RFC 1119: result-expect

libs (panic | error-handling)

• Feature Name: result_expect
• Start Date: 2015-05-13
• RFC PR: rust-lang/rfcs#1119
• Rust Issue: rust-lang/rust#25359

Summary

Add an expect method to the Result type, bounded to E: Debug

Motivation

While Result::unwrap exists, it does not allow annotating the panic message with the operation attempted (e.g. what file was being opened). This is at odds to 'Option' which includes both unwrap and expect (with the latter taking an arbitrary failure message).

Detailed design

Add a new method to the same impl block as Result::unwrap that takes a &str message and returns T if the Result was Ok. If the Result was Err, it panics with both the provided message and the error value.

The format of the error message is left undefined in the documentation, but will most likely be the following

panic!("{}: {:?}", msg, e)

Drawbacks

• It involves adding a new method to a core rust type.
• The panic message format is less obvious than it is with Option::expect (where the panic message is the message passed)

Alternatives

• We are perfectly free to not do this.
• A macro could be introduced to fill the same role (which would allow arbitrary formatting of the panic message).

Unresolved questions

Are there any issues with the proposed format of the panic string?