How to retrieve the underlying error from a Failure Error?

Question

When trying to open a broken epub/ZIP file with epub-rs, the zip-rs crate error (which doesn't use Failure) is wrapped into a failure::Error by epub-rs. I want to handle each error type of zip-rs with an distinct error handler and need a way to match against the underlying error. How can I retrieve it from Failure?

fn main() {
    match epub::doc::EpubDoc::new("a.epub") {
        Ok(epub) => // do something with the epub
        Err(error) => {
            // handle errors
        }
    }
}

error.downcast::<zip::result::ZipError>() fails and error.downcast_ref() returns None.

score 2 · Accepted Answer · answered May 10 '19 at 23:17

You can downcast from a Failure Error into another type that implements Fail by using one of three functions:

use failure; // 0.1.5
use std::{fs, io};

fn generate() -> Result<(), failure::Error> {
    fs::read_to_string("/this/does/not/exist")?;
    Ok(())
}

fn main() {
    match generate() {
        Ok(_) => panic!("Should have an error"),
        Err(e) => match e.downcast_ref::<io::Error>() {
            Some(e) => println!("Got an io::Error: {}", e),
            None => panic!("Could not downcast"),
        },
    }
}

For your specific case, I'm guessing that you are either running into mismatched dependency versions (see Why is a trait not implemented for a type that clearly has it implemented? for examples and techniques on how to track this down) or that you simply are getting the wrong error type. For example, a missing file is actually an std::io::Error:

// epub = "1.2.0"
// zip = "0.4.2"
// failure = "0.1.5"

use std::io;

fn main() {
    if let Err(error) = epub::doc::EpubDoc::new("a.epub") {
        match error.downcast_ref::<io::Error>() {
            Some(i) => println!("IO error: {}", i),
            None => {
                panic!("Other error: {} {:?}", error, error);
            }
        }
    }
}

Thank you, my depency versions were different. – unknown rustacean May 11 '19 at 18:50 — unknown rustacean, May 11 '19 at 18:50

How to retrieve the underlying error from a Failure Error?

1 Answers1