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A proposal for new syntax construct to define errors

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When we define an error class in a module, we do one of the following.

module App
  class Error < StandardError; end
  class ServerError < Error; end
  class ClientError < Error; end
end
module App
  Error = Class.new(StandardError)
  ServerError = Class.new(Error)
  ClientError = Class.new(Error)
end

IMO, the ugliness of the syntax is partly responsible that not many libraries have custom errors of their own, even when it makes sense.

It would be great if we could write this way instead:

module App
  define_error Error                            # inherits StandardError by default
  define_error ServerError, ClientError < Error # inherits App::Error
end

Which would encourage define errors.

I realized that the same could apply to empty class inheritance in general, but errors are much more likely to inherit without adding any features - thus naming specifically define_error here.

If you put ":" before the error names, you can define it by yourself.

  define_error :Error
  define_error :ServerError, :ClientError, super: Error

I agree that it's doable as a gem that way.

I just think if the syntax was a built-in, more libraries would be encouraged to define custom errors, without worrying about external dependency other than ruby itself.

So, ticket filed: http://bugs.ruby-lang.org/issues/7376

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