Error.swift

extension Error {

  var localizedMessage: String {
    if let error = self as? SignIn.Error { return error.underlying?.localizedMessage ?? error.localizedMessage }
    if let error = self as? Network.Error { return error.localizedMessage }
    if let error = self as? Store.Error { return error.localizedMessage }
    return L10n.commonErrorMessage // Fallback error message
  }

  var underlying: Swift.Error? {
    if let error = self as? SignIn.Error { return error.underlying }
    if let error = self as? Network.Error { return error.underlying }
    return nil
  }
}

I think this could be improved by casting errors to Underlying or Localizable like this:

extension Error {

    var localizedMessage: String {
        if let error = self as? Underlying,
           let message = error.underlying?.localizedMessage {
            return message
        }

        if let error = self as? Localizable {
            return error.localizedMessage
        }

        return L10n.commonErrorMessage // Fallback error message
    }

    var underlying: Swift.Error? {
        if let error = self as? Underlying {
            return error.underlying
        }

        return nil
    }
}

This way you can just add new errors and they will work out of the box.

What do you think?

If you want to override a particular error case, you can just add it.

Hi Eduard, thank you for your feedback!

Yes, I agree with you, we can improve the code below.

For the Medium post & to be sure by reading the code that we manage all potential subcases, we've put all cases separately.

The opposite is good too, as you expose, by generalize the behavior and specify it after.
That makes sense ;)