I have a [library](https://github.com/kastiglione/Parse-RACExtensions) that RACifies Parse's ObjC API. There are a number of APIs like this one, that provide a callback with a boolean success value. Personally, I see boolean success as either completion or error, so for those APIs, I don't send the boolean, I either call `-sendCompleted` or I call `-sendError:`. In this case, since the API doesn't provide you with an error, you'd have to generate one of your own. The value to doing it this way is that if you're composing a signal that depends on successful log in, then sending an error means consumers of this signal don't have to wedge error handling into the stream of values.