Is it possible to embed information in errors?

[hasenj]

For example, say I'm parsing something and I hit an unexpected character, so I do something like:

return error.invalidChar;

But I also want the error to carry information about which char was invalid.

Is there a way to do that?

[andrewrk]

The suggestion for this situation is to make your own type with enum and use that.

const Result = enum {
    Ok,
    InvalidChar: usize,
};
fn parse(input: []const u8) -> Result {
    // if it works
    return Result.Ok{};
    // if it has unexpected char
    return Result.InvalidChar { position };
}

[thejoshwolfe]

This is brushing against a general issue which is that zig error codes are used a lot like exception objects (in Java, for example), but zig error codes are missing lots of useful features of exception objects. Another missing feature that would be useful is a stack trace.

[hasenj]

Thanks @andrewrk, I think that makes sense to me.

Regarding stack traces, I think it would be useful if there was a compiler function to get current line number, e.g. @lineinfo() which could return something like a struct { filename: []const u8, line: u64, col: u64 } or something like that.

Though I'm not sure how that would interact with the error enum, or if it even should.

[andrewrk]

@lineinfo() which could return something like a struct { filename: []const u8, line: u64, col: u64 } or something like that.

I agree, I was thinking along the same lines. Want to make a new issue with a proposal? We can use that to track the feature.

[hasenj]

opened #577

[andrewchambers]

The proposed solution will not work with errdefer though?

[andrewrk]

The proposed solution will not work with errdefer though?

In this case you can populate the error data, and then return a general error such as error.SemanticAnalysisFailed. This is the strategy taken by the self hosted compiler, for example.