Proposal: Support for error types

[jurajkulich]

Hello,
At the moment, it's not possible to differentiate errors via error types. This could help determine the error directly instead of using, e.g. strings.Contains.

For example:

if err := env.Parse(c); err != nil {
	if err == env.ErrMissingRequired {
	}
	if err == env.ErrEmpty {
	}
}

[akutuev]

@caarlos0 should I take a look at that improvement?

[caarlos0]

ah, this is a good one indeed

yes @akutuev feel free to take a hit at it

thanks everyone!

[akutuev]

Hello all,

I did some brief investigations and let me put my input here: Due to our new improvement #208 (and its PR #233) we aggregate multiple errors (even with multiple types) to be able to provide a single aggregated message for a user. Therefore to identify the error type we have to iterate over the list of errors.

If it makes sense, The user should:

1. Covert type of the Parse/ParseWithFunc error as AggregateError to get access to the list of errors
2. Iterate over errors to get a specific type of the error

In general, this approach won't lead to breaking changes but might bring some complexity

@caarlos0 @jurajkulich Happy to get your opinion

[caarlos0]

@akutuev that sounds good yes!

I wonder if we should wait to see how the aggregate errors are looking like in go 1.20... haven't looked much into it yet, but maybe it makes this easier...

[caarlos0]

merged #240