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Comment by tsimionescu | Hacker News Reader

I know about ? in Rust - the one thing that isn't very clear to me is how often it is enough. That is, with Go, it's quite typical to do something like:

  result, err := call()
  if err != nil {
    return fmt.Errorf("Error while trying to call: %v", err)
  }

Essentially manually building a stack trace. If just doing "return err", you end up with calls to a REST service failing with messages like `couldn't parse "" as int` even in internal logs, which isn't helpful. With exceptions (in any language except C++), the stack trace is often decent enough.

Does ? add any kind of context implicitly, or do you have to actually pattern match manually to add context?

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There is not (normally) context added automatically. One can use `map_err(|e| ...)` to add context like this:

    let result = call().map_err(|e| SomeError::MoreData(e))?;

Or if using one of the common error libraries to have non-specific error kinds:

    let result = call().context("some data")?;

    let result = call().with_context(format!("something happened: {}", other_thing))?;

For something that more directly matches the behavior of the go code, this (using the `anyhow` crate) is a possible match:

    let result = call().map_err(|e| anyhow::Error::msg(format!("Error while trying to call: {:?}", e)?;

Though one would normally avoid this when using anyhow (or in rust in general) as it means we're flattening the error instead of generating a list of causes.

if you only limit yourself to the standard library, you would need to unwrap and rewrap the error - although admittedly since Result<T, E> is a type like any other, you could add an extension function (through a trait) so that you can add context more easily. If you use use one of the error handling libraries though, you're in luck - adding context is usually a single function call away:

    //without context
    some_function()?;
    //with context, using anyhow
    some_function().context("function returned error")?;
    //with wrap_err, using eyre
    some_function().wrap_err("function returned error eyre")?;

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