Add Optional error codes to identify errors

[callicles]

Hi,

When interacting rust code with other languages, we can't do pattern matching on error types.
As a consequence being able to add a method to define error code according of the type of the error would be great.

Thank you

[Yamakaky]

What do you mean? Do you have an example?

[callicles]

I am building a library that's called from languages. More explicitly, I am writing multiple plugins for IDEs. Each IDE has its own plugin and system language. As such, not to have to rewrite most of the logic, I use rust as a common backend. I use Json to communicate, transmitted with pointers as strings to communicate between the rust and the plugin. One of the shortcoming of that serialization are two things with error-chain I need to be able to serialize the backtrace to transmit it to the calling plugin. And I need some way of identifying errors based on some other thing than the message error itself. That's why I want to add error codes, to be able to send error identification information through the frontier between rust and the plugin. (Hope that's more clear)

[Yamakaky]

You mean that you want to be able to associate some data to an error?

[callicles]

yes, data conditional to the type of the error

[Yamakaky]

You can already do that for normal errors, and you would like to do it for links and foreign_links errors? Any idea about which macro change you would like?

[callicles]

What do you mean, I can do it for normal errors? In the errors block within the error_chain macro?

As for the macro change we could have a codes block that associates foreign_links and links to a string (or usize) code like so:

codes {
    Temp, "#0001";
}

To access it we could have a .code method on the error that returns the corresponding error code.

[Yamakaky]

you can do

errors {
    Temp(code: u16) {}
}

I don't really see what would be the benefit. I thought you where talking about getting the code above to work with links

[callicles]

I was, the Temp was supposed to be a foreign link. In the example I went for the result of the mapping.

However, if we can use the result of the mapping to add custom fields then there might not need any addition to the macro.

[Yamakaky]

something like this then?

foreign_links {
    io::Error, Io<u16>;
}