Strings as errors

This pattern is a way to create new errors without doing much set up. It is definitely the sloppiest way to throw errors. It can be great to use this during prototyping, but maybe not in the final product.

String types do not implement Fail, which is why there are two adapters to create failures from a string:

  • failure::err_msg - a function that takes a displayable type and creates a failure from it. This can take a String or a string literal.
  • format_err! - a macro with string interpolation, similar to format! or println!.
fn check_range(x: usize, range: Range<usize>) -> Result<usize, Error> {
    if x < range.start {
        return Err(format_err!("{} is below {}", x, range.start));
    if x >= range.end {
        return Err(format_err!("{} is above {}", x, range.end));

If you‘re going to use strings as errors, we recommend using Error as your error type, rather than ErrorMessage; this way, if some of your strings are String and some are &'static str, you don’t need worry about merging them into a single string type.

When might you use this pattern?

This pattern is the easiest to set up and get going with, so it can be great when prototyping or spiking out an early design. It can also be great when you know that an error variant is extremely uncommon, and that there is really no way to handle it other than to log the error and move on.

Caveats on this pattern

If you are writing a library you plan to publish to, this is probably not a good way to handle errors, because it doesn‘t give your clients very much control. For public, open source libraries, we’d recommend using custom failures in the cases where you would use a string as an error.

This pattern can also be very brittle. If you ever want to branch over which error was returned, you would have to match on the exact contents of the string. If you ever change the string contents, that will silently break that match.

For these reasons, we strongly recommend against using this pattern except for prototyping and when you know the error is just going to get logged or reported to the users.