[theoh]

The way I see it, each of the data types in a sum type is analogous to a channel. The type of the 'message' conveyed by a value is effectively "Either Control Data".

In-band signalling is messier than out-of-band signalling, so I see out-of-band as the goal. If you look at the various approaches to the semipredicate problem on that Wikipedia page, it looks like, historically, we started out with awkward in-band solutions and the evolution is towards the clarity offered by expanding the return type to send errors out-of-band.

I mentioned this previously on HN, and maybe that earlier comment will clarify what I'm talking about: https://news.ycombinator.com/item?id=18766478

pjc50's way of applying the terms to flow of control is a somewhat different thing.