Rust: Enums to Wrap Multiple Errors

Re:Rust: Enums to Wrap Multiple Errors by Macha on Friday October 08, 2021 @11:36AM (#28800429)

You can match on std::io::ErrorKind, which is available by calling kind() on your error. This effectively has the same result as error inheritance hierarchies in Java and C#.

I don't think you really would treat a failure in File::open and read_to_string() differently in most applications. If you have a function doing file IO you might retry reading a read_to_string() error, but you know that the file exists because you know which function you just called.

If you're at higher level of abstraction, where something is calling both open() and read_to_string() for you, are you really going to handle it differently that frequently whether it's a missing file or a file on an unavailable storage medium?

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Re:Rust: Enums to Wrap Multiple Errors by nagisa on Friday October 08, 2021 @11:51AM (#28800597)

In my personal experience distinct handling of errors like these does indeed come up significantly more rarely. What ends up mattering to me much more often is how these errors are presented to the user, or what ends up in the logs in production. In those cases something like

    error: failed to read contents of `foo`
      caused by: a physical I/O error has occurred (...)

and

    error: failed to open `foo`
      caused by: permission denied (...)

are significantly more actionable than just the underlying I/O errors by themselves. To the point where a person reading these messages has a shot at addressing the problem without any prior knowledge of the code base.

In `anyhow` adding such context via its `context` family of methods seems fairly well accepted. I hope that my comment demonstrates there's no reason why `enum` based approach would need to be any worse than what `anyhow` can achieve.

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Re:Rust: Enums to Wrap Multiple Errors by shepmaster on Friday October 08, 2021 @12:29PM (#28801124)

> how these errors are presented to the user, or what ends up in the logs in production. In those cases something like [...] are significantly more actionable than just the underlying I/O errors by themselves. To the point **where a person reading these messages has a shot at addressing the problem** without any prior knowledge of the code base.

(emphasis mine)

This is a huge belief of mine as well. I call it a "semantic stacktrace".

I actually go as far as to say that _most of the time_ backtraces in errors are an antipattern, as it gives a false sense that an error is actionable ("oh, just look in the code!").

I even try to make it such that each leaf error is constructed in exactly one place so that grepping for the error identifies that one location.

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Re:Rust: Enums to Wrap Multiple Errors by Macha on Friday October 08, 2021 @12:06PM (#28800811)

For your logs you're going to print the error's default message anyway, so you should have the underlying message of the std::io::Error as long as it's listed in the source of your wrapped error.

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Re:Rust: Enums to Wrap Multiple Errors by shepmaster on Friday October 08, 2021 @12:08PM (#28800835)

> you should have the underlying message of the std::io::Error

This is a point of debate[1] among the error-handling working group.

[1]: https://github.com/rust-lang/project-error-handling/issues/4...

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