0000-extended-try.md

• Start Date: 2014-06-25
• RFC PR:
• Rust Issue:

Summary

Extend the try! macro to include an optional second parameter that is a constructor to wrap around the original error in case of failure.

Motivation

try! is a useful macro when dealing with many functions that return Results, but they become useless when the Result type that the programmer wants to return has a different failure type. For example, in a function that uses Io and Regex, two different error types could be generated (IoError, and Regex::Error). The author could not choose either of these errors to return because neither is extendable with the other. Instead it is common for library and application authors to create their own error types that wrap the errors that could possibly be produced. Unfortunately, this means that the try! macro is no longer useful.

Detailed design

This RFC proposes adding another argument to the try! macro that would be used as a constructor to wrap around existing error types. For example:

enum MyError {
  IoFailed(IoError),
  RegexFailed(regex::Error)
}

fn read_then_regex(filename: &str, regex: &str) -> MyError {
   let file = try!(File::open(filename), IoFailed);
   let regex = try!(Regex::new(regex), RegexFailed);
   // do things
}

The desugared version of this example (which is required to implement this pattern today) would look like:

fn read_then_regex(filename: &str, regex: &str) -> MyError {
   let file = match File::open(filename) {
     Ok(a) => a,
     Err(x) => IoFailed(x)
   };
   let regex = match Regex::new(regex) {
     Ok(a) => a,
     Err(x) => RegexFailed(x)
   };
   // do things
}

The motivation for this improvement is the exact same as the motivation for the original try! macro.

The original form of the try! macro would still be valid and would continue to work without any changes.

Drawbacks

Adds confusion. It is not immediately obvious as to what the 2nd argument is for if the reader is not already familiar with it.

Alternatives

• Create another macro that is very similar
  • (named try_or! ?).
• Create another macro that is very similar and place it in an external library