the `ErrorKind`.
        ErrorKind::CorruptMetadata(format!("metadata file {:?} is empty", path))

    // a special mode of `ensure` works for functions that return `ErrorKind`-s
    // and take a single string as argument:
        metadata.len() > 100,
        ErrorKind::CorruptMetadata, // Any FnOnce(String) -> ErrorKind
        "metadata file {:?} is too long", // Rest of arguments are format args.

    // `bail` is like `ensure`, but without the condition and always returns
    // early.
        "validation isn't actually implemented"

fn load(wad: &Path, metadata: &Path) -> Result<()> {
    // `chain_err` stashes the original error as the `cause` of a new error.
    let wad_file = File::open(wad).chain_err(|| ErrorKind::WadIo(wad.to_owned()))?;

    let mut metadata_content = String::new();

    // `chain_inspect_err` stashes the original error as the `cause` of the new
    // error, but it first allows the callback to inspect it.
    let metadata_file = File::open(metadata)
        .and_then(|mut file| file.read_to_string(&mut metadata_content))
        .chain_inspect_err(|_io_error| ErrorKind::MetadataIo(metadata.to_owned()))?;

    validate_metadata(metadata, &metadata_content)?;


Future Plans

This library is a proof-of-concept and was developed as part of the error_chain -> failure migration of rust-doom and inspired by @burntsushi's error handling solution in imdb-rename.

The main issue with the current library is that failchain must be leaked as a public dependency via the Error type-alias. This would be easily solved with a custom derive which implements the necessary From and Fail impl-s e.g.

pub struct Error(BoxedError<ErrorKind>);

// expanding to
impl Fail for Error { /* delegate */ }
impl From<ErrorKind> for Error { /* delegate */ }

The error handling story in Rust is a little up in the air right now, but in my opinion any eventual solution should include these pieces (as error_chain did).


failure is a public dependency of failchain. The library versioning scheme is <failchain_major><failure_version><failchain_minor>. So 0.1015.2 would be version 0.1.2 of failchain with a dependency on version 0.1.5 of failure.

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